How to Do Self Assessment: A Complete Guide for UK Taxpayers

Alan Bermingham

10 Years of Expertise in Fintech Innovation

3 min read

What you need to know...

Self assessment is how millions of UK taxpayers report their income and pay tax to HMRC each year. If you're self-employed, a landlord, or earn income outside of PAYE, you'll almost certainly need to file a return. This guide walks you through the entire process, from working out whether you need to file, right through to paying your bill.

Do I need to complete a self assessment tax return?

Not everyone needs to file. HMRC collects tax automatically through PAYE for most employees, but self assessment covers everyone whose tax situation is more complex.

You need to file if you are self-employed and earned more than £1,000 in profit, earned over £100,000 in the last tax year, received rental income above £2,500, or have untaxed income from dividends, savings interest, or foreign earnings. You'll also need to file if HMRC has written to you asking you to do so, or if you want to claim tax reliefs such as pension contributions or Gift Aid.

If you're unsure, it's always better to check with HMRC than risk a penalty for not registering. Our guide on at what income do I have to start paying tax or file a return covers the thresholds in more detail. You might also want to read what is self assessment if you're starting from scratch, or do PAYE employees need to complete a self assessment tax return if you're employed but think you may still need to file.

Close-up of a person reviewing tax documents and entering information on a laptop.

Step 1: Register for self assessment

Before you can file, you need to register with HMRC. If this is your first time, you'll need to create a Government Gateway account and register as self-employed or for self assessment.

Our guide on how to register for self assessment with a Government Gateway login walks you through the process step by step, and if you're new to working for yourself, how to register as self-employed with HMRC covers everything you need to know about getting set up.

The deadline to register is 5th October after the end of the tax year in which you started receiving untaxed income. If you miss this, you could face a penalty.

Once registered, you'll be issued a UTR number, which you'll need to file your return. If you're not sure what this is or how to find it, our guide on UTR numbers explains everything.

Step 2: Gather your records

Good record-keeping makes filing significantly easier. Before you start your return, pull together your P60 or P45 if you're also employed, your self-employment income and expenses records, rental income statements, bank and savings interest certificates, dividend vouchers, pension contribution records, and receipts for any allowable expenses you plan to claim.

HMRC can request evidence going back up to five years after the filing deadline, so it's important to keep records for at least that long, alongside ensuring you handle household claims correctly by notifying tax credits about children staying in education if your family circumstances change.

Our guide on how long should I keep records for self assessment explains exactly what you need to hold onto and for how long. If you've lost or misplaced some records, maximising tax claims without receipts may help.

Person reviewing receipts and tax records on a laptop while preparing a Self Assessment tax return.

Step 3: Understand your allowable expenses

If you're self-employed, you can reduce your tax bill by claiming allowable business expenses. These must be costs incurred wholly and exclusively for business purposes.

Common examples include office supplies and equipment, business travel and mileage, professional fees and subscriptions, a proportion of home office costs if you work from home, and marketing and advertising costs. You cannot claim personal costs, even if you occasionally use them for work, and registering for indirect taxes also requires understanding the current VAT rate in the UK and what most people get wrong when calculating threshold turnover limits.

If you're unsure what crosses the line, what is a non-deductible expense explains the distinction clearly, whilst those managing low earnings due to health challenges can also explore qualifying for limited capability for work benefits with mental health conditions for additional financial backing.

Step 4: Fill in your tax return

Once your records are in order, you can complete your return online via your Government Gateway account, or use self assessment software to guide you through each section.

Your return will ask you to report employment income, self-employment income and expenses, property income, savings and investment income, and pension contributions, which can be previewed beforehand by using tax scenario simulation tools to test different tax outcomes fast and easily before submitting your data.

For a detailed walkthrough of the form itself, read our guide on how to fill in a self assessment tax return, which breaks down every section of understanding the standard SA100 tax return form required for reporting annual profits.

Using software makes this process considerably faster. Pie connects directly to your bank accounts, categorises your income and expenses automatically, and calculates your tax in real time, so there are no surprises when you come to file. Find out how Pie works and why people choose Pie over traditional methods.

Man reviewing self assessment tax documents at home

Step 5: Know your deadlines

Missing a deadline is costly. The key dates to save are 5th October to register if you are filing for the first time, 31st October for paper returns, 31st January for online returns, and 31st July for payments on account, while landlords must also consider reviewing a complete UK guide on lump sum rent payment tax when receiving advance tenancy payments.

Payments on account can catch people off guard the first time they file. Our guide on payment on account explains how they work and how to budget for them. If you think your income will be lower next year, you may be able to reduce them, read reducing HMRC payments on account for guidance.

Step 6: Pay your tax bill

Once you've filed, HMRC will calculate what you owe. You can pay by bank transfer, debit card, or direct debit, and once your tax calculations are finalized by HMRC, you can proceed with learning how to obtain an SA302 form for your mortgage application to prove your self-employed earnings to lenders.

Our guide on how to pay your self assessment tax bill explains all the available methods and what to do if you're struggling to pay on time.

If you're finding it difficult to pay the full amount, HMRC offers a Time to Pay arrangement that lets you spread payments over time. It's always better to contact HMRC proactively than to miss a payment without explanation.

Common mistakes to avoid with self assessment

What happens if I file late or make a mistake?

Late filing carries an automatic £100 penalty, even if you owe no tax. The fines increase the longer you leave it, and HMRC charges interest on any unpaid tax after 31st January. You can read more about how penalties work in our guide on HMRC late filing penalties explained.

If you make a mistake on your return, you can amend it within 12 months of the original filing deadline.

Common self assessment mistakes to avoid

Many people forget to include all their income sources. Bank interest, side hustle income, and investment gains all count. Others claim expenses without receipts, not realising HMRC can ask for evidence up to six years later.

Filing at the last minute often leads to errors, and missing the registration deadline in your first year can trigger an immediate fine. It's also easy to underpay if you forget to account for payments on account, particularly for sole traders clarifying subcontractor tax rules and working definitions under the Construction Industry Scheme.

Make self assessment simpler with Pie

Self assessment doesn't have to be stressful. Pie is the UK's first personal tax app built for self-employed people, sole traders, and landlords. It connects to your bank, tracks your income and expenses throughout the year, and guides you through filing your return, all in one place.

Get started with Pie today.