How HMRC Self Assessment tax returns work
In the United Kingdom, individuals with income that is not fully taxed at source must file an annual Self Assessment tax return (SA100) with HM Revenue and Customs (HMRC). This includes sole traders, business partners, landlords receiving rental income, company directors with dividend distributions, and individuals with untaxed savings or foreign income.
Your tax return consolidates all your earnings for the tax year (6 April to 5 April). HMRC calculates your total statutory liability for Income Tax and Class 4 National Insurance, deducts any tax you have already paid during the year through PAYE or Construction Industry Scheme (CIS) deductions, and establishes whether you owe a balancing payment or are entitled to an HMRC tax rebate.
Balancing payments and payment deadlines
If your total tax and National Insurance liability exceeds the deductions made during the year, you must pay the remaining balance to HMRC. This is known as your balancing payment. For an online tax return, the balancing payment for any given tax year is strictly due by midnight on 31 January following the end of that tax year.
Failure to submit your return or pay your balancing liability by 31 January triggers an immediate £100 late filing penalty, followed by progressive daily penalties, percentage surcharges, and statutory late-payment interest charged by HMRC.
Understanding payments on account for next year
HMRC requires taxpayers to make advance payments towards their next year tax bill, known as payments on account. You must make payments on account unless your Self Assessment tax bill is under £1,000, or you have already paid more than 80% of your total tax liability at source (for example through PAYE employment deductions).
Each payment on account is equal to 50% of your previous year Self Assessment liability. The first payment on account is due on 31 January (the same day as your balancing payment), and the second payment on account is due on 31 July.
Claiming tax relief for pensions and Gift Aid
Personal pension contributions made under relief at source and charitable donations made under Gift Aid reduce your total tax liability by expanding your basic rate tax band. For every £80 paid into a personal pension or donated to charity, HMRC treats your gross contribution as £100.
Basic rate tax relief (20%) is claimed directly by the pension provider or charity, while higher rate (40%) and additional rate (45%) taxpayers claim the extra 20% or 25% tax relief through their annual Self Assessment tax return.
To check your standard PAYE employment salary deductions, use the UK income tax calculator.
For sole trader business expenses and Class 4 National Insurance, explore the UK self-employed tax calculator.
For company director dividends and dividend tax brackets, visit the UK dividend tax calculator.
Inputs for the tax return calculator
Tax year
The UK tax year for your return, running from 6 April to 5 April.
Tax residency
Select Scotland if you are a Scottish tax resident subject to Scottish Income Tax bands, or England, Wales or Northern Ireland for standard UK rates.
Employment income (P60 / P45)
Total gross salary, wages, and taxable bonuses received from employment before any tax or pension deductions.
Self-employed profit (sole trader)
Net trading profit from self-employment (gross business turnover minus allowable allowable business expenses).
Rental and property profit
Net taxable profit from letting residential or commercial property in the UK or abroad.
Dividend income
Total gross dividends received from UK or foreign limited companies and investments.
Other taxable income
Taxable savings interest, private pensions, state pensions, and taxable state benefits received during the tax year.
Gross pension contributions
Gross contributions paid into personal or workplace pensions under relief at source schemes.
Gift Aid donations (net paid)
Total net monetary donations paid to registered charities under Gift Aid during the tax year.
Tax already deducted (PAYE, CIS, etc.)
Total income tax already deducted at source via PAYE (box 2 on your P60), Construction Industry Scheme (CIS) deductions, or withholding tax.
Payments on account already made
Total advance payments on account already paid towards this tax year (normally paid in previous January and July instalments).
CALCULATOR QUESTIONS
Tax return questions
When is the HMRC Self Assessment tax return deadline?
For online tax returns, the deadline is midnight on 31 January following the end of the tax year. For example, for the 2024-25 tax year ending 5 April 2025, the online filing and balancing payment deadline is 31 January 2026.
What is the difference between a balancing payment and payments on account?
A balancing payment settles the outstanding tax liability owed for the tax year just ended. Payments on account are advance instalments towards the following year tax bill, each calculated as 50% of your previous year Self Assessment liability.
How do I receive an HMRC tax refund?
When you submit your Self Assessment tax return and your tax already paid exceeds your total liability, HMRC issues a rebate. You can provide your UK bank account details (sort code and account number) on the return to receive the refund via direct bank transfer within several days.
Can I reduce my payments on account if my income drops?
Yes. If you anticipate that your profits or untaxed income will be substantially lower in the upcoming tax year, you can submit an application to HMRC (form SA303 or via your online account) to reduce your payments on account.
Do I have to file a tax return if all my income is from PAYE?
Most employees taxed solely through PAYE do not need to file a tax return unless their adjusted net income exceeds £150,000, they or their partner claim Child Benefit and earn over £60,000, or they have untaxed income exceeding statutory allowances.