UK Self Assessment Filing Complete Walkthrough 2026/27
UK Self Assessment 2026/27 - who needs to file, registration deadlines (5 October), tax return sections explained, Payments on Account, late penalties, MTD ITSA April 2026.
Do you need to file Self Assessment 2026/27?
You must file if any of these apply:
- Self-employed with profit above £1,000 (the trading allowance)
- Partner in a partnership
- Rental income above £2,500 (above £10,000 in some cases)
- Untaxed savings/investment income above your PSA
- Capital gains above £3,000 AEA
- Total income above £150,000
- High Income Child Benefit Charge (HICBC) owed (adjusted net income above £60,000 with Child Benefit claim)
- HMRC has issued you a Notice to File
- Foreign income/assets, trust income, ministers of religion, certain occupational categories
Key dates for 2026/27 tax year
| Deadline | Action |
|---|---|
| 5 October 2027 | Register for Self Assessment (6 months after tax year end) |
| 31 October 2027 | Paper return deadline |
| 30 December 2027 | Online return if you want tax collected via PAYE code (for amounts under £3,000) |
| 31 January 2028 | Online return + balancing payment + 1st Payment on Account 2027/28 |
| 31 July 2028 | 2nd Payment on Account 2027/28 |
The 6-step filing process
- Register at gov.uk/register-for-self-assessment - HMRC sends UTR within 10 days
- Gather documents: P60 from employer, P45 from previous jobs, P11D for Benefit in Kind (BIK), bank interest statements, dividend vouchers, rental records, business books
- Login at gov.uk/log-in-file-self-assessment with Government Gateway
- Complete sections relevant to your income (employment, self-employment, property, capital gains, dividends, savings, foreign)
- Submit the return - HMRC calculates tax owed
- Pay by 31 January via Personal Tax Account, bank transfer, debit card, or Direct Debit
Payments on Account - the cash-flow trap
If your Self Assessment bill exceeds £1,000, HMRC requires Payments on Account toward the following year. Each POA equals 50% of current year tax bill. Year 2 hits hard:
- Year 1 (2026/27) tax bill: £5,000 - paid 31 January 2028
- Plus 1st POA 2027/28: £2,500 - same date
- Total demand 31 January 2028: £7,500
- Plus 2nd POA 2027/28: £2,500 - due 31 July 2028
Set aside approximately 30-40% of self-employed profit from day one.
MTD ITSA: changing from April 2026
Making Tax Digital for Income Tax Self Assessment phases in:
- April 2026: Sole traders + landlords with combined gross income above £50,000
- April 2027: Group above £30,000
- Below £30,000: remain on annual Self Assessment
MTD ITSA requires: digital record-keeping via HMRC-recognised software (Xero, FreeAgent, QuickBooks, Sage, Coconut), quarterly updates to HMRC, end-of-period statement, final declaration. Annual Self Assessment paper or online return no longer accepted for affected groups.
Common Self Assessment mistakes
- Missing the registration deadline (5 October following year end) - £100 penalty even if no tax due
- Forgetting Payments on Account - hits year 2 with double tax demand
- Not claiming all allowable expenses - typical sole trader leaves £1,000-£3,000/year unclaimed
- Filing without records - HMRC investigations can demand evidence 5 years back
- Late filing penalties stack - £100 + £900 + £300 + £300 = £1,600 for 12-month-late filer
Related pages
Frequently asked questions
Who has to file a UK Self Assessment tax return?
You must file if any of these apply: self-employed earning over £1,000, partner in a partnership, rental income above £2,500 (£10,000+ in some scenarios), savings/investment income above PA + PSA, foreign income, capital gains above £3,000 Annual Exempt Amount (AEA), total income above £150,000, HICBC charge owed, or HMRC has sent you a notice to file. Check the gov.uk tool: gov.uk/check-if-you-need-tax-return.
When are Self Assessment deadlines for 2026/27?
Registration: by 5 October 2027 (6 months after tax year end). Paper return: 31 October 2027. Online return: 31 January 2028. Balancing payment: 31 January 2028. First Payment on Account 2027/28: 31 January 2028. Second Payment on Account: 31 July 2028. Late filing penalty: £100 immediately at 31 January 2028, more after.
How do Payments on Account work?
If your Self Assessment bill exceeds £1,000, HMRC requires Payments on Account toward next year. Each POA = 50% of current year tax bill. Paid 31 January (balance + POA1) and 31 July (POA2). New self-employed get hit hardest in year 2 - £5,000 bill in year 1 becomes £7,500 cash demand on 31 January year 2 (£5k balance + £2.5k POA1). Set aside ~40% of profit from start.
What if I can't pay my Self Assessment bill?
Apply for Time to Pay arrangement at gov.uk/difficulties-paying-hmrc. Online self-service for debts under £30,000 - spreads payments over 12 months. Interest accrues at 7.5% (HMRC current rate June 2026) but no late-payment penalty if arrangement set up before bill due. Above £30,000 requires phone call to HMRC Time to Pay team.
What is MTD ITSA?
Making Tax Digital for Income Tax Self Assessment - phased in from April 2026 for sole traders + landlords with combined gross income above £50,000. Requires quarterly digital updates via approved software + final declaration. Phase 2 from April 2027 captures £30,000-£50,000 group. Below £30,000 remain on annual Self Assessment. Software: Xero, FreeAgent, QuickBooks, Sage are HMRC-recognised.
What expenses can I claim against self-employed income?
Wholly + exclusively business expenses: business mileage (£0.55/mi first 10k, £0.25 after), home office (simplified £10-£26/month or actual %), business phone usage, professional subscriptions, software, training in current trade, business insurance, accountancy fees. See our allowable expenses guide.
What's the late filing penalty?
Online return late: £100 fixed penalty immediately after 31 January (even if no tax due). After 3 months: £10/day up to 90 days max £900. After 6 months: greater of £300 or 5% of tax due. After 12 months: another £300 or 5%. Penalties stack - 12-month-late filer with £5,000 tax owes £100 + £900 + £300 + £300 = £1,600 in penalties on top of tax + interest.
Can I file my own Self Assessment or do I need an accountant?
Most PAYE-with-side-income or simple self-employed filers can DIY. Accountant worth it when: profits above £80k, multiple income streams, capital gains, rental property, complex business expenses, IR35 contractor. Typical accountant fee £200-£600 for individual SA, £400-£1,200 for sole trader full bookkeeping + SA, £800-£2,000 for Ltd company.
How do I register for Self Assessment?
Go to gov.uk/register-for-self-assessment. Pick correct form: self-employed (SA1), partner (SA401), other reason (SA1). Need Government Gateway account. HMRC sends Unique Taxpayer Reference (UTR) within 10 days. Use UTR + Government Gateway to file online. Register before 5 October following the tax year - so 5 October 2027 for 2026/27 income.