Tax return self assessment guide: everything you need to know
Whether you are self-employed, a landlord, a director, or someone with untaxed income, this guide walks you through every stage of the UK self assessment process — from registering with HMRC to filing your return and paying your bill on time. Estimated reading time is around ten minutes.
What you need to know
- You must file a self assessment return if you are self-employed and earned more than £1,000 before deductions.
- The online filing deadline for 2025/26 returns is 31 January 2027; paper returns must reach HMRC by 31 October 2026.
- Missing the deadline triggers an immediate £100 penalty, with further daily and percentage-based charges building quickly.
- Claiming all allowable expenses — office costs, travel, professional fees, equipment — can significantly reduce your tax bill.
- Making Tax Digital for Income Tax is coming in 2026, replacing annual filing with quarterly digital submissions for many taxpayers.
What is self assessment and who does it affect?
Self assessment is the system HMRC uses to collect Income Tax from people whose tax cannot be collected automatically through PAYE. For most employees, tax is deducted at source by their employer. For everyone else — the self-employed, landlords, company directors, investors, and others with untaxed income — self assessment is the mechanism through which they calculate what they owe and pay it.
This tax return self assessment guide is aimed at anyone who needs to navigate the process, whether you are filing for the first time or looking to make sure you are doing it correctly. The rules are not especially complicated once you understand the structure, but the details matter: wrong figures, missed supplementary pages, or late submission can each produce an unnecessary cost.
The 2025/26 tax year runs from 6 April 2025 to 5 April 2026. Returns for that year are due in the filing window that opens in April 2026. This guide reflects deadlines and rules current as at June 2026, drawing on HMRC guidance and practical experience of completing these returns for hundreds of UK business owners.
Who needs to send a self assessment tax return?
HMRC’s starting position is that self assessment applies to anyone with income that has not been taxed at source. The most common groups who must file are:
- Sole traders and freelancers who earned more than £1,000 from self-employment before deducting any expenses in the relevant tax year.
- Partners in a business partnership, regardless of the level of profit — the partnership itself also files a return.
- Company directors who receive income outside PAYE, such as dividends, or whose company has paid them via a combination of salary and dividends.
- Landlords with rental income from UK property — even if that income is modest and falls below your Personal Allowance.
- Investors and savers with untaxed income from dividends, interest on savings above the Personal Savings Allowance, or capital gains above the Annual Exempt Amount.
- Higher earners (over £100,000) whose Personal Allowance is tapered, or anyone liable to the High Income Child Benefit Charge.
- Those with foreign income or income from trusts and settlements.
Voluntary filing
Not everyone who files is required to. You can submit a self assessment return voluntarily — for example, to claim Gift Aid relief as a higher-rate taxpayer, to prove your self-employment income for a mortgage application, or to pay voluntary Class 2 National Insurance contributions to protect your State Pension entitlement.
New to self assessment? Notify HMRC by 5 October
If you need to file for the first time for the 2025/26 tax year and you have not previously been registered for self assessment, you must notify HMRC by 5 October 2026. Missing this date does not prevent you filing, but it reduces the time available before the January 2027 deadline and may attract a penalty if you file late as a result.
Self assessment deadlines for the 2025/26 tax year
Deadlines in self assessment are hard. HMRC does not grant extensions for forgetting, being busy, or finding the process confusing. Here are the key dates for 2025/26 returns:
| Deadline | Date | What it covers |
|---|---|---|
| Notify HMRC of new liability | 5 October 2026 | Tell HMRC you need to file for 2025/26 if you have not done so before |
| Paper return submission | 31 October 2026 | Paper SA100 must be received by HMRC by 11:59pm |
| Collect tax via PAYE code | 30 December 2026 | Online return must be filed by this date for HMRC to collect underpaid tax through your 2027/28 PAYE code (balances of £3,000 or less only) |
| Online return submission | 31 January 2027 | Online SA100 must be submitted by 11:59pm |
| Payment of tax owed | 31 January 2027 | Balancing payment for 2025/26, plus first payment on account for 2026/27 |
Payments on account
If your tax bill for the year exceeds £1,000 and less than 80% of it was collected at source, HMRC requires you to make two payments on account towards the following year’s tax. The first is due on 31 January and the second on 31 July. First-time filers are often caught off guard by these — your January payment covers both the balance for the year just ended and the first instalment for the year ahead.
