A self assessment tax return, explained simply

If the phrase “self assessment tax return” makes your stomach drop a little, you’re in good company. It sounds official and complicated. In reality it’s HMRC’s way of asking you to declare income they don’t already know about, work out the tax on it, and pay. Once you’ve done one, the mystery mostly disappears.

Here’s the plain-English version, with every date and figure you need.

Which year are we talking about? If you’re reading this in the second half of 2026, the return you’re working on covers the 2025/26 tax year (6 April 2025 to 5 April 2026), and it’s due by 31 January 2027. Figures checked against GOV.UK on 20 July 2026; sources are at the end.

Who actually needs to file one

You’ll usually need to send a return if any of these applied during the tax year:

You were self-employed as a sole trader and earned more than £1,000 (before expenses, not profit)
You were a partner in a business partnership
You had untaxed income, for example from renting out a property, tips, commission, or savings and investments above the allowances
You earned money from a side hustle that isn’t taxed at source, like reselling, freelancing or selling content
You need to pay the High Income Child Benefit Charge, or you have other income HMRC can’t collect through your tax code

If you’re only employed and taxed through PAYE, you probably don’t need to file. If you’re doing anything on the side that brings in real money, you probably do. When in doubt, HMRC has a short online checker, and it takes a couple of minutes.

The £1,000 is gross, not profit. This is the single most common misunderstanding. If you turned over £3,000 reselling and spent £2,400 on stock, your profit was £600, but you were over the £1,000 threshold and you need to register. The trading allowance is measured on income.

Every date that matters

Miss these and the penalties stack up quickly, so put them somewhere you’ll actually see them.

Deadline	What’s due
5 October	Register for Self Assessment, if it’s your first time
31 October	Paper returns must reach HMRC
30 December	File online if you want the bill collected through your PAYE code
31 January	File your online return and pay the tax
31 July	Second payment on account, if you make them

Working through it for the current cycle: the 2025/26 tax year ended on 5 April 2026, so you register by 5 October 2026, file online and pay by 31 January 2027, and if you make payments on account the second one is due 31 July 2027.

Don’t leave registering late. Registering isn’t the same as filing. After you register, HMRC posts you a UTR (a 10-digit Unique Taxpayer Reference), and you can’t file without it. That takes time to arrive. People who register in mid-January regularly miss the filing deadline waiting for a number.

That 30 December date is worth knowing too, and almost nobody does. File by then and if you’re also employed, you can often have the tax collected gradually through next year’s PAYE code instead of paying a lump sum in January. It’s the closest thing to an interest-free instalment plan HMRC offers as standard.

What the penalties actually are

Worth knowing precisely, because the first one surprises people.

For filing late:

How late	Penalty
1 day	£100, automatic
3 months	£10 a day, up to £900
6 months	5% of the tax due, or £300, whichever is greater
12 months	Another 5% of the tax due, or £300, whichever is greater

For paying late, separately: 5% of the unpaid tax at 30 days, again at 6 months, and again at 12 months. Interest also runs on what you owe.

Two things to take from this table.

The £100 applies even if you owe no tax at all. A dormant year, a loss-making year, a year you earned £200: file late and it’s still £100. People who stopped trading and assumed the obligation stopped with it get caught by this constantly. If you no longer need to file, tell HMRC and get the return withdrawn; don’t just ignore it.

Filing and paying are separate penalties. If you can’t pay, still file on time. You’ll take the late payment penalty but avoid the filing one, and HMRC is considerably more helpful about a Time to Pay arrangement when you’ve filed and know your number than when you’ve gone quiet.

If something genuinely went wrong (serious illness, bereavement, a failure of HMRC’s own service), you can appeal on the grounds of reasonable excuse.

What you’ll need before you start

Gather these first and the form takes an evening rather than a weekend:

Your UTR, the 10-digit reference from when you registered
Your National Insurance number
A record of all your income for the year, by source
A record of your allowable expenses
Details of any other income: employment (your P60 or P45), interest, dividends, property
Details of pension contributions and Gift Aid donations, which can reduce your bill
Your student loan plan type, if you have one

The tax is worked out on your profit, which is income minus allowable expenses. Keep those two things separate and clear all year and the return more or less fills itself in.

