How to Do a Self-Assessment Tax Return Without an Accountant
Every January, millions of Britons face the same ritual: the Self Assessment tax return. And every year, thousands pay accountants £150–£300 for what is, for straightforward affairs, a genuinely DIY-able job. If you’re an employee with some freelance income, a landlord with one property, or a crypto investor with modest gains, you can absolutely do your own Self Assessment — HMRC’s online system is far better than its reputation, and this guide walks you through the entire process, step by step, with the pitfalls marked.
First: do you actually need to file?
Not everyone does. You must file if: you’re self-employed with income over £1,000, you have untaxed income over £2,500 (rental, freelance, investments), your crypto gains exceed the £3,000 annual exempt amount, you’re a company director, you earn over £150,000, or HMRC has sent you a notice to file. You don’t need to file if you’re purely employed under PAYE with no extra income — your tax is handled automatically.
The £1,000 trading allowance is the key threshold most people miss: freelance/side-hustle income under £1,000 a year doesn’t even need declaring. Over £1,000, you register and file. Check HMRC’s “do I need to file” tool if you’re unsure — it takes two minutes and gives a definitive answer. And note the deadlines: register by 5 October following the tax year, file online by 31 January, pay by 31 January. Miss the filing deadline and it’s an automatic £100 fine, escalating monthly — the most expensive procrastination in British finance.
Getting set up: registration and records
If you’ve never filed, register for Self Assessment on gov.uk — you’ll receive a Unique Taxpayer Reference (UTR) by post within ~10 working days, then set up a Government Gateway account. Do this by early October at the latest; leaving it to January is the classic disaster.
Then gather your records before touching the form: P60 (from your employer, showing pay and tax), P11D (benefits), bank interest statements, dividend vouchers, rental income and expense records, freelance invoices and expense receipts, and crypto transaction histories. The golden rule: HMRC trusts your figures but can ask for evidence going back six years — so keep everything, organised by tax year. Our guide to freelancer tax details exactly which records the self-employed need, and registering as self-employed covers the setup if that’s your situation.
Filling in the return: section by section
HMRC’s online return only shows you the sections relevant to your answers — it’s less intimidating than it looks. Employment: copy figures straight from your P60 (pay, tax deducted, benefits). Ten minutes. Self-employment: total income, total allowable expenses, resulting profit — the system calculates the tax. Allowable expenses are the key battleground: home office costs, travel, equipment, professional fees, and a proportion of phone/broadband. Claim everything legitimate — gov.uk’s allowable expenses guide lists what’s claimable.
Property income: rental received minus allowable costs (letting agent fees, repairs — not improvements, insurance, mortgage interest subject to the Section 24 restrictions). Capital gains: crypto, shares, property sales — proceeds minus costs, minus the £3,000 annual exempt amount. Other income: savings interest above your personal savings allowance, dividends above the £500 allowance. Work through each section methodically; the system’s running calculation shows your bill building in real time, which demystifies the whole thing.
The parts people get wrong
Payments on account: the biggest shock for first-time filers. If your tax bill exceeds £1,000, HMRC demands next year’s estimated tax in advance — two payments on account (January and July), each 50% of the current bill. Your first January payment can therefore be 150% of what you expected. Budget for it — it’s the #1 cause of Self Assessment panic. Forgetting the Class 2/4 National Insurance: the self-employed pay NICs through Self Assessment too, calculated automatically but often surprising.
Missing allowable expenses: most DIY filers under-claim, not over-claim — the simplified home-office flat rate (£6/week), mileage at 45p/mile, and a reasonable proportion of household bills are routinely missed. Ignoring pension tax relief: higher-rate taxpayers must claim the extra 20% relief through Self Assessment — basic-rate relief is automatic, but the higher-rate top-up isn’t. On a £10,000 pension contribution, that’s £2,000 of relief DIY filers leave behind. Filing late: the £100 fine is automatic, plus interest on unpaid tax — file even if you can’t pay in full (contact HMRC about Time to Pay arrangements).
When you SHOULD get an accountant
DIY has limits — get professional help if: your affairs span multiple income types with complexity (rental + freelance + crypto + foreign income), you’re near the £100,000 threshold where the personal allowance tapers (an accountant’s pension planning can save thousands), you have capital gains with tricky calculations (property, share schemes), HMRC has opened an enquiry, or you simply value your time more than the £150–£300 fee. A good accountant doesn’t just file — they spot reliefs and planning opportunities that cover their fee several times over.
The MoneyHelper tax guides help you judge your own complexity honestly. And remember: even with an accountant, you’re legally responsible for the return’s accuracy — keep your own records regardless. Our comparison of accountant vs DIY helps you decide which camp you’re in.
After you file: what happens next
Once submitted, HMRC processes your return and updates your tax account — usually within days for straightforward filings. If you owe tax, pay by 31 January to avoid interest (currently charged daily on late payments). If you’ve overpaid — common when you’ve had both PAYE employment and freelance income — HMRC refunds you automatically or offsets it against future liabilities. Keep an eye on your Government Gateway account for the updated statement, and note your next year’s deadlines immediately: payments on account fall due the following 31 July, and the cycle begins again. Many DIY filers find year two dramatically easier — the registration is done, the records system exists, and the form holds your previous answers. The first return is the hard one; after that, it’s maintenance.
Conclusion
Doing your own Self Assessment comes down to: confirm you need to file, register by 5 October, gather records by tax year, work through HMRC’s tailored online sections methodically, claim every legitimate expense and relief, budget for payments on account, and file by 31 January. For straightforward affairs — employment plus modest freelance, rental or investment income — it’s a few focused hours, not a professional qualification. The system is designed for ordinary people; the fear is worse than the form. File it yourself, keep the £200, and spend it on something more enjoyable than an accountant’s invoice.
Frequently Asked Questions
How long does a Self Assessment take to complete?
For straightforward affairs with records ready: 1–3 hours. First-timers should budget half a day including gathering documents. Complex affairs (multiple income sources, capital gains) take longer — which is when an accountant earns their fee.
What happens if I file late?
An automatic £100 penalty, plus £10/day after 3 months (up to £900), plus 5% of tax due at 6 and 12 months, plus interest. File on time even if you can’t pay — penalties for late filing are separate from and harsher than late payment.
Can I correct my return after filing?
Yes — you have 12 months from the filing deadline to amend your return online. Found a missed expense in March? Amend it. HMRC expects corrections; it’s not an admission of wrongdoing.
Do I need to file if I only earned £800 freelancing?
No — the £1,000 trading allowance means income under £1,000 doesn’t need declaring or filing. But keep basic records anyway, in case HMRC ever asks.
What are payments on account?
Advance payments toward next year’s tax bill, demanded when your bill exceeds £1,000 — two instalments (January and July) of 50% each. They surprise first-time filers; budget for 150% of your bill in your first January.