Self Assessment Explained

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Self Assessment is the system for people whose tax is not all collected through PAYE. If you are self-employed, a landlord, or you have income HMRC does not know about, you have to tell HMRC about it yourself — by filing a tax return each year. The system is straightforward once you know the dates, but the penalties for missing them are automatic and they grow quickly. This guide explains who has to file, the deadlines, and what happens if you miss them.

Self Assessment in one paragraph

You must register by 5 October after the tax year ends, file a paper return by 31 October or an online return by 31 January, and pay the tax you owe by 31 January. File even one day late and HMRC charges an automatic £100 penalty, even if you owe no tax. Keep your records for at least 5 years after the 31 January deadline.

Who has to file

You must file a Self Assessment return if you are self-employed as a sole trader and your income is over £1,000, if you are a partner in a business partnership, or if you are a landlord with rental income. You must also file if you have untaxed income — for example, from savings or investments above the allowances — if your income is over £100,000, or if you or your partner claim Child Benefit and one of you earns over £60,000.

You can check whether you need to file using HMRC’s free online tool on GOV.UK — it asks a few questions and tells you whether a return is needed. If you are not sure, check: HMRC can charge a "failure to notify" penalty if you should have told them and did not, and the penalty can apply even if you eventually pay all the tax.

Registering for the first time

If it is your first year, you must register for Self Assessment by 5 October following the end of the tax year. For the 2026/27 tax year (which ends on 5 April 2027), the registration deadline is 5 October 2027. You register on GOV.UK, and HMRC sends you a Unique Taxpayer Reference (UTR) — a 10-digit number that identifies you in the Self Assessment system. Keep it safe; you will need it every year.

You will also need a Government Gateway account to file online. Set it up well before the deadline, because identity checks can take time, and HMRC’s helplines are at their busiest in January. If you register after 5 October and do not pay all your tax by 31 January, GOV.UK says you may get a failure-to-notify penalty.

The deadlines

There are four dates to remember. 5 October: the deadline to register if it is your first year. 31 October: the deadline for a paper return — very few people use this now. 31 January: the deadline for an online return, and the deadline to pay the tax you owe for the previous tax year, plus the first payment on account for the current year if you make them. And 31 July: the second payment on account.

For the 2026/27 tax year, that means: register by 5 October 2027, file a paper return by 31 October 2027, or file online and pay by 31 January 2028. The 31 January date is the one that matters most, because it is both the filing deadline and the payment deadline — miss it and you face a filing penalty and interest on the unpaid tax at the same time.

Payments on account

If your Self Assessment bill was £1,000 or more last year, HMRC usually asks you to pay in advance for the current year — payments on account. You pay half of the estimated current-year tax on 31 January, along with last year’s balancing payment, and the other half on 31 July. Each payment is half of the previous year’s bill, less any tax already deducted at source.

Payments on account surprise a lot of first-timers, because the first 31 January can mean paying last year’s tax and half of next year’s at once. If you expect your income to fall, you can ask HMRC to reduce your payments on account — but if you reduce them too far, HMRC charges interest on the shortfall.

Late filing and late payment

The penalties are fixed and automatic. File one day late and you get a £100 penalty, even if you owe nothing. After 3 months, daily penalties of £10 a day start, up to £900. At 6 months, a further penalty of 5% of the tax due or £300, whichever is higher. At 12 months, another 5% or £300. Interest also runs on any unpaid tax from the day after the deadline.

If you cannot pay, tell HMRC: you may be able to agree a Time to Pay arrangement, paying in instalments. And if you realise a return you already filed is wrong, you can amend it online within 12 months of the 31 January deadline. After that, you need to write to HMRC.

Making Tax Digital

Making Tax Digital for Income Tax is HMRC’s programme to move Self Assessment onto quarterly digital updates. If your qualifying income from self-employment and property was over £50,000 in 2024/25, you must use it from 6 April 2026. If it was over £30,000 in 2025/26, you must use it from 6 April 2027. If it was over £20,000 in 2026/27, you must use it from 6 April 2028.

Under Making Tax Digital you keep digital records in software HMRC recognises, send HMRC a summary every 3 months, and file your tax return through that software by 31 January. HMRC does not provide the software itself. If you are affected, the time to choose software and start keeping digital records is well before your start date, not in the January rush.

Sources: GOV.UK, Self Assessment tax returns: deadlines; GOV.UK, Self Assessment penalties; GOV.UK, Making Tax Digital for Income Tax; Budget 2026, section 7.22.

General information only. This page explains how Self Assessment works in plain English. It is not tax advice, and it cannot tell you what you owe or what you should do about your own tax. Always confirm the current rates, rules, and your own position on GOV.UK or with HMRC before you act, and get professional advice for anything complicated. Contact: hello [at] tapetoolhq.com.