Self Assessment Step by Step (2026/27)

1. Overview: what Self Assessment is

Self Assessment is the framework HMRC uses to collect Income Tax (and Class 2 / Class 4 National Insurance, and Capital Gains Tax) from people whose income does not pass through payroll. The system has run in its current form since 1996 and is used by roughly 12 million UK taxpayers each year. Crucially, Self Assessment is a collection mechanism, not a separate tax: the rates, bands and reliefs are the same as those applied through PAYE. The difference is that you, the taxpayer, are responsible for declaring untaxed income and paying the resulting bill rather than your employer handling it through monthly payroll deductions.

Most Self Assessment filers are also PAYE employees. The return picks up the P60 figures from the employer (already taxed at source) and adds the untaxed income on top - rental profit, dividends, sole-trader profit, foreign income, capital gains. HMRC then calculates the marginal tax due on the combined total and bills the difference as the balancing payment due 31 January following the end of the tax year. You do not pay tax twice on the PAYE slice - the return credits the PAYE deducted at source against the total bill. For the full PAYE vs Self Assessment comparison and decision tree see PAYE vs Self Assessment.

2. Who must file Self Assessment

HMRC publishes the definitive checker at gov.uk/check-if-you-need-tax-return. You must file a return for 2026/27 if any of the following apply to you in that tax year.

Self-employed sole trader. If your gross self-employment income exceeds the £1,000 trading allowance, you must register for Self Assessment and Class 2 NIC. This includes freelance work, side gigs, market trading, online sales above hobby thresholds, taxi driving, gardening, tutoring - any activity where you provide services or sell goods on your own account rather than as an employee. Income under £1,000 gross is covered by the trading allowance and requires no registration or return.

Partner in a partnership. Each partner files their own SA return declaring their share of partnership profits, regardless of how the profit was distributed in cash. The partnership itself also files a separate SA800 partnership return through the nominated partner. LLPs (limited liability partnerships) work the same way for tax purposes - the LLP files SA800 and each member files their own SA100 with SA104 supplementary pages.

Company director. Directors who receive untaxed income (typically dividends from their own company) or whose total income exceeds £150,000 file Self Assessment. The old rule requiring all directors to file regardless of income was relaxed by HMRC in 2018 - a director whose only income is a small PAYE salary below £150,000 and who takes no dividends does not need to file. But the moment dividends exceed the £500 allowance, or salary plus dividends pushes total income over £150,000, filing becomes mandatory.

Total income over £150,000. HMRC removed the old £100,000 threshold in April 2023 and replaced it with £150,000. Above that figure from any source - salary, bonus, pension drawdown, rental, dividends, foreign income combined - you must file. People earning £100,000-£150,000 are no longer automatically required to file on income grounds alone, but most still file because at that income level the Personal Allowance taper, HICBC and pension annual allowance taper typically trigger SA on other grounds.

Untaxed income. Rental property income above £1,000 gross (the property allowance equivalent of the trading allowance); foreign income of any amount; dividends above £500 (down from £1,000 in 2023-24 and £2,000 in 2022-23); interest and savings income that exceeds what HMRC can collect through your tax code; and untaxed pension income that has not been brought into PAYE. Crypto disposals also count as untaxed income if they generate capital gains - many filers forget this and trigger penalty assessments when HMRC matches exchange data via the Common Reporting Standard.

Capital Gains above the annual exempt amount. The CGT annual exempt amount (AEA) is £3,000 from 2024-25 onwards, down from £12,300 in 2022-23 and £6,000 in 2023-24. Disposals that produce gains above the AEA must be declared on SA108 - shares, second-hand assets sold above £6,000, second properties, crypto, business asset disposals, foreign assets. UK residential property disposals are reported separately within 60 days via the UK Property Disposal service.

High Income Child Benefit Charge. If you or your partner claim Child Benefit and either of you has adjusted net income above £60,000, the higher earner must file to declare HICBC. The charge tapers between £60,000 and £80,000 (1% of the benefit for every £200 of income over £60,000), reaching 100% clawback at £80,000. The threshold was raised from £50,000 to £60,000 in April 2024 and the upper bound from £60,000 to £80,000 - so the band is now wider but starts later. See HICBC explained for the full mechanics.

