What Is the SA100?
The SA100 is HMRC's core Self Assessment tax return form. It covers a full UK tax year (6 April to 5 April) and summarises your personal tax position – income from all sources, allowances, reliefs, and any tax already paid. It does not stand alone: depending on your circumstances, you'll attach supplementary pages that feed figures into the main return.
HMRC uses the completed SA100 – along with any supplementary pages – to calculate how much tax and National Insurance you owe, or whether you're due a refund.
The SA100 is the main tax return form used by millions of individuals across the UK to declare their income and claim allowances. While largely replaced by the online HMRC portal, understanding the structure of the SA100 is crucial for accurate filing.
Who needs to file one?
You'll generally need to file a Self Assessment return if, in the relevant tax year, you were:
- Self-employed (including sole traders and freelancers)
- A company director receiving income outside PAYE
- A landlord with rental income
- Earning over £100,000 in any source of income
- Liable for the High Income Child Benefit Charge (HICBC)
- Receiving significant savings interest, dividends, or foreign income
- Disposing of assets that may trigger Capital Gains Tax
Employees and pensioners whose tax is fully handled through PAYE generally do not need to file – unless one of the above applies to them.
Partnerships, trusts, and limited companies use separate returns (SA800, SA900, and CT600 respectively) and should not use the SA100.
Supplementary Pages: Which Ones Do You Need?
The SA100 rarely travels alone. The supplementary pages you attach depend entirely on where your income comes from.
| Supplementary Page | When to use it |
|---|---|
| SA102 | Employment income (P60/P45/P11D) |
| SA103S / SA103F | Self-employment – short or full version |
| SA104 | Share of partnership profits or losses |
| SA105 | UK property income and rental receipts |
| SA106 | Foreign income and double tax relief |
| SA107 | Income from trusts or estates |
| SA108 | Capital gains and losses |
| SA109 | Non-residence, split year, or remittance basis |
| SA101 | Additional information not covered elsewhere |
| SA110 | Tax Calculation Summary (complex cases) |
If you're a sole trader with straightforward accounts, you'll likely need just the SA103S alongside your SA100. A landlord-director with overseas investments, on the other hand, could be attaching five or six supplementary pages.
Key Deadlines and Penalties
Missing a Self Assessment deadline is costly – and the penalties escalate quickly.
Submission and payment deadlines
- 5 October – Register for Self Assessment if you're newly required to file (after the end of the tax year in question)
- 31 October – Deadline for paper SA100 submission
- 31 January – Deadline for online submission and balancing payment
- 31 July – Second payment on account (if applicable)
Payments on account apply when your tax bill exceeds £1,000 and less than 80% was collected at source. You'll make two equal payments – in January and July – each representing half your prior year's liability.
⚠️ Important: Missing the 31 January online deadline triggers an automatic £100 penalty, even if no tax is owed. After three months, HMRC adds £10 per day (up to 90 days). At six months and again at twelve months, further percentage-based penalties apply. Late payment also attracts 5% surcharges at 30 days, six months, and twelve months, on top of daily interest.
If you believe you had a genuine reason for missing a deadline – illness, bereavement, technical failure – you can submit a reasonable excuse appeal. HMRC will consider these on a case-by-case basis, but "I forgot" is unlikely to succeed.
You have 12 months from the 31 January filing deadline to amend a submitted return. HMRC may also issue corrections or, in some cases, raise a discovery assessment up to four years later (or 20 years for deliberate errors).
Registering for Self Assessment
If you need to file for the first time, you must register with HMRC by 5 October following the end of the tax year in which the obligation arose. So if you became self-employed in the 2024/25 tax year, you should register by 5 October 2025.
- CWF1 – for those registering as self-employed for the first time
- SA1 – for all other reasons (directors, landlords, high earners, etc.)
Once registered, HMRC will send your Unique Taxpayer Reference (UTR) – a 10-digit number – by post within about 10 working days. You'll then need to set up a Government Gateway account and, for online filing, activate your Self Assessment service using the activation code HMRC posts separately.
If you use an accountant, you can formally authorise them as your agent through HMRC's online system. Once authorised, they can file on your behalf, view your tax account, and correspond with HMRC directly.
How to File Your SA100
There are three ways to submit: HMRC's own online service, commercial tax software, or paper.
Online via HMRC is the most straightforward option for most people with relatively simple tax affairs. The system guides you through each section, performs basic checks, and gives you an instant submission receipt.
Commercial SA100 software is worth considering if your return is complex, you have multiple income streams, or you're preparing for Making Tax Digital for Income Tax (MTD ITSA). Providers recognised by HMRC offer more flexibility than the standard online service – including the ability to attach detailed computations and access previous years' data easily.
Paper filing is still available but must reach HMRC by 31 October. You'll need to print the correct-year SA100 PDF from GOV.UK, complete it in black ink using capital letters, attach your supplementary pages in order, and post it to HMRC's Self Assessment address. Keep a copy and consider using tracked postage.
Completing the SA100: What Goes Where
Whether you're filing online or on paper, the structure of the return is the same. Here's how the main sections work in practice.
Personal details and income
Start with your UTR, National Insurance number, name, address, and contact preferences. If you're due a refund, you'll enter your bank details here.
Employment income comes from your P60 (or P45 if you left during the year) and your P11D for benefits in kind. Self-employment figures – turnover, allowable expenses, and profit – go on the SA103. You can use HMRC's simplified expenses for mileage and use-of-home costs, or claim actuals with supporting records.
For property income, the SA105 covers rental receipts and associated costs. Mortgage interest is no longer fully deductible for residential landlords; instead, you claim a basic rate tax credit. The Rent-a-Room scheme and furnished holiday lettings each have their own rules and boxes.
