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Self Assessment Tax Returns: A Complete Guide
Who needs to file Self Assessment, how to register, what goes on the SA100, deadlines, payments on account and how to avoid common mistakes.
Quick answers
Who needs to file a Self Assessment tax return?
You typically need to file Self Assessment if you're self-employed earning over £1,000, a company director with untaxed income, a partner in a partnership, a landlord with property income above the thresholds, or have significant untaxed savings, investment income or capital gains.
When is the Self Assessment deadline?
Online Self Assessment returns and any tax owed must be filed and paid by 31 January following the end of the tax year. Paper returns have an earlier deadline of 31 October. Late returns trigger an automatic £100 penalty.
What are payments on account?
Payments on account are advance instalments towards next year's Self Assessment tax bill. If last year's tax was over £1,000 and less than 80% was collected at source, you pay two instalments each equal to 50% of last year's bill — due 31 January and 31 July.
Self Assessment Tax Returns
Self Assessment is HMRC's system for collecting Income Tax from people whose tax isn't fully deducted at source through PAYE.
Who must file?
You generally need to file a Self Assessment return if you:
- Are self-employed with gross trading income over £1,000.
- Are a partner in a partnership.
- Are a company director with untaxed income.
- Have rental income over £1,000 (gross) or £2,500 (net).
- Have untaxed savings or investment income over £10,000.
- Have a Capital Gains Tax liability.
- Earn over £150,000 (the new threshold replacing the old £100k rule).
- Need to claim certain reliefs not given through PAYE.
How to register
Register online at gov.uk by 5 October following the end of the tax year you became liable. HMRC issues a Unique Taxpayer Reference (UTR) and an activation code for the online portal — together these can take 2–3 weeks.
Key deadlines
- 31 October — paper returns.
- 31 January — online returns and balancing payment.
- 31 January & 31 July — payments on account (each is 50% of last year's tax) if last year's bill was over £1,000 and less than 80% was collected at source.
What goes on the SA100
The main return is supplemented by pages depending on your circumstances:
- SA103 — self-employment.
- SA104 — partnership.
- SA105 — UK property.
- SA106 — foreign income.
- SA108 — capital gains.
Payments on account explained
If your bill exceeds £1,000, you make two equal payments on account towards next year's tax — one on 31 January, one on 31 July. They're each 50% of last year's tax bill. A balancing payment then settles any difference the following 31 January.
Common mistakes
- Forgetting the trading allowance (£1,000 tax-free for casual income).
- Missing pension contributions that increase your basic-rate band.
- Not declaring side income like crypto, eBay sales above the £1,000 threshold, or freelance work.
- Confusing gross and net rental income.
This is general guidance for the 2025/26 UK tax year and is not personal tax advice. Always check the latest figures on GOV.UK or speak to a qualified accountant for your situation.
Frequently asked questions
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