If you run a business in the UK, whether as a sole trader, a partner, or a limited company director taking dividends, you will almost certainly need to file a Self Assessment tax return. This guide is written for you. It is not a generic overview. It is the definitive self assessment tax return guide for UK business owners who need to get this right, avoid penalties, and understand what is changing under Making Tax Digital.
We are ICAEW qualified accountants. We work with businesses across the UK, from a freelance graphic designer in Camden to a growing construction partnership in the Jewellery Quarter of Birmingham. This guide covers the rules for 2025/26, the forms you need, the deadlines you cannot miss, and the practical steps to file correctly. We include specific numbers, real forms, and a full action checklist. Bookmark this page. You will come back to it every year.
Who Must File a Self Assessment Tax Return in 2025/26
Not everyone needs to file. But if you are a business owner, the chances are high. Here is who HMRC expects to receive a return from.
Sole Traders and Freelancers
If you are self-employed and your gross income from self-employment exceeds £1,000 in a tax year, you must register for Self Assessment and file a return. The £1,000 trading allowance means you do not need to register if your income is below that threshold, but the moment you cross it, you are in scope. You report your income and expenses on the SA103 form (short or full version depending on your turnover).
Partners in a Business Partnership
Every partner in a UK partnership must file their own Self Assessment return. The partnership itself files an SA800 partnership return, and each partner receives a partnership statement showing their share of the profits. You report that share on your personal return. This applies even if the partnership made a loss.
Limited Company Directors
If you are a director of a limited company and you receive income outside of PAYE, you must file a Self Assessment return. Common triggers include:
- Dividends above the £500 allowance for 2025/26
- Director's loan account balances that are not repaid within 9 months and 1 day of the year-end
- Benefits in kind such as a company car or private medical insurance
- Rental income from property held personally
- Capital gains from selling shares or property
Even if your only income is salary through PAYE, if you have dividends or benefits, you must file. Check with your accountant. Many directors file even when not strictly required, to confirm their tax position and protect their filing history.
Landlords and Property Investors
If you receive rental income from UK property, you must file a Self Assessment return if your gross rental income exceeds £2,500. Below that, HMRC may adjust your tax code. Above £10,000, you must file a return regardless. The SA105 form covers property income.
High Earners and Those with Complex Tax Affairs
If your income exceeds £100,000, you must file a return even if all tax is deducted at source. This is because the personal allowance tapers away by £1 for every £2 of income above £100,000. HMRC needs your return to calculate the correct allowance. The same applies if you have to pay the High Income Child Benefit Charge, which kicks in when your income exceeds £50,000.
How to Register for Self Assessment
You cannot file a return until you are registered. Registration is straightforward but time-sensitive. Miss the deadline and you face a penalty.
Registration Deadlines
You must register by 5 October following the end of the tax year in which you first became self-employed or received untaxed income. For the 2025/26 tax year, that means registering by 5 October 2026. If you miss this deadline, HMRC may charge a penalty.
How to Register Online
Go to GOV.UK and search "register for Self Assessment". You will need your National Insurance number, your passport or UK driving licence details, and your address history. HMRC will issue you a Unique Taxpayer Reference (UTR) within 10 working days. You will also receive an activation code for your online account, which arrives by post within 7 to 10 days.
If you are registering as a sole trader, you also need to register for Class 2 National Insurance. This is done through the same process. From 2024/25, Class 2 NICs are effectively zero for most self-employed people, but you still need to be registered.
Registering as a Partnership
If you are starting a partnership, one partner registers the partnership for Self Assessment and receives a UTR for the partnership. Each individual partner then registers separately for their own Self Assessment. The partnership return is separate from the partners' personal returns.
Registering as a Director
Company directors do not register as self-employed. You register for Self Assessment as an individual. HMRC will link your return to your company through your director's details. Your company's payroll and Corporation Tax affairs are handled separately through PAYE and the CT600 Corporation Tax return.
Key Deadlines for 2025/26
HMRC operates two deadlines for paper and online filing. Miss them and the penalties escalate quickly.
| Deadline | What is due | Notes |
|---|---|---|
| 5 October 2026 | Registration deadline for 2025/26 | If you started self-employment in 2025/26, register by this date. |
| 31 October 2026 | Paper return filing deadline | You must post your paper return to HMRC by this date. Late filing penalty applies from 1 November. |
| 31 January 2027 | Online return filing deadline | File by midnight. Also the deadline to pay any tax due. |
| 31 January 2027 | First payment on account due | If your tax bill exceeds £1,000, you pay half of next year's estimated tax now. |
| 31 July 2027 | Second payment on account due | Second instalment of next year's estimated tax. |
If you miss the 31 January deadline, you incur an automatic £100 penalty. After 3 months, daily penalties of £10 per day kick in, up to £900. After 6 months, a further £300 or 5% of the tax due, whichever is higher. After 12 months, another 5%. The penalties can quickly exceed the tax you owe.
