A Simple Guide to Self Assessment

Self Assessment is how HMRC collects tax from people whose income isn’t taxed automatically through PAYE – and missing a deadline is one of the easiest ways to pick up a penalty for no real reason. Here’s how it actually works.

Who needs to file a return

You’ll usually need to register for Self Assessment and file a return if you’re self-employed or a partner in a business, a company director, a landlord with rental income, or you have other untaxed income such as dividends, savings interest, or foreign income above certain levels. You’ll also need to file if you or your partner claim Child Benefit and either of you has income over £60,000, or if HMRC has simply asked you to.

The key dates

5 October following the end of the tax year – deadline to register for Self Assessment if this is your first time filing
31 October – deadline for paper returns
31 January – deadline for online returns, and for paying any tax you owe for the year

The tax year runs to 5 April, so for the 2025/26 tax year (6 April 2025 to 5 April 2026), you’d need to register by 5 October 2026 if it’s your first return, file online by 31 January 2027, and pay whatever you owe by the same date.

Payments on account

If your Self Assessment bill is over £1,000 and less than 80% of your tax was collected at source, HMRC usually asks you to make “payments on account” – advance payments towards next year’s tax bill, each equal to half of your last bill. These are due on 31 January and 31 July, with any balance owed or refund due settled the following 31 January. It’s a common source of confusion in your first year or two of filing, since the January bill can be considerably more than your actual tax for the year once the first payment on account is added on top.

What it costs to be late

Late filing penalties apply even if you don’t owe any tax:

1 day late: an automatic £100 penalty
3 months late: £10 a day for up to 90 days, on top of the £100
6 months late: a further £300 or 5% of the tax due, whichever is higher
12 months late: another £300 or 5% of the tax due, whichever is higher

Late payment is penalised separately from late filing: 5% of the unpaid tax at 30 days, another 5% at 6 months, and another 5% at 12 months, plus daily interest on top for as long as the tax stays unpaid. It’s entirely possible to file on time but still rack up late payment penalties if the bill itself isn’t settled by 31 January.

Note: HMRC is replacing this penalty structure with a new points-based system for late filing, and a revised late payment penalty scale, from April 2027. The figures above are what currently applies.

See our checklist of what to gather before you file, and if you’d like help getting your return done properly and on time, get in touch – or find out more about how we support individuals.

Get in touch

John is the founder of Summed Up Accountancy Services. He qualified as a Chartered Certified Accountant (ACCA) in 2006, worked in practice and then in industry as a Head of Finance, and set up Summed Up in 2019 to give growing businesses and directors straightforward, jargon-free advice.