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🇬🇧 United Kingdom

Self Assessment (SA100)

UK Tax Return for Self-Employed & High Earners

Medium ~30 min TaxSelf-EmployedHMRC

/ What is this form?

Self Assessment is how HMRC collects income tax that cannot be deducted at source through PAYE. You file a tax return each year for the previous tax year (April 6 to April 5), reporting all income, claiming expenses and reliefs, and paying any tax owed.

The deadline for online filing is January 31. Paper returns have an earlier deadline of October 31. A £100 automatic penalty applies for late filing, increasing with time.

Payments on Account — advance payments towards next year's tax — may also be required if your tax bill exceeds £1,000.

/ Who needs this form?

  • Self-employed sole traders with trading income
  • Landlords with rental income above £1,000
  • People earning over £100,000 (personal allowance tapering)
  • Company directors
  • Anyone with income that hasn't had tax deducted at source

/ What you need before you start

UTR (Unique Taxpayer Reference) — issued by HMRC when you register
National Insurance number
Income records: invoices, bank statements, P60
Receipts for allowable business expenses
Details of any other income: savings interest, dividends, rental

/ Step-by-step guide

1 Register for Self Assessment
If you're new to Self Assessment, register with HMRC by October 5 after the end of the tax year. You'll receive a UTR (Unique Taxpayer Reference).
2 Gather Your Records
Collect income records (invoices, bank statements), P60 if employed, interest certificates, rental income, and receipts for allowable expenses.
3 Complete the Return Online
Log in to HMRC's online service and complete the SA100. Add supplementary pages for self-employment (SA103), property (SA105), or foreign income (SA106).
4 Review Calculation
HMRC calculates your tax bill automatically. Review the amount and check for allowable deductions you may have missed.
5 File and Pay by January 31
Submit online by January 31. Pay any tax owed by the same date. Payments on Account (advance payments) may also be due.

/ Key fields explained

Field What to enter Common mistake
Total income All income from all sources: self-employment, employment, rental, savings interest, and dividends. Forgetting to include all income sources — HMRC cross-checks with banks, employers, and pension providers.

/ Common mistakes to avoid

Missing the January 31 online deadline — a £100 penalty is automatic, even for £0 tax owed.
Not registering by October 5 of the year after the tax year — prevents timely filing.
Not keeping records for at least 5 years after the filing deadline.
Not declaring all sources of income.

/ Frequently asked questions

Do I need to file if I'm employed?

Only if you have untaxed income over £1,000, earn over £100,000, have income from property, are a company director, or have received a notice to file from HMRC.