Which Forms Do You Need
Your Self Assessment return is not a single form. It is a core return plus supplementary pages depending on your income sources.
SA100 - The Core Return
Every taxpayer files the SA100. This captures your personal details, employment income, pensions, savings interest, dividends, and any tax already deducted. It also includes the box for the High Income Child Benefit Charge.
SA103 - Self-Employment Pages
If you are a sole trader, you file the SA103. There are two versions:
- SA103S (Short) - for turnover up to £85,000. You report total income and total expenses. No breakdown required.
- SA103F (Full) - for turnover above £85,000 or if you prefer to show a breakdown. You categorise expenses into cost of sales, wages, rent, utilities, travel, and so on.
If you use the cash basis (common for sole traders with turnover under £150,000), you report income when received and expenses when paid. If you use accrual accounting, you report income when earned and expenses when incurred, regardless of payment timing.
SA105 - Property Pages
For rental income from UK property. You report each property's income and expenses separately if you have more than one. The SA105 also captures furnished holiday lettings, which benefit from more favourable tax treatment.
SA800 - Partnership Pages
The partnership itself files the SA800. Each partner then reports their share on their personal return using the partnership statement.
SA108 - Capital Gains Pages
If you sold an asset in the tax year and made a gain above your annual exempt amount (£3,000 for 2025/26), you file the SA108. For residential property disposals, you also need to file a 60-day CGT property return and pay the tax within 60 days of completion.
SA109 - Residence Pages
If you are non-UK resident or split your time between the UK and abroad, you file the SA109 to determine your residence status and which income is taxable in the UK.
What Counts as an Allowable Expense
This is where most mistakes happen. Claiming expenses you cannot deduct, or missing expenses you can, both cause problems. HMRC expects you to claim only expenses that are wholly and exclusively for the purposes of your trade.
Common Allowable Expenses for Sole Traders
- Office costs: stationery, printing, postage, software subscriptions (Xero, QuickBooks, Dext)
- Travel: mileage at 45p per mile for the first 10,000 business miles, 25p thereafter. Train fares, parking, tolls. Not commuting from home to your regular workplace.
- Professional fees: accountant, solicitor, surveyor, bookkeeper
- Premises costs: rent, rates, utilities, cleaning, insurance
- Staff costs: wages, pensions, training
- Advertising and marketing: website hosting, Google Ads, social media, business cards
- Stock and raw materials: cost of goods sold
- Interest on business loans: but not capital repayments
- Subscriptions to professional bodies: ICAEW, ACCA, RIBA, etc.
Working from Home
If you work from home, you can claim a proportion of your household costs. HMRC accepts a simplified rate of £6 per week without needing receipts. Alternatively, you can calculate the actual cost based on the number of rooms used and the hours worked. For a sole trader using one room as an office 40 hours a week, you might claim 10% of your rent, council tax, heating, and electricity. Keep a log of your working hours.
What You Cannot Claim
- Personal clothing, even if worn for work (unless it is a uniform or protective clothing)
- Client entertainment (100% disallowable for sole traders and companies alike)
- Fines and penalties
- Capital expenditure (that goes through capital allowances instead)
- Drawings or personal expenses
How to Calculate Your Tax Bill
Your Self Assessment return calculates your total income from all sources, deducts allowances and reliefs, and applies the appropriate tax rates. Here is how it works for a typical sole trader.
Worked Example: Priya's Freelance Design Business
Priya runs a freelance graphic design business from her home in Stokes Croft, Bristol. For 2025/26, her figures are:
- Gross self-employment income: £63,400
- Allowable expenses: £14,720
- Net profit: £48,680
- Bank interest: £320
- Dividends from a small shareholding: £1,200
Her total income is £48,680 + £320 + £1,200 = £50,200.
Personal allowance: £12,570.
Taxable income: £50,200 - £12,570 = £37,630.
Basic rate band: £37,700. She is within it.
Income tax on self-employment profit: £37,630 at 20% = £7,526.
Dividend tax: £1,200 minus £500 allowance = £700 at 8.75% = £61.25.
Bank interest: £320 minus £500 starting rate for savings (reduced because of her other income) = £0 tax.
Class 4 NICs: 6% on profits between £12,570 and £50,270. £48,680 minus £12,570 = £36,110 at 6% = £2,166.60.
Class 2 NICs: nil from 2024/25.
Total tax and NICs: £7,526 + £61.25 + £2,166.60 = £9,753.85.
Priya must pay this by 31 January 2027. Because her tax bill exceeds £1,000, she also makes payments on account. Her first payment on account (due 31 January 2027) is half of £9,753.85 = £4,876.93. Her second (due 31 July 2027) is the same. So on 31 January 2027, she pays £9,753.85 + £4,876.93 = £14,630.78.
Payments on Account and Balancing Payments
Payments on account catch many business owners out. They are advance payments towards your next year's tax bill, based on the current year's liability. If your tax bill is consistently falling, you can apply to reduce them. If it is rising, you will face a larger balancing payment the following January.