State Pension above the Personal Allowance. If your State Pension alone exceeds the £12,570 Personal Allowance (rare but possible with a deferred-pension uplift), you may need to file. More commonly, retirees file when State Pension plus a private pension plus savings interest combined exceeds the PA - because the State Pension is paid gross (no tax deducted at source) and HMRC needs the return to calculate the right code adjustment.

3. Registration: getting a UTR

Before you can file, you need a Unique Taxpayer Reference (UTR) - a 10-digit number that identifies you to HMRC for Self Assessment. Registration happens at gov.uk/register-for-self-assessment and is a one-off action: you keep the same UTR for life.

Deadline to register. You must register by 5 October following the end of the tax year you first became liable. For the 2026/27 tax year (ending 5 April 2027) that means registering by 5 October 2027. Missing the registration deadline can trigger a separate Failure to Notify penalty in addition to filing and payment penalties, calculated as a percentage of the tax owed. The practical rule is: as soon as you know you will be liable (you start trading, you start renting out a property, you hit the dividend threshold), register straight away. There is no penalty for registering early.

Online registration via gov.uk. The form asks for your name, date of birth, address, National Insurance number, contact details, and the reason for registration (self-employed, partner in a partnership, not self-employed but need to file). The flow differs slightly by category - self-employed registration also includes Class 2 NIC enrolment and lets you optionally register for VAT in the same flow if your turnover is above the VAT threshold.

Government Gateway login. If you do not already have one, registration creates a Government Gateway user ID for you. This is your single sign-on for every HMRC online service - Self Assessment, PAYE, VAT, Personal Tax Account, Marriage Allowance, Pension forecast, and so on. You will need to set up identity verification, usually via a combination of passport / driving licence / payslip / P60 / Credit Reference Agency questions. The verification step can fail if you have a thin credit file or recently moved - have a backup option (such as your passport details) ready.

UTR and activation code by post. HMRC posts your UTR within 10 working days, then a separate activation code within a further 10 working days. The activation code is required the first time you log in to the SA online service - it confirms the postal address on the account. The code expires after 28 days from issue. If it lapses, request a new one through the Government Gateway. Plan at least three weeks between registration and first filing - longer if you are abroad and post is slow.

Self-employed: register for Class 2 NIC. The self-employed registration flow at gov.uk automatically sets you up for Class 2 NIC as well as Income Tax Self Assessment. Class 2 is now voluntary above the Small Profits Threshold (since April 2024) but still worth paying - it protects your State Pension qualifying year for under £4 a week, the cheapest way to buy a State Pension year. The Class 2 election sits on the SA103 supplementary pages and you tick a box each year to confirm whether you want to pay voluntarily.

4. UK tax year structure

The UK tax year runs from 6 April to 5 April - a quirk that dates back to the 1752 calendar reform when Britain switched from the Julian to the Gregorian calendar. Every Self Assessment milestone references this fiscal year, not the calendar year, so it pays to internalise the dates.

Worked example: the 2025/26 tax year. Ran from 6 April 2025 to 5 April 2026. Any income you received in that 12-month window is reportable on the 2025/26 SA return. The return becomes available shortly after the tax year ends (HMRC opens online filing in early-to-mid April 2026), and the deadlines for the 2025/26 return are:

• 5 October 2026 - register for SA if it is your first 2025/26 filing year.
• 31 October 2026 - paper SA100 deadline.
• 30 December 2026 - online filing deadline if you want HMRC to collect a bill under £3,000 through your 2027/28 PAYE code.
• 31 January 2027 - online filing + balancing payment + first payment on account toward 2026/27.
• 31 July 2027 - second payment on account toward 2026/27.

The 2026/27 tax year (which is the current SalaryTax engine year) runs 6 April 2026 to 5 April 2027. The return for 2026/27 is due online by 31 January 2028, with the balancing payment and first PoA on the same date, second PoA due 31 July 2028. For the full deadline calendar including VAT, PAYE, CIS and Corporation Tax, see tax deadlines 2026/27.

5. Filing methods

There are four routes for getting your return into HMRC. The right one depends on the complexity of your tax affairs and whether you keep books in commercial software.

Online via HMRC Personal Tax Account. The default and most common route - around 96% of returns are filed online via HMRC's free service. Log in at gov.uk with your Government Gateway credentials, navigate to Self Assessment, pick the year, fill in the sections relevant to your income, and submit. The deadline is 31 January following the tax year end. HMRC's free filing service supports SA100, SA101 (additional income), SA102 (employment), SA103 (self employment short and full), SA104 (partnership), SA105 (property), SA108 (capital gains) and SA109 (residence and remittance). It does not handle SA107 (trust income) - you need commercial software or an agent for that.

