What do you need to file your Self Assessment tax return?

You've got until 31 January 2027 to file your tax return for the 2025/26 tax year.

You've been able to file since 6 April 2026, so if you haven't got round to it yet, it's worth starting sooner rather than later to avoid the deadline creeping up on you.

When is the deadline for the 2025 to 2026 tax return?

HMRC's guidance gives two filing deadlines for the tax year that ended on 5 April 2026. A paper return must reach HMRC by 11:59pm on 31 October 2026. An online return must be submitted by 11:59pm on 31 January 2027. The tax you owe is due by 11:59pm on 31 January 2027 either way.

Two earlier dates matter to some people. If you want HMRC to collect the bill through your tax code, the online return has to be in by 30 December 2026. And if you have not sent a return before, HMRC says you must tell it by 5 October that you need to complete one for the previous tax year, which you do by registering for Self Assessment.

What do you need to have ready?

Here's what you'll need to have ready:

1. Income details - from employment, self-employment, or rental property
2. Expenses - anything incurred in relation to that income
3. Charitable donations and pension contributions made over the year
4. Dividends and interest received
5. Investments (EIS, SEIS, VCT), residence, overseas income/gains, UK gains, crypto sales, etc

HMRC's record keeping guidance names the documents behind most of that list. For employment, that is your P60, your P45 if you left a job, and form P11D for expenses and benefits such as a company car or health insurance. For savings and investments, it is bank and building society statements, statements of interest, and the dividend vouchers you get from UK companies. For rental income, it is the dates the property was let, the rent received, and the allowable expenses you paid to run it. For overseas income, it is evidence of what you earned and proof of any tax already paid, in the UK or overseas.

If you are self-employed, HMRC says you must keep records of all sales and income and all business expenses, with proof such as receipts, bank statements and sales invoices. You do not send the records in with the return, but you need them to work out your profit and to show HMRC if asked. If some are missing, see what happens if you lose a receipt.

Crypto belongs on the list because HMRC says you may need to pay Capital Gains Tax when you sell cryptoasset tokens, exchange them for a different type, use them to pay for goods or services, or give them away. There is more on gains in tax when you sell stocks and shares, and if Child Benefit is part of your picture, see the High Income Child Benefit Charge.

Do you need software to file a Self Assessment return?

For most people filing a 2025 to 2026 return, no. Gov.uk says you can file your Self Assessment tax return online through HMRC's own service, and you can save your entry and go back to it later. A paper SA100 form is also available.

HMRC's online service cannot be used in some cases, including a return for a partnership, a trust or an estate, or if you lived abroad as a non-resident. In those cases HMRC says to use commercial software or the other forms.

Making Tax Digital for Income Tax, which does require compatible software, began on 6 April 2026 for the first group of sole traders and landlords. HMRC's guidance says you still need to submit a Self Assessment tax return as you normally do for the tax year before you start using it.

What happens if you file or pay late?

HMRC's late filing penalties start with an initial £100. After 3 months there are additional daily penalties of £10 per day, up to a maximum of £900. After 6 months there is a further penalty of 5% of the tax due or £300, whichever is greater, and the same again after 12 months. Paying late brings penalties of 5% of the tax unpaid at 30 days, 6 months and 12 months, and interest on the amount owed.

Get ahead of it now, get it done, and take the January deadline pressure off your plate completely.

Deadlines, penalties and filing methods change, and what belongs on your return depends on your own income, so this is general information rather than advice on your circumstances. If you would rather hand the return over, talk to us.