Savings, dividends, and investment income
The personal savings allowance (£500 for higher-rate taxpayers, £1,000 for basic-rate) means many people won't owe tax on modest bank interest – but it still needs to be declared. The dividend allowance is £500 for 2024/25. Anything above these thresholds is taxable at the relevant rate, and the figures need to appear in the SA100 even where no additional tax is due.
Capital gains
If you've sold shares, property, crypto assets, or other chargeable assets during the year, you'll need the SA108. Note that residential property gains must also be reported to HMRC within 60 days of completion via a separate online return – this is in addition to, not instead of, the SA108.
Reliefs, pension contributions, and Gift Aid
Pension contributions made to relief-at-source schemes (most personal pensions) should be entered as the net amount – HMRC will gross them up. Those in net pay arrangements don't need to claim relief, as it's applied before the payroll deduction. Higher and additional rate taxpayers can reclaim the extra relief through their return.
Gift Aid donations can also increase the amount of income subject to the higher-rate threshold – effectively reducing your tax bill. You can even carry back donations made in the current year to the previous tax year, if you file before the return deadline for that prior year.
💡 Tip for directors and high earners: If your adjusted net income exceeds £100,000, your personal allowance begins to taper away at a rate of £1 for every £2 over the threshold. Making additional pension contributions or Gift Aid payments can bring adjusted net income back below the threshold and restore your full allowance – potentially saving up to £5,048 in tax.
HICBC, student loans, and Marriage Allowance
If you or your partner received Child Benefit and either of you earned above £60,000 in 2024/25 (the threshold was raised from £50,000 in April 2024), the High Income Child Benefit Charge applies. The charge is 1% of the benefit for every £200 of income above £60,000, with the full benefit clawed back at £80,000. It's one of the more frequently missed obligations on the SA100.
Student loan repayments are calculated automatically once you declare your income and select your plan type. Marriage Allowance – which allows a lower-earning spouse to transfer £1,260 of their personal allowance – can also be claimed here, and can be backdated up to four years.
Payments, Refunds, and Managing Your Bill
Your tax liability is calculated from the figures in your return. Income Tax is applied in bands (20%, 40%, 45%), and Class 4 National Insurance applies to self-employed profits above the lower profits limit.
If you can't pay on time, contact HMRC before the deadline to arrange a Time to Pay plan. These are typically available for debts under £30,000 and let you spread payments over up to 12 months. Ignoring a debt is always worse than engaging with HMRC early – penalties and interest continue to accumulate regardless.
Refunds can be paid directly to your bank account (enter details on the return) or offset against future liabilities. Small underpayments are sometimes coded out through PAYE the following year rather than paid directly.
Record-Keeping and Common Mistakes
HMRC can enquire into any return up to 12 months after the filing deadline, and longer in cases of suspected fraud or carelessness. Self-employed individuals must keep records for at least five years after the 31 January filing deadline; everyone else must keep them for at least 22 months after the end of the tax year.
The most common errors on SA100 returns include:
- Using the wrong UTR or National Insurance number
- Omitting bank interest (HMRC receives data from financial institutions and will notice)
- Confusing gross and net figures for pension contributions or Gift Aid
- Forgetting to declare the High Income Child Benefit Charge
- Failing to include payments on account in cash flow planning
Finally, be cautious about unsolicited contact. HMRC will never ask for your bank details by email or text, and it will never threaten immediate arrest for unpaid tax. If you receive a suspicious message claiming to be from HMRC, report it to [email protected].
Making Tax Digital: What's Coming
MTD for Income Tax Self Assessment (MTD ITSA) will require sole traders and landlords with qualifying income above £50,000 to keep digital records and submit quarterly updates to HMRC from April 2026. The threshold drops to £30,000 from April 2027. This doesn't replace the annual return but adds an ongoing reporting requirement throughout the year. If your business is approaching these thresholds, now is the time to review your bookkeeping software.
Frequently Asked Questions
Do I need to file an SA100 if all my income is taxed through PAYE? Generally no – but you will if your income exceeds £100,000, you have untaxed income from other sources, or you're liable for the High Income Child Benefit Charge. HMRC may also issue a notice to file even if you believe you don't need to; once a notice is issued, you must file or risk automatic penalties.
How do I report cryptoasset gains or income? Disposals go on the SA108 Capital Gains pages. If you've received crypto as income – through mining, staking, or as employment-related rewards – it's taxable as income and may need to appear on the SA100 main return or SA103, depending on the nature of the activity.
Can I amend my return after submitting it? Yes, within 12 months of the 31 January filing deadline for the relevant tax year. Amendments are made online or by writing to HMRC. If you've already received a refund based on the original return and the amendment reduces your entitlement, HMRC will collect the difference.
What if I need to file returns for previous years? You can file online for tax years going back several years, though you may need to contact HMRC to reactivate access for older years. Paper returns are also accepted for prior years using the correct-year PDF from GOV.UK.
Where to Get Help
HMRC's SA150 guidance notes – available as a PDF download alongside each year's SA100 – explain every box in detail. Specific helpsheets cover areas like foreign income, capital gains, and residence. The Self Assessment helpline (0300 200 3310) handles general queries, though wait times during peak season can be lengthy.
If your return involves significant capital gains, foreign income, share schemes, or overseas residency, professional advice is almost always worth the cost. Look for a qualified accountant registered with ICAEW, ACCA, CIOT, or ATT, and check they carry professional indemnity insurance. For straightforward returns, many online accountancy services offer fixed-fee SA100 filing.