Self-Assessment is the system HMRC uses to collect Income Tax from people and businesses who have income that is not deducted automatically from wages and pensions through PAYE (Pay as You Earn). The SA100 is the main detailed document where you report all your income, claim reliefs, and help HMRC calculate what tax you owe or what they should repay you.
The SA100 doesn't work alone. Depending on your circumstances, you'll need supplementary pages, additional sections that collect detailed information about specific types of income like employment, self-employment, property, or foreign earnings. These pages feed into your main return, creating a complete picture of your finances for the tax year.
Are You on HMRC's Self-Assessment List?
You must send a Self-Assessment Tax Return if, during the last tax year (6 April to 5 April), any of the following mandatory criteria applied:
Mandatory Filing Criteria
- HMRC Request - You must send a return if HMRC specifically asks you to.
- Self-Employment - You were self-employed as a 'sole trader' and earned more than £1,000 before claiming any tax relief. If your total turnover from all your businesses was over £1,000, you must complete the 'Self-employment' pages.
- Partnerships - You were a partner in a business partnership.
- High Income Child Benefit Charge (HICBC) - You had to pay the HICBC and were not paying it through PAYE. This charge applies if your adjusted net income was over £60,000 and you or your partner received Child Benefit payments.
- Capital Gains Tax (CGT) - You had to pay CGT when you sold or disposed of an asset that had increased in value. This includes situations where you sold or disposed of chargeable assets worth more than £50,000, or your chargeable gains before taking off any losses were more than £3,000.
Circumstances Involving Untaxed Income
You may also need to send a tax return if you received untaxed income from specific sources:
- Property Income - Money from renting out a property, or if your total UK property income exceeds £1,000.
- Miscellaneous Earnings - Tips and commission.
- Investment Income - Income from savings, investments, and dividends.
- Foreign Income - Foreign income, including untaxed foreign interest over £2,000 or foreign dividends over £500.
Specific Taxpayer Categories Requiring Alternative Filing
Certain categories of taxpayers cannot use the official online service and must therefore file using commercial software or alternative forms. These categories include, but are not limited to:
- Individuals who lived abroad as non-residents
- Those dealing with returns for a trust or an estate
- Individuals who get income from a trust or are a Lloyd’s underwriter or are a religious minister
Voluntary Filing
Even if a person is not mandatorily required to file based on their income, they may choose to complete a return for specific administrative or benefit purposes:
- To claim certain Income Tax reliefs
- To prove you are self-employed (for example, to claim Maternity Allowance or Tax-Free Childcare)
- To pay voluntary National Insurance contributions
Understanding the SA Forms
What is the SA100 & What Information Does It Collect
The SA100 is structured to capture your complete tax position for a single tax year, which runs from 6 April to the following 5 April. The Self-Assessment form opens with personal details like your name, address, date of birth, and National Insurance number (NINO). These identifiers ensure HMRC processes your return correctly and gives you the right allowances.
The core sections collect different types of income. You'll report interest from banks and building societies, dividends from UK companies, pensions and state benefits, and any other UK income that doesn't fit on supplementary pages. Each section asks for gross amounts (before tax) and any tax already deducted, allowing HMRC to calculate what you still owe or what they should refund.
Tax reliefs occupy their own section. Here you claim relief for pension contributions, charitable giving through Gift Aid, and Blind Person's Allowance if applicable. The form also handles Marriage Allowance, a mechanism that lets you transfer part of your personal allowance to your spouse or civil partner if your income is low enough.
Toward the end, you'll find sections for Student Loan repayments, the High-Income Child Benefit Charge (if your income exceeds £60,000 and you or your partner claim Child Benefit), and instructions for how you want HMRC to handle any refund. The form links to supplementary pages through simple checkboxes on page 2, where you indicate which additional sections you're submitting.
Which Supplementary Pages Do You Need for Your Income
Employment Pages (SA102)
These are required if you were employed full-time, part-time or casually, if you're a company director receiving income, or if you held an office like company secretary and were paid for it. Agency workers who work for one person through another company also use these pages. You'll need a separate employment page for each job, directorship or office you held during the year.
Self-Employment Pages (SA103S) & (SA103F)
Self-employment pages come in two versions. The short pages (SA103S) are for straightforward businesses with a turnover under £90,000. The full pages (SA103F) handle more complex situations, turnover of £90,000 or more, or cases where you need to adjust your profits. You'll need separate pages for each business if you run more than one. Note that if your self-employment receipts total £1,000 or less, the Trading Income Allowance exempts them from tax entirely, so you don't need to report them unless you want to voluntarily pay Class 2 National Insurance, claim loss relief, or recover tax deducted under the Construction Industry Scheme.