What you’ll actually pay

For 2026/27, on profits in England, Wales and Northern Ireland (Scotland sets its own income tax bands):

Band	Profit	Income Tax	Class 4 NI
Personal allowance	Up to £12,570	0%	0%
Basic rate	£12,571 to £50,270	20%	6%
Higher rate	£50,271 to £125,140	40%	2%
Additional rate	Over £125,140	45%	2%

Class 2 National Insurance no longer produces a bill for most people. At £7,105 of profit or more it’s treated as paid, so your State Pension record builds without you paying anything. Below that you owe nothing either, but you can pay voluntarily at £3.65 a week to avoid a gap in your record.

One trap in the higher band: the personal allowance tapers away above £100,000, dropping by £1 for every £2 of income above it and reaching zero at £125,140. That creates an effective marginal rate of around 60% on that slice of income, which is worth knowing before you decide when to invoice a large job.

A worked example

You made £42,000 profit as a sole trader in 2026/27, with no other income.

Income Tax: the first £12,570 is free. The remaining £29,430 is taxed at 20% = £5,886
Class 4 NI: 6% on the £29,430 above £12,570 = £1,765.80
Total: £7,651.80, an effective rate of about 18.2%

That’s the figure to set aside across the year, not the one to discover in January.

Payments on account: the surprise nobody warns you about

This is the part that ruins otherwise good first years, so it gets its own section.

If your Self Assessment bill is £1,000 or more, and less than 80% of your tax was already collected at source, HMRC asks you to pay towards next year’s bill in advance. It comes in two instalments, on 31 January and 31 July, each usually half of last year’s bill.

Here’s what that does to a first January. Say your first year’s tax bill is £7,651.80.

31 January 2027: the £7,651.80 you owe for 2025/26, plus £3,825.90 as your first payment on account for 2026/27 = £11,477.70
31 July 2027: another £3,825.90

So the first January isn’t the number you calculated. It’s roughly 150% of it.

It’s not a penalty, it’s not extra tax, and it isn’t a mistake. You’re paying earlier, not more, and it evens out from year two. But if you budgeted for £7,651.80 and £11,477.70 comes out, that’s a genuine crisis, and it’s an entirely predictable one.

The habit that solves it: every time you get paid, move roughly 25 to 30% into a separate savings pot you don’t touch. In year one, aim for the higher end. When the bill lands, the money is already there.

If your income has genuinely dropped, you can apply to reduce your payments on account. Be careful, though: reduce them too far and HMRC charges interest on the shortfall.

Making Tax Digital is coming, and it has a date on it

If you’re self-employed or have property income, Making Tax Digital for Income Tax will change how you file: from one annual return to quarterly updates through compatible software.

The timetable is keyed on your qualifying income:

Qualifying income over	Measured in	You’re in from
£50,000	2024/25	6 April 2026
£30,000	2025/26	6 April 2027
£20,000	2026/27	6 April 2028

Qualifying income is gross self-employment and property income combined, before expenses. The threshold has moved around over the years, so check the GOV.UK page rather than an article, including this one, when your date gets close.

The practical takeaway is simple: keeping your numbers in software rather than a shoebox is becoming the rule rather than good practice.

How to make next year painless

The people who dread self assessment are almost always the ones scrambling through a shoebox in late January. The people who find it boring are the ones who kept decent records all year. Be the second kind.

Log income and expenses as they happen, not in one panicked session. A year-old expense with no receipt and no memory attached is one you won’t claim, which means you get taxed on it.
Keep digital copies of receipts. Thermal receipts fade to blank within a couple of years, so a photo is often a better record than the paper. Keep records for at least five years after the 31 January deadline for that year.
Reconcile against your bank monthly, so nothing slips through and duplicates get caught.
Watch your running profit through the year, so the final number isn’t a shock and your set-aside percentage is based on something real.

This is where a tool that tracks income, expenses and running profit as you go earns its keep. Sedonis keeps your numbers tidy and private throughout the year, so in January you’re reading off a total rather than rebuilding twelve months from memory. Free to start.

The short version

Register by 5 October, file and pay by 31 January, keep income and expenses separate, and set aside 25 to 30% as you earn. More in year one, because payments on account are coming.

Do the boring bit steadily and the return stops being a thing you fear. It becomes a form you fill in and forget.

Next steps

What you can claim as a sole trader · the expenses side, in detail
Self-employed tax calculator · your own numbers, in a few seconds
Free expenses tracker · laid out in the categories the return asks for
How to start a business in the UK · the full nine-step guide

Figures checked against GOV.UK on 20 July 2026: Self Assessment deadlines, penalties, Income Tax rates, self-employed National Insurance, payments on account, Making Tax Digital for Income Tax. Rates and thresholds change, usually each April.

General information, not tax advice. If your situation is at all unusual, an accountant will cost less than getting it wrong.