How to Fill In a Self Assessment Tax Return: Step by Step for 2025/26

To fill in a Self Assessment tax return you sign in to your HMRC online account with your Government Gateway ID and Unique Taxpayer Reference (UTR), answer the "tailor your return" questions so only the relevant pages appear, then enter your income and reliefs section by section before checking the calculation and submitting. For the 2025/26 tax year (6 April 2025 to 5 April 2026) the online filing deadline is 31 January 2027, which is also the date any tax is due.

Key points

• The online return is not one long form — the tailoring questions switch pages on and off, so most people only complete three or four sections.
• You need your UTR and Government Gateway login before you start. Registering for a new UTR takes time, so leave a clear few weeks.
• Figures come from documents you already have: P60, P45, P11D, bank interest statements, dividend vouchers, rental records and your own bookkeeping.
• The return covers a full tax year, not a calendar year, and the filing deadline is separate from the payment deadline only in name — both fall on 31 January.
• You can save and return as many times as you like, and you can amend a filed return for up to 12 months after the deadline.

Before you start

First, check you actually need to file. HMRC expects a return if you were self-employed with gross trading income above £1,000, were a partner in a partnership, had untaxed income that cannot be collected through your tax code, made capital gains above the £3,000 annual exempt amount, owe the High Income Child Benefit Charge, or received a notice to file. If you have had a letter asking for a return, you must file it even if you owe nothing.

Our Self Assessment checker runs through the triggers in a couple of minutes if you are not sure.

Get your login sorted

You need two things: a Government Gateway user ID and a 10-digit UTR. If you have filed before, both already exist — the UTR is on previous returns, payment reminders and in the HMRC app. If this is your first tax return, you have to register for Self Assessment first, and HMRC posts the UTR to you. Allow around 10 working days for it to arrive, longer if you are outside the UK.

The paperwork checklist

• Employment: P60 and any P45s, plus a P11D for benefits like a company car or medical cover.
• Self-employment: total sales for the year and your expenses, split into HMRC's categories. Add up capital purchases (tools, equipment, a van) separately for capital allowances.
• Property: rent received, allowable expenses, and finance costs such as mortgage interest kept separately because relief is given as a 20% tax reducer, not a deduction.
• Savings and investments: interest certificates from banks and building societies, dividend vouchers or consolidated tax certificates.
• Reliefs: personal pension contributions (the gross figure), Gift Aid donations, professional subscriptions, work mileage not reimbursed.
• Other: student loan plan number, State Pension and private pension figures, taxable benefits, Child Benefit received if your income is over £60,000.

Signing in and opening the return

Sign in at GOV.UK, choose Self Assessment from your business tax account, and select "Complete your tax return" for the 2025/26 year. Expect an identity check — usually a code sent to your phone or authenticator app.

The first screens confirm your name, address, date of birth and National Insurance number. Correct anything out of date here, because your address determines whether you are taxed at Scottish rates on non-savings income. Then come the tailoring questions: were you employed, self-employed, a partner, a landlord, did you have foreign income, capital gains, and so on. Answer these carefully — a wrong "no" here is the most common reason people finish a return with a whole income source missing.

Section by section

Employment

Enter pay and tax deducted exactly as shown on your P60 or P45, one page per employer. Use the figures on the form rather than your own total from payslips. Benefits in kind go in the boxes that match your P11D, and expenses you paid for work but were not reimbursed — professional fees, mileage shortfalls, a flat-rate uniform allowance — go in the employment expenses boxes.

Self-employment

Give your trade description, then turnover and expenses. Under £90,000 of turnover you can enter a single total for expenses; above that you must itemise. Enter capital purchases under capital allowances, where the Annual Investment Allowance lets you deduct up to £1,000,000 of qualifying plant and machinery in the year of purchase. If you started or stopped trading during the year, enter the dates — the 2025/26 return asks for these.

Property

Rental income and expenses go on the property pages. Residential finance costs sit in their own box and produce a 20% reduction in tax rather than reducing profit. If you let a furnished room in your own home, the Rent-a-Room scheme exempts the first £7,500 a year, and the £1,000 property allowance can be claimed instead of actual expenses where expenses are small.

Interest, dividends and gains

Enter untaxed UK interest gross. Dividends go in the dividend box — the dividend allowance is £500, and the allowance is applied automatically by the calculation, so do not try to net it off yourself. Report disposals on the capital gains pages if your gains exceed the £3,000 annual exempt amount or your total proceeds were more than £50,000. Directors of their own limited company should note that the return now asks for dividends received from close companies and the percentage shareholding.

Reliefs and the final pages

Pension contributions to a personal pension are entered gross (what you paid plus the basic-rate top-up). This is how higher-rate and additional-rate taxpayers claim the extra relief — nothing happens automatically. Gift Aid donations go in the charity boxes and also reduce adjusted net income, which matters around £60,000 and £100,000. The closing pages cover student loan repayments, the High Income Child Benefit Charge (which applies between £60,000 and £80,000 of adjusted net income), Marriage Allowance, and whether you want any tax under £3,000 collected through next year's tax code instead of as a lump sum — that option needs the return filed by 30 December.

A worked example

A sole trader with profit of £38,000 and no other income, filing for 2025/26:

So the January bill is £9,917.70, with a further £3,305.90 due on 31 July 2027. Scottish taxpayers use the same boxes, but HMRC applies Scottish rates to non-savings income, so the Income Tax line differs.

Common mistakes

• Answering the tailoring questions too quickly and leaving out an income source entirely.
• Entering net pension contributions instead of the gross figure, which understates the relief.
• Missing bank interest — it is taxable once it exceeds your Personal Savings Allowance even though it is paid gross.
• Forgetting payments on account already made and panicking at a bill that has effectively been part-paid.
• Treating mortgage capital repayments as an expense on rental property. Only the interest element counts, and only as a 20% reducer.
• Not pressing submit. A saved return is not a filed return; you need the submission receipt and reference number.

Submitting and paying

Before you submit, open "View your calculation" and sanity-check the totals against your own numbers. Then save a PDF copy for your records. Submission is confirmed on screen with a reference — keep it.

Key dates for the 2025/26 return:

• 5 October 2026 — register for Self Assessment if 2025/26 was your first year of self-employment or untaxed income.
• 31 October 2026 — paper returns. HMRC no longer publishes the SA100 for download, so you must request one by phone.
• 30 December 2026 — deadline to ask for tax under £3,000 to be collected through your 2027/28 tax code.
• 31 January 2027 — online filing, the balancing payment and the first payment on account.
• 31 July 2027 — second payment on account.

Payments on account are due when your Self Assessment bill exceeds £1,000 and less than 80% of your tax was collected at source. Pay by bank transfer, debit card or Direct Debit — HMRC does not accept personal credit cards. Late filing triggers an automatic £100 penalty even if no tax is owed, with escalating penalties after three, six and twelve months, and interest runs on unpaid tax from 1 February.

If you spot an error afterwards, you can amend the return online until 31 January 2028. Full dates and penalty detail are in our guide to Self Assessment deadlines.

What to do next

Work out the number before HMRC does. Our Self Assessment tax return tool walks through the same sections and shows the tax, National Insurance and payments on account you should expect, and the self-employed tax calculator is a quick way to pressure-test a profit figure before you type it into the real return.