Paper return SA100. The traditional postal form with the early 31 October deadline. HMRC no longer automatically posts SA100 to most filers - if you want a paper return you must request one from the Self Assessment helpline. Penalties for paper returns filed after 31 October are the same as for late online returns (£100 fixed plus escalating daily and milestone charges) even if you submit before the 31 January online deadline. Useful mainly for taxpayers without reliable internet access or for people who need the SA107 trust pages, which the HMRC online service does not support.

Commercial software. Self-employed filers with bookkeeping in FreeAgent, QuickBooks, Xero, Sage or similar can submit directly through the software. This is especially convenient because the software already holds your profit and loss figures and pre-populates the SA103 - no retyping. The cost varies from free (FreeAgent for NatWest / RBS / Mettle business banking customers) to roughly £30-£50 per month. From April 2026, all MTD ITSA filers will be required to use commercial software for quarterly updates - see the MTD section below.

Agent filing (accountant). You authorise an accountant via Form 64-8 (or the digital equivalent via the Agent Services Account) and they file on your behalf. Agents use specialist software (CCH, Iris, Taxfiler, BTCSoftware) that supports every SA supplementary page including SA107. Cost typically £150-£500 per individual return depending on complexity. Worth it if you have multiple income sources, a property portfolio, foreign income, capital gains, or have historically filed late - agents have direct lines to HMRC that taxpayers do not.

6. Step-by-step online filing walkthrough

The following eight steps cover a standard PAYE-plus-side-income online return through HMRC's free service. Allow 90 minutes to two hours for your first filing - faster in subsequent years once you know which sections apply.

Step 1: log in via Government Gateway. Go to gov.uk and search "sign in to your personal tax account". Enter your Government Gateway user ID and password. You will be prompted for the two-factor code sent to your registered phone or authenticator app. If you have lost access to the account, follow the recovery flow - this can take 5-10 working days, so do not leave it until 30 January.

Step 2: select Self Assessment for the relevant year. From the Personal Tax Account home, click Self Assessment, then "Tell HMRC about your income" for the tax year you are filing. The service shows all years still open for amendment - you can amend a return for 12 months after the original filing deadline, so 2025/26 returns can be amended until 31 January 2028.

Step 3: review pre-populated PAYE income. HMRC auto-fills the employment section from your employer's Real Time Information (RTI) submissions. Cross-check against your P60. If figures differ (which happens occasionally with mid-year job changes or BIK adjustments), ask your employer for a corrected P60 first - do not just overwrite the figures on the SA return without supporting evidence, because HMRC will query large discrepancies.

Step 4: complete sections for unfilled income. Work through the tailored questionnaire HMRC presents at the start of the return - it asks yes/no questions about each possible income source (self-employed, property, dividends, foreign, capital gains, etc) and unlocks the supplementary sections you answer yes to. Use the printed numbers from your bookkeeping, dividend vouchers, P11D, P60, P45 and any Investment Income Certificates from your platform. Crypto exchanges issue gain/loss statements at year end - download these and reconcile to your own records before transcribing to SA108.

Step 5: claim reliefs. Higher-rate and additional-rate taxpayers must claim the extra 20% or 25% pension tax relief on personal contributions via Self Assessment - the basic 20% has already been added to the pension by the provider under Relief at Source. Gift Aid donations also trigger an extra-rate top-up that you must claim on the return. EIS, SEIS, VCT and Social Investment Tax Relief credits go here too, as does Marriage Allowance transfer if either spouse is below the PA. The return prompts you for each in turn - tick the boxes that apply and fill in the amount.

Step 6: review calculation summary. Once all income and reliefs are entered, HMRC's service computes the tax due in real time and presents a summary: total tax owed, Class 2 + Class 4 NIC, Capital Gains Tax, less PAYE already paid at source, less any reliefs - equals the balancing payment. Triple-check the figures. The calculation step also shows whether payments on account toward next year will be required, and the amount of each PoA. Print or save the PDF - it is the only formal record of the calculation HMRC will send.