How many people actually meet the deadline?
For the 2024/25 tax year, just over 11.48 million people filed their return by 31 January 2026, with around 97% choosing the online route. Despite that, an estimated one million taxpayers missed the deadline. If you are in that group for any year, act as quickly as possible — penalties escalate sharply the longer a return remains outstanding.
What are the penalties for filing or paying late?
Self assessment penalties are structured to become progressively more painful the longer you delay. This is not coincidental — the system is designed to incentivise early action. Here is how the charges build:
- Day one: An automatic £100 fixed penalty applies the moment the deadline passes, even if you owe no tax at all.
- After three months: Daily penalties of £10 per day begin, running for up to 90 days. That adds up to a maximum of £900 on top of the initial £100.
- After six months: An additional penalty of 5% of the tax owed, or £300, whichever is greater.
- After twelve months: A further 5% of tax owed or £300, whichever is greater. In cases where HMRC considers the information withheld deliberately, this can rise significantly higher.
Late payment penalties and interest
Separate from the filing penalty, HMRC also charges interest on unpaid tax from the day it is overdue. Late payment surcharges of 5% apply at 30 days, 6 months, and 12 months past the payment deadline. Interest rates follow the Bank of England base rate plus 2.5 percentage points, so the cost of deferring payment is not trivial.
Penalty appeals
HMRC does accept appeals where there is a reasonable excuse — a serious illness, a bereavement, or a genuine technical failure outside your control, for example. Difficulty understanding the tax system or forgetting a deadline are not accepted. If you believe you have grounds for appeal, you need to act promptly and provide clear evidence.
What income and expenses to include in your return
Accuracy is the most important quality a self assessment return can have. Understating income or overstating expenses carries real risk — HMRC cross-references the information you provide against data from employers, banks, HMRC’s own records, and third-party sources.
Income you must declare
Your return should include all sources of taxable income in the relevant tax year, not just the obvious ones. Common sources that are missed include:
- Freelance or consultancy income alongside employed earnings
- Rental income from residential or commercial property
- Dividend income above the dividend allowance (£500 for 2025/26)
- Savings interest above the Personal Savings Allowance
- Capital gains from selling shares, property, or other assets
- Foreign income, including pension income from abroad
- Tips and commission not captured through PAYE
Supplementary pages
The main SA100 form covers your core income, but depending on your circumstances you may need to complete additional supplementary pages alongside it. The most common are:
- SA103 — self-employment income and expenses
- SA105 — UK property income
- SA108 — capital gains
- SA101 — additional information (certain reliefs, pension income, etc.)
Forgetting a supplementary page is one of the most common errors we see — not because people are careless, but because the main return form does not always make it obvious that an additional page is needed.
Allowable expenses
If you are self-employed, you can deduct allowable business expenses from your income before calculating your tax liability. These include costs that are wholly and exclusively for business purposes: office and stationery costs, business travel (but not commuting), professional subscriptions and fees, accountancy costs, equipment and software, and a proportion of your home costs if you work from home regularly. Keeping clear records throughout the year makes claiming these far more straightforward at filing time.
Making Tax Digital for Income Tax: what is coming
From April 2026, Making Tax Digital for Income Tax Self Assessment (MTD for ITSA) is being phased in, and it represents the most significant change to how self-employed people and landlords report their income to HMRC in a generation.
Who is affected first?
