Your first Self Assessment, without the panic

Nobody is born knowing what a UTR is. If this is your first Self Assessment (new contractor, new landlord, new side income, or newly self-employed), here's the whole process in plain English, in the order it actually happens.

Step 1: work out if you actually need to file

The common triggers: self-employment income over £1,000 · rental profit over £2,500 or rent over £10,000 (between £1,000 and £2,500 you usually just tell HMRC; see the landlords' guide) · dividends or savings interest over £10,000, or other untaxed income of £2,500 or more · a side business alongside your PAYE job · certain umbrella/agency situations (contractors' guide) · HMRC simply writing to tell you to file. When in doubt, check: filing unnecessarily wastes time, but missing a required return costs money.

Step 2: register, by 5 October after the tax year

You must register for Self Assessment by 5 October after the end of the tax year you need to file for (tax years run 6 April to 5 April). The same date applies if you registered before but didn't need to send a return for the previous tax year; you may need to reactivate your account instead. Registration gets you a UTR (a 10-digit Unique Taxpayer Reference), which is your identity for everything that follows. Register online and the UTR is in your personal tax account within 72 hours. Register by post and HMRC usually post the UTR in 15 working days (21 if you're abroad), and it may take longer at busy times. Without it, nothing can be filed, so don't leave registration until January. If you do register after 5 October, HMRC give you 3 months from their letter or email to file, but the tax is still due by 31 January.

Step 3: gather the records as you go

• Employment: your P60 (and P45 if you changed jobs), plus any P11D for benefits.
• Self-employment: invoices, business expenses, mileage, tidy monthly beats heroic in January.
• Property: rent received, agent statements, repair bills, mortgage interest statements.
• Everything else: dividend vouchers, bank interest summaries, pension contributions, Gift Aid.

Step 4: the deadlines that matter

Online filing and payment are both due 31 January following the end of the tax year. File even one day late and there's an automatic £100 penalty, even if you owe nothing or are due a refund. Pay late and interest starts immediately.

The first-timer ambush: payments on account

Mistakes first-timers actually make

• Registering in January instead of by 5 October, leaving no time to get the UTR and pay by 31 January.
• Forgetting PAYE income belongs on the return too (the return covers everything, not just the new income).
• Claiming improvements as repairs, or the full mortgage payment instead of the interest credit.
• Missing allowable deductions: pension contributions and Gift Aid extend your basic-rate band.
• Not keeping records, which HMRC can ask for years later.

This guide is general information, not tax advice, and reflects our understanding of the rules as at September 2026. Your circumstances may differ, so please get specific advice before acting.