Step 7: submit and download confirmation. Click Submit. HMRC issues an instant on-screen confirmation with a unique submission reference. Download the PDF confirmation - it is your proof of filing if HMRC later claims they did not receive it. The submission is recorded with a timestamp; a submission at 23:58:59 on 31 January counts as on time, but the service does occasionally lag under load, so do not cut it that fine.

Step 8: pay any balancing payment + PoA. Payment is separate from filing - submitting the return does not pay the bill. Pay via the gov.uk pay-your-tax page using Direct Debit (set up at least 5 working days in advance), debit card, Faster Payment from your bank, CHAPS, or at a Post Office. The deadline for both balancing payment and first PoA is 31 January. Late payment triggers 5% surcharges at 30 days, 6 months, and 12 months, plus daily interest at base rate plus 2.5%. If you cannot pay, call HMRC's Time to Pay helpline to arrange instalments - they are usually flexible for genuine cashflow problems.

7. Self-employment supplementary pages SA103

SA103 covers sole-trader and freelancer profit and loss. It comes in two flavours: SA103S (short) for simple businesses with turnover under £85,000 and no complex adjustments, and SA103F (full) for everyone else. You declare turnover, allowable expenses, capital allowances and the resulting taxable profit.

Cash basis vs accruals. Since April 2024 the cash basis (revenue when received, expenses when paid, no debtor / creditor adjustments) is the default for new sole traders. You can elect into accruals if your business has stock, work-in-progress, or you want unrestricted loss relief. Cash basis is simpler but caps interest deductions at £500 and restricts loss carry-back. Tick the cash-basis box on SA103 each year - you can switch back and forth between methods, though there are anti-fragmentation rules that prevent abuse.

Allowable expenses categories. The SA103 groups expenses into fixed categories: cost of goods bought for resale, car / van / travel expenses, rent / rates / utilities of business premises, repairs, professional fees, phone / internet / postage / stationery, advertising, interest and bank charges, depreciation (cash basis only - replaced by capital allowances under accruals), and other business expenses. The "wholly and exclusively" test applies: an expense must be incurred wholly and exclusively for the purposes of the trade. Mixed-use expenses (home office, personal mobile, family car used for business) must be apportioned reasonably.

Trading allowance £1,000. A simplified approach for very small traders - claim £1,000 against gross income instead of actual expenses. You pick one or the other, not both. Useful if your actual expenses are well under £1,000 - claiming the allowance saves you the bookkeeping. Beware: claiming the trading allowance gives up your ability to claim a loss. If you have a loss year, switch to actual expenses so you can carry the loss forward.

Capital allowances and AIA. Plant and machinery purchases (laptops, tools, equipment, business vehicles other than cars) attract capital allowances. The Annual Investment Allowance gives 100% first-year relief on the first £1,000,000 of qualifying expenditure each year - full deduction in year one rather than depreciation over multiple years. Cars have a separate writing-down allowance regime (18% / 6% pools depending on CO2 emissions). Model with the capital allowances calculator.

Class 2 + Class 4 NIC. SA103 automatically calculates Class 4 NIC at 6% / 2% on profits above the lower profits limit, and includes the optional Class 2 weekly tick box for voluntary State Pension protection. The amounts feed into the overall tax bill and are payable alongside Income Tax on the balancing payment due 31 January. Model your full take-home with the self-employed calculator.

8. Property income supplementary pages SA105

SA105 covers UK property income. Use SA106 for overseas property. The page asks for total rents received, allowable expenses, finance costs (special rules apply since 2020), and the resulting taxable profit. Property is taxed as investment income, not trading income - so it does not attract Class 2 or Class 4 NIC. See HMRC's primary guidance at gov.uk/guidance/income-tax-when-you-rent-out-a-property-working-out-your-rental-income.

Income and expenses. Declare gross rents received in the tax year. Allowable expenses include letting agent fees, repairs (not improvements - those go to capital), buildings and contents insurance, council tax / utilities if paid by the landlord, accountancy fees, and ground rent / service charge for leasehold flats. Travel to inspect the property and HMRC-approved mileage allowance for landlord use of personal car. The cash basis is now the default for property businesses with rental income under £150,000.

Section 24 mortgage interest credit. Since 6 April 2020, residential landlords can no longer deduct mortgage interest from rental income. Instead, they receive a 20% basic-rate tax credit on the interest, applied after the tax calculation. This pushed many higher-rate landlords into a higher effective tax rate because gross rents are now in the tax calculation, potentially tipping them into the 40% or 45% bracket. The SA105 has separate boxes for the interest amount - HMRC computes the credit automatically. The rule does not apply to commercial property, furnished holiday lets that meet the FHL conditions, or property held through a limited company.

