Find the money you can keep
UK tax return builder
A free step-by-step guide to your Self Assessment, then a builder that fills in the real boxes with your own figures.
What the builder does
It asks you plain questions, puts your answers in the right boxes on the real HMRC pages, works out the tax and National Insurance, and shows the reliefs you have not claimed. It starts completely empty, so no figure is ever invented. Save it privately when you are signed in, or print it for your accountant.
Seven steps, in order
Step 1
Check whether you have to file at all
You need a return if you were self-employed and turned over more than 1,000, rented property out, had untaxed income, made capital gains above the allowance, earned over 150,000, or claimed child benefit on high earnings. If none of that applies, you may not need one.
HMRC: check if you need to send a returnStep 2
Get your UTR and Government Gateway login
Register first if this is your first return. The Unique Taxpayer Reference arrives by post and can take up to 10 working days, so start well before January. Register by 5 October after the tax year ends.
HMRC: register for Self AssessmentStep 3
Gather your figures before you open anything
P60 and P45, P11D for benefits, your business income and costs, rent received and expenses, bank and dividend statements, pension contributions, Gift Aid donations, and any student loan deductions.
Step 4
Fill in only the pages that apply to you
SA102 for employment, SA103S for self-employment, SA105 for property, SA108 for capital gains, SA109 if you are not resident, and SA101 for savings, reliefs and pension charges. The builder asks plain questions and fills the boxes for you.
Step 5
Claim every relief you are owed
Pension contributions, working from home, professional subscriptions, mileage, marriage allowance, Gift Aid and trading losses. This is where most refunds come from, and most people miss at least one.
Step 6
Check the calculation before you send it
The builder shows an SA302 style calculation: income, allowances, tax at each band, National Insurance and the payments on account HMRC will ask for next. Read it once and make sure it matches what you expected.
Step 7
File and pay by 31 January
Online filing and payment for the previous tax year are both due by 31 January. Paper returns are due 31 October. Late filing is 100 pounds straight away, then daily penalties, plus interest on late tax.
HMRC: Self Assessment deadlinesReady when you are
Most people finish the questions in under 15 minutes. You can stop and come back, and a saved return can only ever be read by the account that filled it in.