Self Assessment is the system HMRC uses to collect tax from people whose tax cannot all be dealt with automatically through PAYE or another deduction system.
For many people, employment or pension tax is collected before they receive their income. Other people need to tell HMRC about income, gains or other circumstances by completing a Self Assessment tax return.
A tax return brings together the information HMRC needs to calculate your tax position for a particular tax year.
The UK tax year runs from 6 April to the following 5 April.
HMRC: Self Assessment tax returns
Who needs to complete a Self Assessment tax return?
You may need to complete a tax return for several reasons.
HMRC says you must normally send a return if, during the tax year, you were self-employed as a sole trader and received more than £1,000 before expenses, were a partner in a business partnership, had Capital Gains Tax to pay, or had certain other tax charges that were not collected another way.
You may also need a return if you received untaxed income from sources such as property, savings, investments, tips, commission or foreign income.
HMRC: Who must send a tax return
You must also complete a return if HMRC sends you a notice requiring one, even if you believe you have no additional tax to pay.
Not everyone with income outside PAYE needs Self Assessment. The answer depends on the type and amount of income and your wider tax position.
When do you need to register?
If you need to complete a Self Assessment return and HMRC does not already expect one from you, you normally need to tell HMRC by 5 October following the end of the relevant tax year.
For example, if you first need to complete a return for the 2025-26 tax year, which ended on 5 April 2026, the normal notification date is 5 October 2026.
HMRC: Self Assessment deadlines
Registration gives HMRC the information it needs to set up your Self Assessment record. If you are registering for the first time, HMRC will normally issue a Unique Taxpayer Reference, commonly called a UTR.
A UTR is different from your National Insurance number and should be kept safely because it identifies your Self Assessment record.
What information goes on a tax return?
The information required depends on your circumstances.
Someone with employment income and a small amount of bank interest will need much less information than someone who is self-employed, owns rental property, sells investments and trades cryptoassets.
A return may include:
- employment and pension income;
- self-employment income and expenses;
- property income and expenses;
- bank interest and dividends;
- capital gains;
- foreign income;
- pension contributions;
- charitable donations;
- tax already deducted;
- other claims, allowances or reliefs.
You should keep records that support the figures you enter on the return. HMRC requires taxpayers to keep records so they can complete their returns correctly.
Good records also make it much easier to explain a figure later if HMRC asks questions.
When is the tax return due?
For most individuals, the online Self Assessment filing deadline is 31 January following the end of the tax year.
For the 2025-26 tax year:
- the tax year ended on 5 April 2026;
- the normal paper return deadline is 31 October 2026;
- the normal online filing deadline is 31 January 2027;
- tax due through Self Assessment is normally payable by 31 January 2027.
HMRC: Self Assessment deadlines
You do not need to wait until January to submit the return.
HMRC allows you to file once the tax year has ended. Filing earlier can give you more time to understand the amount due and plan for payment.
HMRC: Sending a Self Assessment tax return
How is the tax calculated?
The tax return brings together your relevant income, gains, tax deducted, allowances and reliefs.
The calculation then works out whether:
- you have more tax to pay;
- you have already paid the correct amount; or
- you have paid too much and may be due a repayment.
Self Assessment does not mean that all of your income is taxed again.
For example, if your employer has already deducted Income Tax through PAYE, the return includes both the employment income and the tax already deducted.
HMRC then considers that information as part of the overall calculation.
What are payments on account?
Payments on account often cause confusion when someone receives their first significant Self Assessment bill.
They are advance payments towards the following year’s tax bill.
HMRC normally requires payments on account where the previous year’s Self Assessment tax liability was £1,000 or more and you did not already pay more than 80% of the tax outside Self Assessment.
Each payment is normally half of the previous year’s relevant tax liability.
The first payment is due on 31 January and the second on 31 July.
For example, suppose your 2025-26 Self Assessment calculation shows £4,000 of tax that falls within the payments-on-account rules.
You may need to pay:
- £4,000 balancing the 2025-26 liability; and
- £2,000 as the first payment on account for 2026-27;
by 31 January 2027.
A further £2,000 payment on account would normally fall due on 31 July 2027.
This can make the first January bill look much higher than expected, even though part of the payment relates to the following tax year.
If you reasonably expect the following year’s liability to be lower, it may be possible to reduce the payments on account. Care is needed because HMRC can charge interest if you reduce them too far.
What if you make a mistake on your tax return?
A tax return can usually be amended after submission.
HMRC generally allows you to amend a Self Assessment return within 12 months of the normal filing deadline.
For example, a 2025-26 return normally has a filing deadline of 31 January 2027, so the normal amendment window runs to 31 January 2028.
HMRC: Correcting a Self Assessment tax return
Older errors may need a different correction or disclosure route.
The correct approach depends on the year involved, the type of error and whether HMRC has already contacted you.
If you discover an error, it is normally better to establish the correct figures before sending an amendment rather than making a quick adjustment based on incomplete information.
What happens if you file late?
HMRC can charge penalties for late Self Assessment returns.
The initial late-filing penalty is £100. Further penalties can arise if the return remains outstanding for more than three, six and twelve months. Interest and separate late-payment penalties can also apply where tax remains unpaid.
HMRC: Self Assessment penalties
This means a return can create penalties even where relatively little tax is due.
If you know that a return is overdue, submitting it sooner can prevent further late-filing penalties from building up.
What happens if you cannot pay the tax?
Filing the return and paying the tax are separate obligations.
You should still submit the return even if you cannot immediately pay the full amount.
If you have difficulty paying, HMRC may agree a payment arrangement depending on the circumstances.
Ignoring the return because you cannot pay usually makes the position worse because filing penalties, payment penalties and interest can arise separately.
Can a tax adviser complete Self Assessment for you?
Yes.
An authorised tax adviser can prepare the return, calculate the tax position, deal with relevant HMRC correspondence and submit the return after receiving your approval.
You remain responsible for providing complete and accurate information.
A good Self Assessment process therefore involves more than entering figures into software. The underlying information needs to make sense and support the tax position being reported.
What about Making Tax Digital?
Making Tax Digital for Income Tax now applies to some sole traders and landlords.
From 6 April 2026, individuals with qualifying self-employment and property income above the relevant threshold may need to keep digital records and send quarterly updates to HMRC using compatible software.
Making Tax Digital does not remove the annual tax-return process. It changes how some taxpayers keep records and report information during the year.
You can read more in Making Tax Digital for Income Tax: Who Needs to Use It and What Changes?.
The main point
Self Assessment is the process used to bring together the parts of your tax affairs that HMRC cannot deal with automatically.
The important steps are straightforward: keep reliable records, identify what needs to go on the return, meet the filing deadline, review the calculation and pay the tax by the appropriate date.
The return can become more complicated where you have several income sources, property, investments, capital gains, cryptoassets or earlier-year issues, but the basic process remains the same.
Need help with Self Assessment?
Sat Tax supports UK individuals with Self Assessment and personal tax.
We can review the information relevant to your return, prepare the tax calculation, explain the final position and submit the return to HMRC once you have reviewed and approved it.
If you need help with your annual tax return, get in touch with Sat Tax to discuss your circumstances and the work required.
This article is for general information only and is not personal tax advice. Tax treatment and reporting requirements depend on your circumstances and the tax rules in place at the relevant time.