Replacement of domestic items relief. When you replace a sofa, washing machine, bed, carpets or similar domestic items in a rented property, you can claim the cost of the replacement (less any proceeds from selling the old one) as an expense. The relief applies only to like-for-like replacements - upgrading a £200 sofa to a £600 one means you can only claim £200 of the cost. Initial furnishings when you first let the property are capital, not revenue, and cannot be claimed through SA105.

9. Capital Gains SA108

SA108 handles capital gains and losses on chargeable disposals in the tax year. See HMRC's primary CGT guidance at gov.uk/capital-gains-tax. You include disposals of shares (other than ISA-wrapped), second properties, business assets, crypto, foreign assets, and chattels (second-hand goods) over £6,000.

Disposal records. For each disposal, record acquisition cost (the original purchase price), acquisition date, disposal proceeds, disposal date, and allowable incidental costs (legal fees, dealing commissions, estate agent fees, stamp duty). The gain is proceeds minus cost minus incidentals. Share matching rules apply - same-day rule, then 30-day bed-and-breakfast rule, then Section 104 pool weighted average. Crypto follows the same pool rules.

Annual Exempt Amount £3,000. The first £3,000 of net gains in 2024/25 onwards is tax-free. It was £6,000 in 2023/24, £12,300 from 2020/21 to 2022/23. You cannot carry forward unused AEA - use it or lose it. If you have an unrealised gain in shares, selling enough each tax year to use the AEA can recycle the cost basis without triggering tax (subject to the 30-day bed-and-breakfast rule, which prevents immediate repurchase).

Rates. CGT rates changed during 2024-25. For disposals before 30 October 2024 the rates were 10% / 20% (basic / higher rate) for non-residential assets and 18% / 24% for residential property. From 30 October 2024 (Autumn Budget 2024) all assets follow the higher 18% / 24% rates regardless of asset class. Business Asset Disposal Relief rate moved from 10% to 14% on 6 April 2025, then rises to 18% from 6 April 2026. Investors' Relief was cut from a £10m lifetime cap to £1m. Model with the capital gains tax calculator.

Reliefs. Principal Private Residence (PPR) relief - your main home is fully exempt for the years you lived in it plus the final 9 months. Letting Relief - now restricted to landlords who shared occupation with the tenant. Business Asset Disposal Relief (formerly Entrepreneurs' Relief) - reduced rate on qualifying business disposals up to a £1m lifetime cap. Holdover relief - defers gain on gifts of business assets. Each is claimed via boxes on SA108 with supporting calculations.

Residential property 60-day report. UK residential property disposals must be reported to HMRC via the separate UK Property Disposal service within 60 days of completion, with the CGT paid at the same time. This is in addition to the SA108 declaration on the annual return - the 60-day return is essentially a pay-on-account credited against the final SA bill. Failing to file the 60-day return triggers a £100 fixed penalty plus daily charges, even if no CGT is due (for example if the gain falls within the AEA).

10. Payments on account

Payments on Account (PoA) are HMRC's mechanism for collecting tax in advance from filers whose bills consistently exceed what PAYE collects at source. The first time you owe more than £1,000 on Self Assessment (and less than 80% of your total tax was collected at source), HMRC asks for two PoA instalments toward the following tax year - 50% of the previous year balance each.

Schedule. First PoA due 31 January (same day as the balancing payment for the prior year). Second PoA due 31 July. The two PoAs add up to 100% of the prior-year SA liability and are credited against the following year tax bill when it is finalised. If your actual liability is higher, the balancing payment 31 January after that year ends settles the shortfall. If actual is lower, HMRC refunds the excess (with a small amount of interest at the official rate).

Reduce PoA if income is falling. If you know your income for the new year will be lower (for example you are winding down a side business, retired mid-year, or moved from self-employment to salaried work), you can apply online or via the SA helpline to reduce the PoA. The decision is at your own risk - if you reduce too aggressively and the actual bill turns out higher than the reduced PoA, HMRC charges interest on the shortfall from the original 31 January and 31 July dates, not from the eventual balancing payment. Conservative reductions are usually safer than aggressive ones.

