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 income over £1,000 (landlords' guide) · untaxed income like dividends or significant savings interest · 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, before 5 October
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). Registration gets you a UTR (a 10-digit Unique Taxpayer Reference) which arrives by post and is your identity for everything that follows. Without it, nothing can be filed, so don't leave registration until 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
This is the one that hurts. If your first bill is over £1,000 (and less than 80% of your tax was collected at source), HMRC also wants two advance payments towards next year: half on 31 January with your bill, half on 31 July. Your first January can effectively cost 150% of the tax you expected. Budget for it from day one and it's fine; discover it on 30 January and it isn't.
Mistakes first-timers actually make
- Registering in January instead of October, so the UTR doesn't arrive in time.
- 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.
Want it done right the first time?
Fixed fees, published up front. Send us your details and we prepare, check and file. You approve before anything goes to HMRC.
See the Self Assessment serviceThis guide is general information, not tax advice, and reflects our understanding of the rules as at June 2026. Your circumstances may differ, so please get specific advice before acting.