Self Assessment is the system HMRC uses to collect income tax and National Insurance from people whose tax cannot be deducted automatically at source through PAYE. If you earn income from self-employment, a property, investments, overseas sources, or as a company director, chances are you need to complete a Self Assessment tax return each year.

The tax return itself is a summary of your income and allowable expenses for a given tax year (6 April to 5 April). HMRC uses it to calculate how much tax and National Insurance you owe — or whether you are due a refund. For the 2024-25 tax year, the key dates are running through autumn 2026 and into January 2027.

This tax return self assessment guide covers the full picture: who is required to file, what the deadlines are, how to work through the return itself, and what happens if things go wrong. If any section raises questions specific to your situation, the best move is to speak with a qualified accountant rather than guess — but for the majority of straightforward cases, this guide gives you the knowledge to act with confidence.

HMRC does not automatically send everyone a tax return — you are expected to know whether you need to file one and to register yourself. Broadly, you must complete a Self Assessment tax return if, during the relevant tax year, any of the following applied to you:

• You were self-employed (sole trader or partner in a business partnership) and your taxable income exceeded £1,000
• You were a director of a limited company (even if your only income was a salary below the personal allowance)
• You earned rental income from a UK or overseas property
• Your total income exceeded £100,000 — at this level, your personal allowance starts to be tapered away and the difference must be settled through Self Assessment
• You received income from savings, dividends, or investments above the relevant tax-free allowances
• You or your partner received Child Benefit and either of you earned over £60,000 during the year (the High Income Child Benefit Charge threshold as of 2024-25)
• You had income from abroad, a trust, or an estate
• HMRC sent you a notice to file, even if none of the above applied — once you receive a notice, filing becomes mandatory

If you are a PAYE employee with no additional income streams, you almost certainly do not need to file unless HMRC has specifically asked you to. If you are unsure, HMRC’s online checker at gov.uk is a good starting point, though it cannot account for every edge case.

One important distinction: being registered for Self Assessment and needing to file a return are slightly different things. If you were previously registered but no longer meet any of the criteria above, you should formally notify HMRC so they close your Self Assessment record — otherwise you will continue to receive filing obligations and penalties for non-submission.

Missing a Self Assessment deadline is costly and avoidable. Here are the key dates for the 2024-25 tax year (covering income earned between 6 April 2024 and 5 April 2025):

Payments on account

If your Self Assessment tax bill exceeded £1,000 for 2024-25 and less than 80% of your tax was collected at source, HMRC will ask you to make two advance payments towards your 2025-26 tax bill. The first falls on 31 January 2027 (alongside your 2024-25 settlement), and the second on 31 July 2027. This often catches first-time filers off guard — your first bill can effectively be one and a half years’ worth of tax in a single payment.

Before you can file online, you need a Government Gateway user ID and a Unique Taxpayer Reference (UTR). If you are registering for Self Assessment for the first time, the process works as follows:

For the self-employed

Register as self-employed via HMRC’s online service. You will need your National Insurance number, your business start date, the nature of your trade, and your contact details. HMRC will issue your UTR by post within ten working days (occasionally longer during busy periods). You will also be automatically registered for Class 2 National Insurance — though Class 2 was abolished from April 2024, so this is now handled differently through your Class 4 NIC calculation on the tax return itself.

For company directors

Directors registering for the first time should use HMRC’s Self Assessment registration service and select ‘I am not self-employed’ if your directorship is your only reason for registering. Your UTR will again arrive by post. If you already have a UTR from previous self-employment, you simply re-activate your account rather than registering afresh.

For landlords and those with other income

The same Government Gateway route applies. You will be asked to state the reason you need to file — in this case, property income or another untaxed source.

One practical note: if you are registering close to the 5 October deadline, post can be slow. HMRC can sometimes provide your UTR more quickly if you call them directly, but this is not guaranteed. If you have missed the registration deadline, register immediately and explain the circumstances — HMRC may consider this when assessing any penalty, though it is not an automatic defence.

The main Self Assessment return (SA100) acts as a hub, with supplementary pages added depending on your income sources. Most people filing online are guided through the relevant sections automatically. Here is what each common income type requires:

Self-employment income (SA103)

You will record your total business turnover and allowable expenses. The simplified expenses scheme (flat rates for home working, vehicles, and living premises) is available to sole traders and can save considerable record-keeping effort. You must also include any capital allowances claimed on equipment or vehicles.