Worked example. Self-employed taxpayer files 2025/26 SA return with £8,000 total tax owed. Balancing payment £8,000 due 31 January 2027. HMRC also asks for a first PoA toward 2026/27 of £4,000 (50% of the prior year) due 31 January 2027 - so total payment 31 January 2027 is £12,000. A second PoA of £4,000 toward 2026/27 falls due 31 July 2027. When the 2026/27 return is filed in January 2028, assume the actual liability is £9,000. PoAs already paid total £8,000, so the balancing payment 31 January 2028 is £1,000 - plus the first 2027/28 PoA of £4,500 (50% of £9,000), total 31 January 2028 payment £5,500. The pattern continues indefinitely while you remain in Self Assessment.

PoA does not apply if more than 80% of your total tax was collected at source via PAYE (typical for an employee with small side dividends), or if your prior-year SA liability was under £1,000. In those cases the balancing payment is the only SA payment - no PoA required.

11. Late filing + late payment penalties

Self Assessment has two parallel penalty regimes: one for late filing of the return, one for late payment of the tax owed. They run independently - missing both the filing and the payment deadline triggers both sets of charges. For the full HMRC penalty reference see HMRC penalties guide and HMRC's page at gov.uk/self-assessment-tax-returns/penalties.

Late filing penalties.

• 1 day late: automatic £100 fixed penalty, regardless of how much tax is owed (even if zero or a refund is due).
• 3 months late: daily penalties of £10 begin, running for 90 days - up to a maximum of £900 on top of the £100 fixed.
• 6 months late: a further £300 or 5% of the tax due, whichever is greater.
• 12 months late: another £300 or 5% of the tax due, whichever is greater. If HMRC suspects deliberate behaviour, the 12-month penalty can rise to 70% or 100% of the tax due.

Late payment penalties (these accumulate on top of any filing penalty if both are late).

• 30 days late: 5% of the tax outstanding.
• 6 months late: a further 5%.
• 12 months late: another 5%.
• Daily interest at the Bank of England base rate plus 2.5%, running from the original due date until full payment. At base rate 4.75% as of May 2026, the interest rate is 7.25% per annum.

Appeals and reasonable excuse. HMRC accepts a "reasonable excuse" appeal for genuine circumstances outside your control - serious illness or hospitalisation, bereavement of a close relative, fire / flood / theft, HMRC systems failure, postal disruption when relying on a paper return. Appeals must be lodged within 30 days of the penalty notice. Insufficient funds, ignorance of the deadline, or "the accountant did it" are not reasonable excuses. If the excuse fails, you can escalate to the First-tier Tax Tribunal - but for a £100 fixed penalty the tribunal route rarely makes economic sense.

12. Common mistakes

HMRC publishes statistics on the most common Self Assessment errors. The following six trip up tens of thousands of filers every year. Check each before submitting.

Forgetting dividend income. The dividend allowance has fallen from £2,000 (2022-23) to £1,000 (2023-24) to £500 (2024-25 onwards). Many filers remember the old threshold and assume small dividends below £1,000 do not need reporting - they do. Every dividend voucher from your broker or your own limited company counts. HMRC matches dividend data via the Common Reporting Standard for foreign holdings and via Companies House for UK closely-held companies, so omissions are routinely spotted.

Forgetting student loan repayments. If you have a Plan 1, 2, 4, 5 or Postgraduate Loan, the SA return has a tick box plus questions about your loan plan. PAYE employers already deduct the right amount from salary, but if you have additional self-employed or property income that pushes you over the threshold, more is due via Self Assessment. Many filers tick "no" because deductions came off payslips and miss the SA top-up. Model with the student loan calculator.

Missing higher-rate pension relief. Personal pension contributions paid net (Relief at Source) get the basic-rate 20% added automatically by the provider. Higher- and additional-rate taxpayers have to claim the extra 20% or 25% themselves on the SA return - it is not automatic. A higher-rate taxpayer contributing £8,000 net (£10,000 gross) to a SIPP can reclaim £2,000 of additional Income Tax relief. Most people who forget the claim miss it for years before an accountant spots it. Backdate up to four prior tax years if you find you missed it.

