Self Assessment Deadlines: The Complete UK Guide

If you are self-employed, a landlord, a company director or anyone with income that is not taxed at source, Self Assessment is the system HMRC uses to collect your tax. Missing its deadlines triggers automatic penalties — even if you owe no tax at all. This guide walks through every date that matters.

How the UK tax year works

The UK tax year runs from 6 April to 5 April the following year. The tax return you file reports everything you earned in that window. So the 2025/26 tax year ended on 5 April 2026, and the deadlines below relate to that year.

The key deadlines

• 5 October — register for Self Assessment if this is your first return (for the tax year that ended the previous April).
• 31 October — paper tax return filing deadline.
• 31 January — online tax return filing deadline, and the date your balancing tax payment is due.
• 31 January and 31 July — payments on account are due, if they apply to you.

Most people file online, which means 31 January is the date to remember. It is both the filing deadline and the payment deadline — a double hit that catches thousands of people out every year.

What are payments on account?

If your Self Assessment bill is over £1,000 and less than 80% of your tax was collected at source (for example through PAYE), HMRC usually asks you to prepay next year's bill in two instalments. Each payment on account is normally 50% of the previous year's tax bill.

That means in your first year with a large bill, you can face roughly 150% of one year's tax in a single January: the balancing payment for the year just gone, plus the first payment on account for the year ahead. If your income drops, you can apply to reduce payments on account — but reduce them too far and HMRC charges interest on the shortfall.

Penalties for late filing

• 1 day late: an automatic £100 penalty, even if no tax is owed.
• 3 months late: £10 per day, up to a maximum of £900.
• 6 months late: a further 5% of the tax due or £300, whichever is greater.
• 12 months late: another 5% of the tax due or £300, whichever is greater — and in serious cases up to 100% of the tax.

Penalties and interest for late payment

Late payment attracts its own penalties on top of filing penalties: 5% of the unpaid tax at 30 days, 6 months and 12 months. HMRC also charges interest from the date the tax was due until it is paid in full. If you cannot pay, contact HMRC before the deadline about a Time to Pay arrangement — it is far cheaper than ignoring the bill.

Practical tips to stay on the right side of the deadline

• File early. You can submit your return from 6 April onwards, and you do not have to pay until 31 January.
• Keep digital records as you go rather than reconstructing a year in January.
• Set aside a percentage of every payment you receive in a separate account for tax.
• If January cash flow is tight, remember HMRC's Budget Payment Plan lets you spread payments weekly or monthly.