Self Assessment tax returns: who files, when, and what a mistake costs

Self Assessment is the system HMRC uses for income that nobody deducts tax from before you receive it. You report that income and any allowable costs on a tax return each year, HMRC calculates what you owe, and you pay it. Employees whose tax is handled through PAYE usually never see it. Sole traders, landlords, partners and some higher earners do.

Who needs to file a Self Assessment tax return?

You must send a return if you were a sole trader and earned more than £1,000 in the tax year, counted before you take off anything you can claim relief on. You must also send one if you were a partner in a business partnership, if you had to pay Capital Gains Tax on something you sold, or if you owe the High Income Child Benefit Charge and do not pay it through PAYE. Untaxed income such as rent, tips, savings interest, dividends or foreign income can also trigger a return (GOV.UK, who must send a return).

A tax year runs from 6 April to 5 April. The return you file by January 2027 covers 6 April 2025 to 5 April 2026. GOV.UK offers a short online tool to check whether you need to file, and if HMRC itself writes to ask for a return, you must send one by the date on the letter.

What are the Self Assessment deadlines?

There are three dates to hold in your head, and one of them is easy to miss because it falls in October. The table sets them against the return for 2025 to 2026, using the dates GOV.UK publishes.

If you register after 5 October, HMRC sets a different return deadline, three months from the date on its letter or email. Registering late can still lead to a penalty, so do it as soon as you know you need to. New registrations also need a Unique Taxpayer Reference from HMRC before you can file, and GOV.UK points you to a guide on when to expect it, so do not leave registration to the last week.

What happens if you file or pay late?

Penalties for a late return are automatic and they stack. According to GOV.UK, the first is a £100 charge. After three months HMRC adds £10 a day, up to a maximum of £900. After six months it adds 5% of the tax due or £300, whichever is higher, and it adds the same again at twelve months.

Paying late is penalised separately. GOV.UK states 5% of the unpaid tax is added at 30 days, again at six months and again at twelve months, and interest runs on the amount owed. Because the two sets of penalties are independent, filing on time but paying late still costs you, and the reverse is also true.

What is Making Tax Digital for Income Tax and does it apply to you?

Making Tax Digital for Income Tax changes how some people report. Instead of one return after the year ends, you keep digital records in compatible software and send HMRC an update each quarter, then submit your year-end return. GOV.UK says you need to use it from 6 April 2026 if your total annual income from self-employment and property is over £50,000. That test looks at gross income before expenses, and for the April 2026 start HMRC uses the figures on your 2024 to 2025 return.

HMRC's published plan lowers the line to £30,000 from April 2027, with a further drop to £20,000 from April 2028 for those whose qualifying income exceeded that in 2026 to 2027. Employment pay and pension income do not count towards the test, only self-employment and property. A landlord with a salary on top can still be caught by the rental total alone.

• Check your qualifying income now, before the thresholds fall, so a lower line does not surprise you.
• Ask what software the accountant uses and whether the fee covers four quarterly updates as well as the annual return.
• Keep records as you go. Quarterly updates are built from the income and expenses you log through the year.
• You still file a year-end return, so the January deadline does not disappear.

What does an accountant do on a Self Assessment return?

A good accountant does more than type your figures into HMRC's form. They decide which costs are allowable, apply capital allowances to equipment, work out whether you should make payments on account, and check whether pension contributions or Gift Aid change the bill. For a landlord or someone with a second income, they pull the sources together into one return that matches the paperwork.

Many people file simple returns themselves without trouble. Help earns its fee when income comes from several sources, when you sold an asset, when you have gone over £100,000 and lost part of your Personal Allowance, or when you are heading into Making Tax Digital for the first time. If your setup is a trading business, the question of whether to stay a sole trader is covered in our sole trader or limited company comparison, and companies have a separate route described in the limited company accounts guide.

How do you choose and check an accountant for your tax return?

Anyone can call themselves an accountant, so the first step is a register check. Chartered and certified accountants are listed with their professional body: ICAEW, ACCA, CIMA, AAT, ATT or CIOT. Use the body's own online register to confirm the name, the firm and that the membership is current, and that the person is allowed to work in tax. Firms are matched to those registers before they are listed here, and you can browse by area from the cities page or read more in the guides.

1. Look the firm or individual up on the register of their professional body and confirm membership is current.
2. Ask for a fixed fee in writing, naming the tax year and what is included.
3. Ask how they handle Making Tax Digital quarterly updates if your income is over the threshold.
4. Ask for their own deadline for your records, so a January return is not left until 30 January.
5. Ask whether they hold professional indemnity insurance and how they will tell you about HMRC letters.
6. Confirm you will receive a copy of the filed return and the calculation.

Whichever accountant you pick, you remain legally responsible for the return. Read the figures before they are sent, and keep your records for the period HMRC requires, which GOV.UK sets out on its record-keeping pages.