Not declaring crypto disposals. HMRC has treated crypto as a chargeable asset for CGT purposes since 2018. Every disposal - sale to GBP, swap between tokens, use to buy goods, gift to anyone other than a spouse - is a CGT event. The 30-day bed-and-breakfast rule applies just like shares. Through the Common Reporting Standard and direct data-sharing agreements with major exchanges, HMRC now receives transaction-level data on UK residents. Omitted crypto gains trigger nudge letters and assessments going back up to 20 years (deliberate behaviour) or 6 years (careless).

Not splitting foreign income correctly. If you have foreign rental, dividend or pension income, declare it on SA106 (foreign supplementary pages) - not on the main SA100 or the UK property SA105. Foreign Tax Credit Relief gives you a credit for tax paid abroad (subject to treaty limits), but only if you correctly identify the income as foreign on SA106. Mixing UK and foreign on the same page forfeits the credit and risks double taxation.

Mortgage interest treated as a deduction. For residential lets, mortgage interest stopped being a deductible expense on 6 April 2020. It is now a 20% tax credit applied after the tax calculation, not a deduction from rental profit. Treating it as a deduction overstates allowable expenses, understates profit, and triggers HMRC enquiries when they cross-reference your return against bank-reported interest paid. The rule still applies as a deduction for commercial property, qualifying FHLs, and limited-company landlords - but not for personal residential lets.

13. MTD for ITSA preparation

Making Tax Digital for Income Tax Self Assessment (MTD ITSA) is the biggest change to Self Assessment since the system began in 1996. It replaces the annual return with quarterly digital updates plus a final declaration, all submitted through HMRC-recognised software. The official guidance is at gov.uk/government/publications/making-tax-digital-for-income-tax.

Phased rollout.

• 6 April 2026: self-employed and landlords with combined gross income over £50,000.
• 6 April 2027: combined gross income over £30,000.
• Future (unscheduled): extension to over £20,000.

Combined gross income means the sum of self-employed turnover and gross property rents - so a £30,000 sole trader plus a £25,000 landlord crosses the £50,000 threshold and is in scope from April 2026. Companies, partnerships and trusts are not currently in scope - the rollout is sole-trader and individual landlord only.

Software requirement. Quarterly updates and the final declaration must be submitted through HMRC-recognised software - HMRC's free online filing service does not support MTD ITSA quarterly updates. The HMRC software-vendor list (available at gov.uk) includes FreeAgent, QuickBooks, Xero, Sage, Iris, FreshBooks, Pandle, Hammock (property-specific) and several others. Costs range from free (FreeAgent bundled with some business bank accounts) to £30-£50 per month for mid-tier subscriptions.

What you submit. Quarterly: a digital summary of income and expenses for each separate business and property activity, due one month after the quarter end. The standard quarters are 6 Apr - 5 Jul, 6 Jul - 5 Oct, 6 Oct - 5 Jan, 6 Jan - 5 Apr (with calendar-quarter alignment available as an election). Annually: a final declaration (replacing the SA100) due 31 January after the tax year end, pulling together all quarterly updates plus any other income (employment, dividends, interest, capital gains) and claiming reliefs.

Preparation. If you are in scope from April 2026, start digital bookkeeping now. Move from spreadsheets to MTD-compatible software, set up bank feeds, and run your books in real time rather than catching up at year end. Quarterly deadlines are unforgiving - HMRC has published a new penalty regime with points-based late-filing penalties (£200 charge after a threshold of points accumulated). For the full MTD ITSA timeline and software vendor comparison see MTD ITSA 2026 guide.

14. Frequently asked questions

Do I need to file Self Assessment if I'm employed via PAYE?

Usually no. PAYE-only employees with no other income and total earnings below £150,000 do not need to file. You must file if you have untaxed side income above £1,000, dividends above £500, rental income above £1,000, foreign income, capital gains above £3,000, child benefit clawback (HICBC) at £60,000+, or total income above £150,000. You can also file voluntarily to claim higher-rate pension tax relief, Gift Aid top-up, or EIS / SEIS / VCT credits when those amounts exceed what HMRC can collect through your tax code.

What is the £100 fixed Self Assessment penalty?

HMRC issues an automatic £100 penalty the day after the 31 January online filing deadline, even if you owe no tax or are due a refund. The penalty applies for missing the filing deadline, not for unpaid tax. It is not appealable simply because you forgot - HMRC requires a reasonable excuse such as serious illness, bereavement, or a major technology outage. From 3 months late, daily £10 penalties begin and run for 90 days (max £900). At 6 months and 12 months, a further £300 or 5% of the tax due (whichever is greater) is added. File the return as soon as possible even if you cannot pay, because filing penalties accumulate independently of late-payment penalties.

