Self Assessment is how HMRC collects tax that isn't taken automatically through PAYE. If you're self-employed, a landlord, a company director or have other untaxed income, this is how you report it — and it's far less daunting once you know the steps.
1. Check if you need to file
You usually need to file a return if, in the tax year, you were self-employed and earned over £1,000, received rental income, had untaxed income from savings, investments or abroad, earned over £150,000, or need to pay the High Income Child Benefit Charge. If you're unsure, it's worth checking — filing when you don't need to is better than missing one you did.
2. Register and get your UTR
If it's your first time, register with HMRC by 5 October following the end of the tax year. You'll be sent a Unique Taxpayer Reference (UTR) and set up a Government Gateway account — do this early, as the codes arrive by post.
3. Know your deadlines
- 5 October — register for Self Assessment (first time only).
- 31 October — deadline for paper returns.
- 31 January — deadline for online returns and to pay any tax you owe.
- 31 July — second payment on account, if you make them.
4. Gather what you need
Your UTR and National Insurance number, records of self-employment income and expenses, P60/P45 and P11D from employment, dividend and interest statements, rental income and costs, pension contributions and Gift Aid, and details of any other income or reliefs.
5. Enter your income and expenses
Complete the main return (SA100) plus the supplementary pages that apply to you — for example SA103 for self-employment, SA105 for UK property, or SA102 for employment. Claim every allowable expense and relief you're entitled to.
6. Check the calculation, then submit
HMRC (or your software) works out the tax due automatically. Review it line by line, confirm the declaration, and submit. You'll get an on-screen confirmation and a submission reference — keep it.
7. Pay what you owe
Pay your balancing payment by 31 January. If your bill is over £1,000, you'll usually also make payments on account — two advance instalments towards next year's tax, due 31 January and 31 July.
Avoid the penalties
Miss the online deadline and there's an automatic £100 penalty, with more added the longer you leave it, plus interest on unpaid tax. Filing a few weeks early removes the last-minute stress entirely.
File your return the easy way
Basetax walks you through each section, fills the right forms, and files directly with HMRC.
This guide is general information, not personal tax advice. Deadlines and rules can change — ask our team about your situation.