The initial phase applies to self-employed individuals and landlords with combined qualifying income above £50,000 per year. From April 2027, the threshold drops to £30,000. Further expansion is planned, though the government has not confirmed a timetable for lower income thresholds at the time of writing.
What changes in practice?
Instead of one annual return, affected taxpayers will be required to submit quarterly digital updates to HMRC — essentially four times a year — using MTD-compatible software. These updates summarise income and expenses for each quarter. At the end of the year, a final declaration replaces the current SA100, confirming or adjusting the information submitted quarterly.
What stays the same?
The underlying tax calculations, allowances, reliefs, and payment deadlines remain the same. MTD for ITSA changes the reporting mechanism, not the rules about what is taxable. Tax is still due by 31 January following the end of the tax year.
Why this matters now
If your income is approaching £50,000, or you are a landlord with meaningful rental income, you should be thinking about this now rather than scrambling when the mandate arrives. Moving to compatible software — such as Xero, QuickBooks, or FreeAgent — before you are required to is considerably less stressful than doing it under pressure.
How to complete and file your return
Filing a self assessment return involves several distinct stages. Here is the process from start to finish.
Register for self assessment with HMRC
If you have never filed before, you need to register first. Self-employed individuals register at gov.uk using the SA1 or CWF1 form. Once registered, HMRC sends your Unique Taxpayer Reference (UTR) by post — allow up to ten working days. You will also need to create a Government Gateway account if you do not already have one.
Gather your income and expense records
Before you open the return, collect everything you need: P60s and P11Ds from any employment, bank statements showing savings interest, dividend statements, rental income records, invoices and receipts for business expenses, and any capital gains disposals. Having everything to hand before you start prevents mistakes caused by estimating figures.
Log in and complete the SA100
Access the return through your Government Gateway account at gov.uk. Work through the SA100 section by section. Where your circumstances require supplementary pages — self-employment, property income, capital gains — these are added within the same online session. HMRC’s system calculates your tax liability automatically as you enter figures.
Check and claim all allowances and reliefs
Before you submit, review your entries for anything you may have missed: pension contributions, Gift Aid donations, marriage allowance transfers, capital losses brought forward, and allowable business expenses. These are not applied automatically — you must actively claim them. A missed relief is money left on the table.
Review your tax calculation carefully
The system shows your calculated tax liability before submission. Check that the figure looks proportionate to your income. If something appears significantly higher or lower than expected, work back through the entries to find the discrepancy. It is far easier to correct a mistake before submission than to amend a filed return.
Submit the return and pay by the deadline
Submit the return before 11:59pm on 31 January 2027 for online filing. Then arrange payment of any balance owed by the same deadline. HMRC accepts payment by bank transfer, debit card, or direct debit. Allow enough time for the payment to clear — same-day bank transfer is the safest option close to the deadline.
Common self assessment mistakes to avoid
These are the errors we see most frequently when reviewing returns that clients bring to us after completing them themselves.
Using the wrong UTR or National Insurance number
Your Unique Taxpayer Reference and National Insurance number must both be entered exactly as they appear on your HMRC correspondence. A single digit transposition means the return is associated with the wrong account. This is more common than it sounds, and it can cause delays, duplicate penalty notices, and a genuine headache to resolve with HMRC.
Forgetting to declare all income sources
HMRC receives data from banks, investment platforms, employers, and Companies House. If you receive dividends, savings interest, or freelance income alongside your main employment and omit it, the discrepancy is likely to be flagged. Every taxable income source must appear on the return, even if the resulting additional tax turns out to be small.
Missing supplementary pages for specific income types
The SA100 is the core form, but self-employment income (SA103), rental income (SA105), and capital gains (SA108) each require their own supplementary page. Filing the SA100 without the relevant supplementary pages produces an incomplete return — and HMRC will follow up.
Not correcting errors within the twelve-month window
If you spot a mistake after submission, you can amend the return within twelve months of the filing deadline. After that window closes, corrections require a formal written request to HMRC and are harder to process. If you realise you have made an error, act on it promptly rather than hoping it goes unnoticed.