UK Property Pages (SA105)
These are needed when you receive rental income from UK property, income from furnished holiday lettings in the UK or European Economic Area, or income from letting furnished rooms in your home (unless you provided substantial services like meals, which makes it a trade reportable on self-employment pages). Like self-employment, there's a Property Income Allowance of £1,000; if your total property income doesn't exceed this amount, it's exempt unless it comes from a connected party or you choose to report it anyway.
Foreign Pages (SA106)
This captures income arising outside the UK. This includes foreign interest over £2,000, foreign dividends over £500, overseas pensions and social security benefits, foreign property income over £1,000, and income from offshore trusts or funds. If your only foreign income is interest up to £2,000 and dividends up to £500, you can report these directly on page 3 of the SA100 instead of completing the full foreign pages. The foreign section also lets you claim Foreign Tax Credit Relief or relief under double taxation agreements.
Capital Gains Pages (SA108)
These are required when you've sold or disposed of assets triggering a taxable gain. You must complete them if the total value of assets you disposed of exceeds £50,000, if your total gains before losses exceed £3,000, if you want to claim allowable capital losses, or if you're claiming the remittance basis for foreign gains. Recent disposals of UK property while non-resident, or incomplete Capital Gains Tax payments on Real Time Transaction returns, also require these pages.
The 'Trusts etc' Pages (SA107)
Specialist pages handle fewer common situations. These pages are for beneficiaries of trusts (excluding bare trusts) or settlors whose trust income is deemed to be theirs. The
'Residence, Remittance Basis etc' Pages (SA109)
This applies if you're not a UK resident, arrived in the UK during the tax year, want to claim split-year treatment, or have foreign income you're reporting on the remittance basis. The 'Additional information' pages cover interest from UK securities, life insurance policy gains, share schemes, and various specialist reliefs, including Married Couple's Allowance.
Getting Ready to Fill the Return
What Documents Do You Need Before You Start
Begin by gathering your personal information. You'll need your
- full name
- current address with postcode
- date of birth
- National Insurance number
- and Unique Taxpayer Reference (the ten-digit reference HMRC issued when you registered for Self-Assessment)
Note: Your National Insurance number appears on payslips, your P60, or letters from HMRC or the Department for Work and Pensions. It follows the format of two letters, six numbers, and a final letter (for example, AB123456C).
If you were employed, collect your P60, any P11D, and P45 if you changed jobs. Payslips are useful as backup.
For self-employment, have your profit and loss account or complete business records showing all income and expenses for the year.
For savings and investments, gather bank statements showing interest, dividend vouchers, and broker statements. ISA income is tax-free and not included on the return.
Landlords should prepare rental accounts showing rent received and allowable expenses. If you claim furnished holiday lettings relief, keep evidence that the property qualifies.
For pensions, keep certificates for any personal pension contributions and P60s for pension income.
Tips: Do not send these documents to HMRC unless requested. Keep them safely stored to support your return if HMRC asks for evidence later.
Completing Personal Information
What Personal Details Must You Provide on the SA100
Box 1 asks for your date of birth. This is essential because HMRC uses it to apply age-related allowances. Enter it in the format shown.
Box 2 covers your name and address. If anything is wrong or missing, correct it in the “Issue address” section. If you moved during the year, record the date in box 2. Your address determines whether Scottish, Welsh or rest-of-UK tax rates apply.
Box 4 asks for your National Insurance number if it is not already printed. This links your return to your tax record, so HMRC must have it.
How Do You Tell HMRC About a Change of Address or Other Updates
Record an address change and the date you moved directly in box 2, especially if you moved between tax jurisdictions.
Other changes, such as a new name after marriage or deed poll, should be entered in the “Any other information” box on page TR 7. Significant updates like starting or stopping self-employment or becoming a director will appear naturally in the relevant sections, but you can highlight them in the additional information box if needed.
Declaring Income
How Do You Report Employment Income on the Tax Return
Use the figures from your P60, which shows your total pay before deductions and the tax already taken under PAYE. Each job needs its own SA102 form. Enter your gross pay and tax deducted exactly as shown.
If you received taxable benefits, use your P11D (or your P60 if benefits were payrolled). Include all benefits such as company cars, medical insurance, or loans.
Employment expenses are claimable only if they are necessary for your job and not for personal use. Most go in the Additional Information pages. Typical examples include required professional subscriptions and unreimbursed business mileage.
How Do You Report Self-Employment Income
Self-employment reporting centres on your turnover and allowable expenses. Declare your turnover (total sales before expenses) and then your allowable expenses, which must be wholly for business. Common costs include materials, wages, rent, insurance, and apportioned motor expenses.