Property income (SA105)

Rental income from UK property — whether residential, commercial, or furnished holiday lets — is reported here. You can deduct allowable expenses such as repairs, letting agent fees, and insurance premiums. Note that mortgage interest relief for residential landlords is now restricted to a 20% basic-rate tax credit rather than full deduction, following changes phased in since 2017.

Employment income

Your P60 from your employer covers your PAYE salary and tax deducted. If you received any benefits in kind (company car, private medical insurance, and so on), your P11D will show these figures. As a company director paying yourself a salary and dividends, both must be included.

Dividend income

The annual dividend allowance for 2024-25 is £500. Dividends above this are taxed at 8.75% (basic rate), 33.75% (higher rate), or 39.35% (additional rate). If you are a director taking dividends from your own limited company, these must be reported accurately — it is one of the most common areas where Self Assessment errors arise.

Pension contributions and Gift Aid

Both of these extend your basic-rate tax band, potentially pulling income that would otherwise be taxed at 40% back into the 20% band. They are easy to overlook but can make a meaningful difference to your final bill. Keep records of all pension contributions made outside of employer schemes and all Gift Aid declarations made during the year.

The penalty regime for Self Assessment is strict and has no sympathy for circumstances that were not entirely beyond your control. Understanding what you face if you miss a deadline is useful motivation to file on time.

Late filing penalties

• Day 1 (missed deadline): An automatic £100 fixed penalty. This applies even if your tax bill is zero — the penalty is for not filing the return, not for unpaid tax.
• 3 months late: An additional £10 per day, up to a maximum of £900.
• 6 months late: A further penalty of 5% of the tax due, or £300, whichever is greater.
• 12 months late: Another 5% or £300 charge, and in cases where HMRC believes the non-filing was deliberate, the penalty can be up to 100% of the tax owed.

Late payment penalties

If your tax bill is not paid by 31 January 2027, interest starts accruing immediately. After 30 days, a 5% surcharge is added; further surcharges apply at 6 and 12 months.

Can you appeal?

You can appeal a penalty if you have a genuine ‘reasonable excuse’ — serious illness, a bereavement close to the deadline, or HMRC’s own systems failing, for example. What does not qualify: simply not knowing you had to file, not having received a letter from HMRC, or being too busy. Over 1.1 million people missed the 2023-24 filing deadline, and HMRC’s position is that the obligation is well-publicised. If you receive a penalty you believe is unfair, appeal promptly — you have 30 days from the penalty notice date.

Making Tax Digital for Income Tax (MTD for ITSA) is the most significant change to Self Assessment in a generation. Rather than filing one annual return, affected taxpayers will submit quarterly digital updates of their income and expenses, followed by a final declaration at the year end.

Who does it affect and when?

MTD for ITSA applies to self-employed individuals and landlords whose combined gross income from those sources exceeds £50,000 per year from April 2026, and those earning above £30,000 from April 2027. Those earning above £20,000 will be brought in from April 2028.

If you fall into one of these groups, you will need to use HMRC-recognised software (such as Xero, which DG Accountancy uses for all clients) to maintain digital records and submit quarterly updates via the MTD platform. The traditional Self Assessment return does not disappear entirely — you still make an annual final declaration — but the quarterly submissions replace most of what currently sits in the annual SA100.

What this means in practice

For many sole traders and landlords, MTD for ITSA is a prompt to move from spreadsheets or paper records to cloud accounting software. Done properly, this actually makes record-keeping easier, because bank feeds pull transactions in automatically rather than requiring manual data entry. The quarterly submission itself is relatively lightweight if your records are up to date.

If you are currently inside the £50,000 threshold for April 2026 or the £30,000 threshold for April 2027, it is worth starting to prepare now rather than rushing to comply at the last moment. Getting your bookkeeping into a digital platform early means you have a year or more of clean records before the obligation kicks in — and it makes your current Self Assessment returns more accurate in the meantime.

If you are filing online for the 2024-25 tax year, here is the process from start to finish. Online is the preferred route for most people — it gives you until 31 January 2027 rather than 31 October 2026, and the system calculates your tax bill automatically.

These are the errors that come up repeatedly in practice — and most of them are entirely preventable.