When professional help is worth it
For straightforward cases — a sole trader with one income stream, modest expenses, and no complications — self assessment is genuinely manageable as a DIY exercise. HMRC’s online system is functional, and this guide gives you the framework to work through it.
Where professional support tends to pay for itself are situations such as these:
- You have income from multiple sources: employment, self-employment, property, and investments, each with different tax treatment.
- You have capital gains to report, particularly where reliefs such as Business Asset Disposal Relief or Private Residence Relief may apply.
- You are a company director filing both a personal return and company accounts in the same period.
- You have made pension contributions and want to claim higher-rate relief correctly.
- HMRC has opened a compliance check or sent an enquiry notice.
At Wings Online Filings, a self assessment return is a fixed £150 one-off fee — prepared by a qualified accountant, checked for errors, and filed on your behalf. No hourly rates, no surprises.
Related guides and services
Explore further reading and the services most relevant to self assessment filers.
Frequently asked questions
Do I need to file a self assessment return if I am employed?
Most employees have their tax collected via PAYE and do not need to file. However, you must file if you also have self-employment income above £1,000, rental income, untaxed investment income, or earnings above £100,000 that reduce your Personal Allowance. Employees who receive tips or commission not processed through payroll may also need to file.
What happens if I miss the 31 January self assessment deadline?
An automatic £100 penalty applies immediately after the deadline, regardless of whether any tax is owed. After three months, daily penalties of £10 accrue for up to 90 days. Further percentage-based charges follow at six and twelve months. Interest also runs on any unpaid tax from the due date. The penalties escalate quickly, so filing late is always better than not filing at all.
Can I amend a self assessment return after I have submitted it?
Yes. You can amend a filed return within twelve months of the filing deadline — so for a 2025/26 return filed in January 2027, the amendment window runs until 31 January 2028. Amendments are made through your Government Gateway account. After the twelve-month window closes, corrections require a formal written request to HMRC.
What is a Unique Taxpayer Reference and where do I find it?
Your Unique Taxpayer Reference (UTR) is a ten-digit number HMRC assigns when you register for self assessment. It appears on any correspondence HMRC sends you, including your original registration letter, previous tax returns, and reminder notices. If you cannot locate it, you can retrieve it through your Personal Tax Account on gov.uk or by calling HMRC directly.
How do payments on account work in self assessment?
If your tax bill exceeds £1,000 and less than 80% was collected at source, HMRC requires advance payments towards your next year’s bill. Each payment on account is half the previous year’s liability. The first is due on 31 January alongside your balancing payment, and the second on 31 July. First-time filers are often surprised by the January bill being larger than expected for this reason.
What is Making Tax Digital for Income Tax and when does it start?
Making Tax Digital for Income Tax Self Assessment (MTD for ITSA) requires self-employed individuals and landlords with qualifying income above £50,000 to submit quarterly digital updates to HMRC instead of a single annual return. It begins in April 2026 for those above the £50,000 threshold, with the threshold dropping to £30,000 from April 2027. MTD-compatible software such as Xero, QuickBooks, or FreeAgent will be required.
Final thoughts
Self assessment does not have to be stressful, but it does reward preparation. Knowing who needs to file, understanding the deadlines, keeping clean records throughout the year, and checking your return carefully before submission are the four habits that prevent the majority of problems.
This tax return self assessment guide has covered the essentials: who is required to file, the key dates for 2025/26, how penalties escalate, what income and expenses belong in your return, and the changes that Making Tax Digital will bring. The fundamentals are consistent year on year — what changes are the thresholds, allowances, and occasionally the rules around specific income types.
If your return is straightforward, you now have the framework to complete it yourself. If your situation involves multiple income sources, property disposals, or anything that requires careful judgment, a qualified accountant will usually save you more than the fee costs. We are here if that is useful.