Keep full records of sales, purchases, bank statements, receipts, and mileage logs as HMRC may request them.
If your profits exceed £12,570, HMRC will automatically add Class 2 and Class 4 National Insurance to your calculation.
If your total self-employment and miscellaneous income is under £1,000, the Trading Allowance may cover it, but you can still report if you want to claim a loss or NI credits.
How Do You Report UK Property Income
Report all rent and property-related receipts before deductions. Allowable expenses include agent fees, insurance, repairs, utilities you pay, and service charges.
Mortgage interest is not deductible but attracts a 20% tax credit instead.
Qualifying furnished holiday lets benefit from special rules, including full interest relief.
The £1,000 Property Income Allowance applies if total property income is under £1,000, unless it’s from a connected party.
How Do You Report Foreign Income
Small amounts of foreign income can be entered directly on the SA100:
- Foreign interest up to £2,000
- Foreign dividends up to £500
Convert amounts to pounds using the rate on the day received.
Larger amounts, overseas rentals, and claims for Foreign Tax Credit Relief require the SA106 pages. You can offset foreign tax paid against your UK tax liability.
How Do You Report Savings & Investment Income
Most UK bank interest is paid gross, so enter it in box 2. ISA interest is ignored completely.
UK dividends go in box 4, include the full amount, even if reinvested. The first £500 is tax-free, but you still report all dividends as they affect your tax bands.
Property Income Distributions from REITs or PAIFs are not dividends and belong in box 17.
How Do You Report Pensions & Taxable Benefits
Enter your State Pension based on the amount you were entitled to receive, not the amount actually paid. State Pension lump sums go separately in box 9.
Other pensions, annuities, and taxable lump sums go in box 11, with tax deducted in box 12.
Certain overseas government pensions are only 90% taxable.
Taxable Incapacity Benefit, contribution-based ESA, and Jobseeker’s Allowance each have their own boxes.
Box 16 covers other taxable state benefits such as Bereavement Allowance and Carer’s Allowance.
Reporting Capital Gains
When Do You Need to Report Capital Gains
You must complete the SA108 capital gains pages if:
- The total proceeds from all assets you sold exceed £50,000, even if you made an overall loss. This threshold is based on the amount you received, not the gain
- Your chargeable gains before losses exceed £3,000 (the 2024–25 Annual Exempt Amount). You must report them even if your gains fall to zero after the allowance
- You want to claim a capital loss, either for this year or to carry one forward
- You are claiming or paying tax on the remittance basis, remitting foreign gains, have carried interest, or disposed of UK property while non-resident
Property sales have extra rules. If you filed a Real Time Transaction return but didn’t pay the full Capital Gains Tax, you must also complete SA108 to finalise the position. Non-resident disposals of UK land or property must also be reported here.
Chargeable assets include shares and securities (outside ISAs or certain share schemes), property that is not your main residence, business assets, and personal items worth over £6,000 (cars are exempt)
Provide your separate calculations showing purchase cost, sale proceeds, allowable costs, and how you arrived at the gain or loss. Only enter the final figures on the capital gains pages.
Claims, Allowances & Adjustments
What Reliefs & Allowances Can You Claim on the SA100
Pension Contributions
Report contributions to registered pension schemes operating ‘relief at source’ in box 1 as the gross amount (your payment plus basic rate relief). Higher or additional rate relief is calculated automatically by HMRC. Contributions not using relief at source, such as certain retirement annuity contracts or employer schemes where tax wasn’t deducted, go in boxes 2 and 3. You can claim relief up to 100% of your UK taxable earnings or £3,600 gross for relief at source schemes. Exceeding the Annual Allowance requires reporting on the Additional information pages.
Gift Aid
Enter actual payments in box 5. One-off donations in 2024-25 go in box 6 to adjust your 2025-26 tax code. Boxes 7 and 8 allow shifting payments between tax years. Gifts of qualifying shares or securities go in box 9, while gifts of land or buildings go in box 10.
Marriage & Blind Person’s Allowance
Marriage Allowance lets you transfer £1,260 of your personal allowance to a basic-rate spouse/civil partner, reducing their tax by up to £252. Blind Person’s Allowance adds £3,070 if registered as blind or severely sight-impaired. Surplus allowances from a spouse/civil partner can be claimed in boxes 15 or 16.
How Do You Report Student Loan or Postgraduate Loan Repayments
Tick box 1 if repayment began before 6 April 2025. Report PAYE deductions for Student Loans in box 2 and Postgraduate Loans in box 3. Use P60s from all employments; voluntary payments are excluded. HMRC calculates additional repayments on non-PAYE income automatically.
