Self Assessment Tax Return 2026/27 — Step-by-Step Filing Guide

Self Assessment is how HMRC collects income tax from anyone whose tax is not fully deducted at source. For the 2026/27 tax year — 6 April 2026 to 5 April 2027 — the return is due online by 31 January 2028, with registration for first-timers by 5 October 2027. The online system calculates your tax automatically once you enter income and expenses correctly, so the real work is gathering documents and completing the right sections. Check your calculation as you go with the Self Assessment Calculator→.

This guide walks through registration, documents, the core and supplementary pages, the key 2026/27 figures, payment and the mistakes that trigger penalties.

Step 1 — Register and Get Your UTR

Register on GOV.UK, choose your reason (self-employed, partner, or not self-employed), and HMRC posts your 10-digit Unique Taxpayer Reference. Keep the UTR safe — you need it for every return and every contact with HMRC — and activate your Government Gateway account, which is the login for filing.

Step 2 — Gather Your Documents

Work from gross figures — interest and dividends before any tax deducted at source — because the return asks for the gross amounts and HMRC applies the allowances and credits automatically.

Step 3 — Complete the Sections

The core pages cover personal details, employment income and untaxed income. Supplementary pages add the detail:

Step 4 — The Key 2026/27 Figures

Enter pension contributions and Gift Aid donations in their own sections — both extend your basic rate band and can reduce your bill — and claim capital allowances in the self-employment pages if you bought equipment.

Step 5 — Submit and Pay

Review the tax calculation summary before submitting, then pay by Direct Debit, Faster Payments transfer, debit card or cheque. Payments on Account are normally required when the relevant Self Assessment tax due is at least £1,000 and less than 80% of the tax was deducted at source. Each instalment is usually half of the previous year's relevant bill: 50% by 31 January 2028 and 50% by 31 July 2028, as advances toward 2027/28. They do not cover capital gains tax or student-loan repayments.

Worked Example — The Self-Employed Plumber

Marco has £45,000 of self-employment profit and no other income, and this simplified example assumes he is taxed under the England, Wales or Northern Ireland bands, has no student-loan or other liability, and has no deductible adjustments beyond the stated figures. His taxable income is £32,430 after the Personal Allowance. Income tax at 20% is £6,486. Class 4 National Insurance is charged at 6% on profits between £12,570 and £50,270 — £32,430 at 6% = £1,945.80 — and Class 2 is no longer a compulsory charge for the self-employed, so his estimated combined income-tax and Class 4 liability is £8,431.80.

His payments on account are half of the bill: £4,215.90 by 31 January 2028 and £4,215.90 by 31 July 2028, with a balancing payment the following January if 2027/28 profits change the position.

Common Mistakes

Foreign Income, Student Loans and Less Common Pages

The less common sections trip up otherwise careful filers. Foreign interest and dividends are declared on SA106, where you can also claim Foreign Tax Credit Relief if eligible. Student-loan repayments are calculated from the return only when the relevant plan details are recorded correctly. Capital losses from previous years are entered on the capital gains pages and used under the applicable loss rules; the software's calculation should still be checked against the underlying figures.

If you receive a P800 or a simple assessment, check it against your own return before accepting it — HMRC's automatic calculations occasionally omit reliefs you have claimed. And if HMRC queries a figure, the enquiry letter will ask for the records behind it, so everything you enter should trace back to a document you still hold. The return is a self-assessment in the full sense: HMRC's calculation is a courtesy, and the accuracy of what you submit is your responsibility. Filing early gives you time to resolve any query before the January surge rather than under its pressure.

After You Submit — Enquiries and Amendments

Filing is not the end. You can amend a return within 12 months of the deadline — so for 2026/27, amendments are possible until 31 January 2029 — which covers forgotten income or a corrected expense figure. HMRC's enquiry window runs alongside: most enquiries open within 12 months of filing, and a discovery assessment can reach back four years for an innocent error, six for carelessness and 20 for deliberate concealment. If HMRC enquires, the process is a structured request for the records behind your figures, and cooperating with the deadline in the letter keeps the enquiry proportionate.

The best defence is the return itself: complete, accurate and traceable. Amending proactively within the 12-month window when you spot a mistake is treated far more favourably than waiting for an enquiry, and it avoids the penalty surcharges that attach to unprompted discoveries. Keep the records that support every figure — the same records that satisfy the five-year retention rule are the ones that answer an enquiry letter. An enquiry is not a presumption of guilt; most are random or risk-scored, and a well-documented return closes the file quickly.

Frequently Asked Questions

Reconcile the return before submission

Do not treat the online calculation as a substitute for a source reconciliation. Compare the return with bank statements, invoices, dividend vouchers, pension certificates, P11D or payroll records, property schedules and foreign-income statements. Check that gross receipts have not been entered net of tax, that transfers between your own accounts are not counted as income, and that expenses are allocated to the correct trade or property activity. Keep a note for every estimate or apportionment.

After submission, save the submitted return, calculation, payment reference and any confirmation screen. If HMRC later issues a statement that differs from your working paper, compare the tax year, payments already made, payments on account and coding deductions before paying again. If an error is found, preserve the original figures and use the correction or disclosure route that matches the error; do not silently overwrite the evidence.

Check your calculation with the Self Assessment Calculator→, read the Self Assessment Deadlines guide for the penalty calendar, and see the Record Keeping guide before you file anything.

Official sources

• GOV.UK: Who must send a tax return
• GOV.UK: Register for Self Assessment
• GOV.UK: Self Assessment deadlines
• GOV.UK: Correct a Self Assessment return
• GOV.UK: Self Assessment tax return forms
• GOV.UK: Payments on account
• GOV.UK: Self Assessment penalties
• GOV.UK: How long to keep self-employed records