Can I file a paper return after 31 October?

No. The paper SA100 deadline is 31 October following the end of the tax year. After that, HMRC will only accept the return online (deadline 31 January). Filing a paper return after 31 October triggers the same automatic £100 penalty as missing the online deadline, even if you file before 31 January. The only exception is if you are unable to file online for accessibility or technical reasons, in which case HMRC may issue a paper return with a different deadline - request this in advance through the SA helpline.

What's the difference between Payment on Account and Balancing Payment?

The Balancing Payment is the final settlement of your actual tax bill for the completed tax year, due 31 January after that year ends. Payments on Account (PoA) are advance instalments toward the next tax year, equal to 50% of the previous year balance each. The first PoA is due 31 January (alongside the balancing payment), the second on 31 July. PoA is required when your prior-year SA bill exceeded £1,000 and less than 80% of your tax was collected at source via PAYE. If actual income falls, you can apply to reduce PoA - but if you over-reduce and end up owing more, HMRC charges interest on the shortfall back to the original due dates.

Do I need to register for Self Assessment each year?

No. Registration is a one-off setup that issues your Unique Taxpayer Reference (UTR). Once registered, you keep the same UTR for life and HMRC issues a Notice to File each April for the tax year just ended. You only re-register if HMRC has previously removed you from Self Assessment because your circumstances changed (for example you stopped trading) and you now need to file again. If your situation changes mid-year and you become liable for the first time, you must register by 5 October following the end of that tax year.

Can I claim back higher-rate pension relief without filing Self Assessment?

Sometimes. If your higher-rate pension relief due is under £2,500 a year, you can claim by writing to HMRC or by phone, and HMRC adjusts your PAYE tax code to give relief through payroll. Above £2,500 of relief, or if you already file Self Assessment for any other reason, you must claim through the SA return. Salary sacrifice pension contributions do not need any claim - the relief is automatic because the contribution comes off gross pay before tax. The claim only applies to relief at source (RAS) contributions to personal pensions or workplace SIPPs.

What if I miss the 31 January deadline?

You incur the £100 fixed penalty immediately. File as soon as possible to stop further penalties accumulating - daily £10 penalties begin from 1 May (3 months late) and cap at £900 after 90 days. At 6 months and 12 months a further £300 or 5% of tax due is added at each milestone. If you owe tax, late-payment penalties run separately: 5% surcharge at 30 days, 6 months, and 12 months late, plus daily interest at the Bank of England base rate plus 2.5%. If you have a genuine reasonable excuse (serious illness, bereavement, HMRC system failure), appeal within 30 days of the penalty notice.

How do I tell HMRC I no longer need to file Self Assessment?

Call the Self Assessment helpline on 0300 200 3310 or use the online service in your Personal Tax Account to deregister. You must still file a final return for the year your circumstances changed (for example the year you stopped self-employment or stopped letting property). HMRC will confirm in writing that they have closed your Self Assessment record. Until you receive confirmation, you remain liable to file - including the £100 fixed penalty if a Notice to File has been issued and you fail to submit. Keep the confirmation letter as evidence in case HMRC re-opens your record by mistake.

Can I file Self Assessment jointly with my spouse?

No. UK Self Assessment is individual, not joint. Each spouse files their own return for their own income, even on shared assets like a let property. For jointly-owned property, both file SA105 declaring their share - 50/50 by default for married couples or civil partners, with the option to declare an unequal split via HMRC Form 17 if the legal ownership is unequal. Capital gains on jointly-owned assets are also split equally by default. The only joint-style elections in UK tax are Marriage Allowance (one spouse transfers £1,260 of Personal Allowance to the other) and Form 17 for property income splits.

Is the trading allowance the same as the Personal Allowance?

No. The £1,000 trading allowance is a separate small-trader simplification that lets you receive up to £1,000 of gross self-employment income each tax year without registering for Self Assessment or paying any tax. The Personal Allowance (£12,570 in standard form) is the slice of your total taxable income on which Income Tax is 0%. The trading allowance only applies to trading income, sits before the Personal Allowance, and is gross-not-net (a £900 dog-walking business with £600 of expenses still falls under the allowance). You can claim either the £1,000 allowance OR your actual expenses on the SA103 - not both.

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