How Do You Deal With the High-Income Child Benefit Charge
Applies if adjusted net income exceeds £60,000 and you or your partner received Child Benefit. Enter total payments for 2024-25 in box 1, number of children in box 2, and stop date if applicable in box 3. HMRC calculates 1% of Child Benefit for every £200 over £60,000; full clawback occurs at £80,000+. You can choose to pay the charge via Self-Assessment instead of your 2025-26 tax code by ticking the box on page TR 6.
Checking & Submitting the Return
How Do You Check the Accuracy of Your Self-Assessment Form
- Ensure personal details are complete, including date of birth and National Insurance number
- Tick boxes on page 2 for every supplementary page you are submitting and attach those pages
- Verify totals from multiple sources (e.g., several employments, bank accounts, dividend payments)
- Confirm all figures relate to the 2024-25 tax year: check dates on all source documents
- Ensure tax deducted entries match supporting documents (P60, dividend vouchers, pension statements)
- Check supplementary page cross-references; ensure all indicated pages are actually attached
- Review expenses, especially for self-employment and property, avoiding personal or capital items
- Sign and date the declaration; an unsigned return is invalid
How & When Should You Submit a Paper Tax Return
The deadline for paper returns is 31 October 2025. Post to the HMRC address shown on your return using a service that provides proof of posting, ideally tracked. Early submission is recommended to avoid delays and allow time to correct issues before penalties apply.
When Do You Need to File Online Instead of Using Paper
The online deadline is 31 January 2026. You must file online if you miss the paper deadline or want to use HMRC PAYE code collection for amounts under £3,000 (if submitted by 30 December 2025). Online filing calculates tax immediately, validates entries, flags errors, and allows saving partial returns to finish later. HMRC prefers online filing for efficiency and accuracy.
Paying Your Tax
When Do You Need to Pay the Tax That HMRC Calculates
The deadline for your 2024-25 tax bill is 31 January 2026, whether filing online or on paper. Missing this date incurs interest and potential penalties. If you file a paper return by 31 October 2025, HMRC will calculate your tax on time. Later filings—paper after 31 October or online between November and January—require you to calculate the tax yourself or wait for HMRC’s calculation.
Payments can be made via online/telephone banking, CHAPS, debit or corporate credit card, or at your bank/building society. Personal credit cards are not accepted. Refunds from debit card payments go back to the same card. Class 2 National Insurance contributions are also due on 31 January 2026 and must be paid separately, as they cannot be collected through PAYE.
What Are Payments on Account & Who Must Make Them
Payments on account are advance payments for next year’s tax. If your 2024-25 Self-Assessment liability exceeds £1,000 and less than 80% was collected through PAYE, you must make two payments on account for 2025-26. Each equals half your 2024-25 tax bill (excluding capital gains and student loans), due 31 January 2026 (with your balancing payment) and 31 July 2026.
If you expect your 2025-26 tax to be lower, you can apply to reduce payments on account, but underpayment attracts interest. Budget carefully, as January may require paying both your balancing payment and the first advance payment.
Upcoming Change: Making Tax Digital (MTD) for Income Tax
If you work for yourself as a sole trader or earn income from property and your gross qualifying income as per the Self-Assessment Tax Return for the tax year 2024/25 exceeds £50,000, then tax reporting for you is changing. From April 2026, you’ll need to follow Making Tax Digital for Income Tax (MTD ITSA). This is the biggest change to Self-Assessment in over 30 years.
Instead of completing your tax return all at once, you will record your income and expenses digitally throughout the year using HMRC-recognised software. You will then send quarterly updates to HMRC, keeping your records up to date, and submit a final tax return by 31 January each year.
Key Points to Prepare for MTD ITSA
Staying prepared helps avoid last-minute stress and ensures your tax return is accurate. Explore our MTD resources and support to understand software options, reporting requirements, and deadlines.
Conclusion
Filing a Self-Assessment Tax Return can be complex, particularly if you have multiple income sources or claim various reliefs and allowances. Understanding which supplementary pages to use, keeping accurate records, and submitting your return on time are essential to avoid penalties and interest.
From April 2026, Making Tax Digital for Income Tax will change how self-employed individuals and property owners report their income. Keeping digital records throughout the year and submitting quarterly updates will become the norm, making preparation and ongoing record-keeping more important than ever.
Let's Start a Conversation
At UK Property Accountants, we provide dedicated Self-Assessment services, helping you navigate the complexities of tax reporting, meet deadlines, and stay fully compliant. Our team can guide you, ensuring your return is accurate and submitted on time.
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