How to prepare and file your 2025-26 Self Assessment tax return

Before you start

If you are not yet sure you need to file, start with our guide to who needs a Self Assessment return for 2025-26. This article assumes you do.

You need a Unique Taxpayer Reference (UTR), a 10-digit number HMRC issues when you register for Self Assessment. If you are registering now, allow time: HMRC says confirmation can take up to 10 working days, or 21 if you are abroad, and the UTR comes by post. If you were registered once before but did not need to file for 2024-25, you may need to reactivate your existing record rather than register afresh. Filing without reactivating it can hold your return up.

To file online you also need HMRC sign-in details, and you may be asked to prove your identity when you sign in.

Gather your records first

Having the paperwork together before you start makes the return quicker to complete and easier to check. Collect these first:

• Employment: your P60 for the year, any P45 if you changed jobs, and any P11D showing benefits or expenses
• Self-employment: a record of every sale and business expense, bank statements for the account the business uses, and the start or end date if the business began or stopped in the year
• Property: rent received, letting agent statements, repair and running costs, and mortgage interest
• Savings and investments: interest statements from banks and building societies, and dividend statements
• Pensions: the P60 from each pension provider and the amount of any State Pension
• Capital gains: for each disposal, the dates of purchase and sale, what you paid and received, the costs of buying, selling or improving the asset, and a copy of any 60-day property return you have already made
• Foreign income: evidence of the income and of any foreign tax paid on it
• Reliefs: personal pension contributions and Gift Aid donations
• Other: Child Benefit received if your income was over £60,000, your student or postgraduate loan plan, and recent HMRC letters and statements

Keep those records afterwards. If you were self-employed or a partner, keep them for at least five years after the 31 January filing deadline, which for 2025-26 means until at least the end of January 2032. Otherwise the minimum is 22 months after the end of the tax year, so until the end of January 2028 if you file on time.

What is new on the 2025-26 return

Two sets of questions became compulsory from 2025-26:

• If you ran a business: the date it started or stopped, if that happened in the year
• If you were a company director: whether any of your companies was a close company and, for each one that was, its name and registered number, the dividends you received from it (nil is a valid answer) and the highest percentage of its shares you held during the year

Each directorship generally needs its own employment page, even for a dormant company. Gather the company numbers and dividend figures before you start.

Paper or online

A paper return for 2025-26 must reach HMRC by 31 October 2026, and an online return is due by 31 January 2027. Filing online gives you three more months, lets you save your work and come back to it, and makes later corrections simpler.

HMRC’s own online service cannot handle every return. Partnership returns, returns by non-residents, people with income from a trust, Lloyd’s underwriters and ministers of religion, among others, need commercial software or paper forms.

The pages you may need

The main return is the SA100. Supplementary pages cover particular kinds of income:

Online, the same ground is covered as sections of the return. On paper, the notes for each page explain which version to use.

Working through the return

A few habits make the work more reliable:

• Check your personal details first, including your address and your student loan plan.
• Match every figure to a document. Pay and tax from the P60, interest from the bank statement, rent from the agent’s statement. If you cannot point to the source, find it before you file.
• Use provisional figures only when you must. If a figure cannot be finalised in time, enter your best estimate, flag it as provisional and amend the return when the real number is known.
• Read the calculation before you submit. It shows how the tax has been worked out. After you submit, it may not be viewable for up to 72 hours, so save a copy first if a payment is due in that window.

Payments on account and the balancing payment

If your bill comes to £1,000 or more and no more than 80% of your tax was collected at source, HMRC will usually ask for two payments on account towards the next year, each half of this year’s bill, due on 31 January and 31 July. Class 4 National Insurance is included. Capital Gains Tax and student loan repayments due through the return are paid with the 31 January balancing payment instead.

Here is how the first two years can look for someone new to Self Assessment, with figures chosen only to show the mechanics:

The first January is the heaviest: £6,000 in this example, against a bill of £4,000. If you expect your income to fall, you can ask HMRC to reduce the payments on account, online or on form SA303. If you reduce them too far, interest is charged on the difference.

How to pay

Your payment reference is your UTR followed by the letter K. The ways to pay, and how long each takes to reach HMRC, are set out in HMRC’s guide to paying your Self Assessment bill. In outline:

• Same or next day: Faster Payments through online or telephone banking, approving a payment through your bank’s app, CHAPS, a debit card online, or at your bank with an HMRC paying-in slip. Personal credit cards are not accepted, and a corporate card carries a fee.
• Three working days: Bacs, or a Direct Debit you have used with HMRC before.
• Five working days: a Direct Debit set up for the first time.

31 January 2027 falls on a Sunday. Unless you pay by Faster Payments or card, make sure the money arrives by Friday 29 January.

If you would rather spread the cost of the next bill, a Budget Payment Plan lets you pay weekly or monthly by Direct Debit, provided you are up to date with previous payments.

Correcting a mistake

You can amend a return within 12 months of the filing deadline: until 31 January 2028 for 2025-26. Online, you wait 72 hours after submitting and then reopen the return through your HMRC account. On paper, you send corrected pages marked “amendment” with your name and UTR. If you used commercial software, the provider’s own process applies.

After that window closes, you write to HMRC instead. Tax you have overpaid can be claimed back for up to four years after the end of the tax year, which for 2025-26 means by 5 April 2030. If you find income from earlier years that was never declared, Linx Tax Defence explains how voluntary disclosures work.

If you cannot pay in full

Pay what you can by the deadline and contact HMRC about the rest. HMRC may agree a payment plan, known as Time to Pay, if it accepts the instalments are affordable. You will need your UTR, your bank details and figures for your income and spending, and HMRC will expect any savings to be used to reduce the debt. Interest is charged on the unpaid balance for as long as it takes. Linx Tax Defence’s guide to HMRC Time to Pay arrangements goes into more detail.

If you are going to be late

These penalties apply to the 2025-26 return:

• Late filing: £100 straight away; after three months, £10 a day up to a maximum of £900; after six months, a further 5% of the tax due or £300, whichever is more; after twelve months, the same again
• Late payment: 5% of the unpaid tax at 30 days, again at six months and again at twelve months, plus interest

They apply even if you joined Making Tax Digital for Income Tax on 6 April 2026, because the new points-based penalties start from the year you join. If a partnership return is late, every partner is penalised. Linx Tax Defence’s guide to late filing penalties and penalty points goes into more detail.

Having it done for you

If you would rather hand the return over, our Self Assessment service covers preparing, checking and filing it. If the real difficulty is keeping your business records in order through the year, see bookkeeping.