When Payments on Account Apply
You make payments on account if your tax and Class 4 NICs bill exceeds £1,000. The first payment is due on 31 January alongside the balancing payment for the previous year. The second is due on 31 July.
How to Reduce Payments on Account
If you know your income will be lower next year, you can submit a form to HMRC to reduce your payments on account. Be careful. If you reduce them and your income is higher, HMRC charges interest on the underpayment. The form is available through your HMRC online account.
Making Tax Digital for Income Tax Self Assessment (MTD ITSA)
This is the biggest change to Self Assessment in a generation. From April 2026, MTD for ITSA becomes mandatory for sole traders and landlords with total income above £50,000. From April 2027, it extends to those with income above £30,000. From April 2028, to those above £20,000.
What MTD ITSA Means for You
Instead of filing one annual return, you will need to send quarterly updates to HMRC using MTD-compatible software. You also file an end-of-year statement. The quarterly updates are summaries of your income and expenses. HMRC uses them to calculate your estimated tax liability in real time.
Software such as Xero, QuickBooks, FreeAgent, and Sage are already MTD-compatible. If you use spreadsheets, you will need bridging software to submit the data. HMRC has published a list of compatible software on GOV.UK.
Who Is Exempt
If you are digitally excluded, for example because of age, disability, or living in an area with poor broadband, you can apply for an exemption. Trusts, partnerships with corporate partners, and some other entities are also excluded for now. But the direction of travel is clear. Everyone will eventually be on MTD.
How to Prepare
If you are not already using digital bookkeeping, start now. Choose your software. Set up your chart of accounts. Begin recording your income and expenses digitally. The transition will be smoother if you are already in the habit. We cover this in more detail on our bookkeeping and compliance page.
Common Mistakes and How to Avoid Them
We see the same errors year after year. Here are the most common, and how to avoid them.
Missing the Registration Deadline
The 5 October deadline is absolute. If you miss it, you cannot file on time. The penalty is automatic. Register as soon as you start trading, even if you have not yet earned anything.
Forgetting to Declare All Income
HMRC receives data from banks, platforms, and employers. They know about your interest, dividends, and side income. If you omit it, you risk a compliance check and penalties. Declare everything, even if it seems small.
Claiming Personal Expenses as Business Expenses
The "wholly and exclusively" rule is strict. A suit for work is not allowable. A meal with a client is not allowable (client entertainment is disallowable). A laptop used 50% for personal use means you claim 50% of the cost. Keep records.
Not Keeping Adequate Records
HMRC can ask for receipts up to 5 years after the filing deadline. If you cannot produce them, your claim is disallowed. Use Dext or similar software to scan and store receipts. Keep a mileage log. Keep bank statements.
Filing Late
The 31 January deadline is not a suggestion. The £100 penalty applies even if you have no tax to pay. File early. You can file as soon as the tax year ends on 5 April. Do not wait until January.
Action Checklist for Filing Your Self Assessment Return
Use this checklist every year. Tick each item off as you complete it.
- Register for Self Assessment by 5 October following the end of the tax year (if not already registered).
- Gather all income records: sales invoices, bank statements, dividend vouchers, interest statements, P60, P45, P11D.
- Gather all expense records: receipts, invoices, mileage logs, bank statements, credit card statements.
- Choose your software. Use Xero, QuickBooks, FreeAgent, or Sage for digital bookkeeping. Use BrightPay for payroll if you have employees.
- Complete your SA100 core return and any supplementary pages (SA103, SA105, SA108, SA800).
- Calculate your tax and Class 4 NICs. Check your payments on account.
- File online by 31 January. File earlier if you can.
- Pay your tax by 31 January. Set up a direct debit or pay by bank transfer using your HMRC online account.
- If you owe more than £1,000, make your first payment on account by 31 January and your second by 31 July.
- Keep all records for at least 5 years after the 31 January deadline.
- If your income exceeds £50,000, prepare for MTD ITSA from April 2026. Choose MTD-compatible software now.
- Review your tax position with your accountant. Check if you are overpaying or missing reliefs.
When to Speak to a Qualified Accountant
This guide covers the core rules, but every business is different. If you have multiple income sources, complex expenses, capital gains, or overseas income, the rules multiply. A mistake on your Self Assessment return can cost you thousands in penalties and interest. It can also trigger an HMRC compliance check, which is time-consuming and stressful.
We are ICAEW qualified accountants based in the UK. We work with sole traders in the Northern Quarter of Manchester, partnerships in Leith in Edinburgh, and directors in Canary Wharf. If you need help with your Self Assessment return, contact us. We will review your position, file your return correctly, and help you plan for the future.
If you are considering incorporating your business, read our incorporation guide. If you are dealing with capital gains, see our exit and capital gains page. For everything else, our fundamentals section covers the basics of UK business tax.