Every year, millions of people sit down with the self assessment tax return form, take one look at it, and put the kettle on instead. We get it. Boxes, references, supplementary pages; it can feel like HMRC designed it as a puzzle.
Here's the truth though. Filling your self assessment tax return form is a process, not a mystery. Once you know what goes where, and what documents to have next to you before you start, most returns are far more manageable than they look.
This guide walks you through the whole thing step by step, flags the mistakes we see most often as accountants, and tells you when it's worth handing the job over.
First Things First: What Is the Self Assessment Tax Return Form?
The main self assessment tax return form is called the SA100. It's how you report income to HMRC that hasn't already been taxed, so HMRC can work out what you owe (or what they owe you).
You'll usually need to fill one in if you're:
Self-employed as a sole trader and earned more than £1,000
A partner in a business partnership
A landlord with rental income
Earning untaxed income, such as dividends, savings interest over your allowance, or side income
A higher earner affected by things like the High Income Child Benefit Charge
Claiming certain tax reliefs or repayments
Some company directors need to file too, depending on how they take their income. If you run a limited company and you're not sure, that's a common question we answer, and our blog covers director pay in more detail.
The supplementary pages
The SA100 is the main form, but it rarely travels alone. Depending on your income, you may also need supplementary pages, such as:
SA102 for employment income
SA103 for self-employment
SA104 for partnership income
SA105 for UK property income
SA106 for foreign income
SA108 for capital gains
If you file online, the system adds the right pages automatically based on your answers. One less thing to worry about.
Before You Start: Register and Gather Your Documents
Register with HMRC (if it's your first time)
You can't just fill in the form and send it off. First, you need to register for Self Assessment with HMRC, and the deadline for that is 5 October after the end of the tax year you need to report.
Once registered, HMRC sends you a Unique Taxpayer Reference (UTR), a ten-digit number you'll need for everything. It can take a couple of weeks to arrive, so don't leave registration until January.
Get your paperwork together
Think of this like cooking. The recipe is easy when the ingredients are already chopped and on the counter. Trying to find your bank interest statements halfway through the form is where the swearing starts.
Have these to hand before you open the form:
Your UTR and National Insurance number
P60 or P45 if you were employed during the year
P11D if you received benefits in kind
Records of self-employment income and expenses
Bank and building society interest statements
Dividend vouchers
Rental income and expense records, if you're a landlord
Pension contribution and Gift Aid records
Details of any capital gains, such as selling shares or property
Good records all year round make this stage painless. If your paperwork is currently a carrier bag of receipts, our bookkeeping service exists for exactly this reason.
How to Fill In Your Self Assessment Tax Return Form: Step by Step
Step 1: Choose online or paper
Around 97% of people now file online, and it's easy to see why. The online return calculates as you go, flags missing sections, and gives you an extra three months compared with paper.
For the 2025/26 tax year (6 April 2025 to 5 April 2026):
Paper deadline: 31 October 2026
Online deadline: 31 January 2027
Tax payment deadline: 31 January 2027
To file online, log in through your Government Gateway account at GOV.UK.
Step 2: Complete your personal details
Name, address, date of birth, UTR, National Insurance number. Simple, but check it carefully. A wrong UTR or NI number can cause real delays.
Step 3: Tell HMRC about your income sources
The form asks a series of yes/no questions about where your money came from during the tax year: employment, self-employment, property, savings, dividends, pensions, foreign income and so on. Your answers determine which supplementary sections you'll complete.
Be thorough here. Even small amounts of taxable savings interest need to be declared if they're over your allowance. "I forgot about that account" is not a defence HMRC accepts.
Step 4: Enter your income figures
Work through each relevant section, entering income for the tax year. A few tips:
Use whole pounds. Round income down and expenses up (yes, HMRC lets you round in your favour).
Take figures from official documents, not memory. Your P60, not your best guess.
For self-employment, enter turnover and allowable expenses. Only claim expenses that were wholly and exclusively for the business.
Step 5: Claim your reliefs and allowances
This is the part people rush, and it's the part that saves money. Common ones include:
Pension contributions (higher-rate taxpayers often miss the extra relief)
Gift Aid donations
Marriage Allowance
The £1,000 trading allowance or property allowance, where relevant
Step 6: Check, submit, and keep proof
The online system runs a validation check before you submit. Read the summary carefully; once submitted, changes mean filing an amendment. When you're happy, submit and save your confirmation reference.
Then pay any tax due by 31 January. If you owe more than £1,000, watch out for payments on account, which are advance payments towards next year's bill. They catch a lot of first-time filers off guard.
Common Mistakes to Avoid
After years of reviewing returns, the same errors come up again and again:
Missing the registration deadline, then scrambling for a UTR in January
Forgetting small income sources, like savings interest or dividend income
Claiming personal costs as business expenses, which invites HMRC scrutiny
Ignoring payments on account, leading to a January bill twice the size expected
Leaving it until the last week of January, when HMRC's helplines are jammed and mistakes multiply
Miss the deadline and there's an automatic £100 penalty, even if you owe nothing. Leave it three months and daily penalties start stacking up, with interest charged on unpaid tax on top.
Heads Up: Making Tax Digital Is Changing Things
From April 2026, Making Tax Digital for Income Tax began rolling out. If you're self-employed or a landlord with qualifying income over £50,000, you now need to keep digital records and send quarterly updates to HMRC using compatible software. The threshold drops to £30,000 from April 2027 and £20,000 from April 2028.
If that's you, or will be soon, moving to cloud accounting now is far easier than scrambling later. It's something we help clients set up all the time, and it makes filling your self assessment tax return form dramatically easier, because the numbers are already there.
When to Hand It Over to an Accountant
Plenty of people with simple affairs file their own return, and that's fine. But it's usually worth professional help if you have:
Multiple income streams
Rental property
Capital gains
Foreign income
A limited company alongside personal income
Or simply better things to do with your January
At Force Accounting, personal tax returns are one of our core services. We work on fixed, transparent fees, so you know the cost before we start, and we're proactive about deadlines, so you'll never be the person panicking on 30 January. We're based in Staffordshire and work with clients across the UK through cloud accounting tools, always in plain English.
The Short Version
The SA100 is the main self assessment tax return form; supplementary pages cover specific income types.
Register with HMRC by 5 October, file paper returns by 31 October, and file online (and pay) by 31 January.
Gather your documents before you start, declare all income, and don't skip the reliefs section.
Watch for payments on account and the £100 late filing penalty.
Making Tax Digital is changing how many people report income, so get ahead of it.
Rather skip the form-filling altogether? Get in touch with Force Accounting and we'll handle your self assessment from start to finish, for a fixed fee agreed upfront.
FAQs
What is the SA100 self assessment tax return form?
The SA100 is HMRC's main tax return form for individuals. It reports your taxable income, gains, and reliefs for the tax year so HMRC can calculate the tax you owe or the refund you're due.
When is the deadline for filling in your self assessment tax return form?
For the 2025/26 tax year, paper returns must reach HMRC by 31 October 2026, while online returns are due by midnight on 31 January 2027, which is also the payment deadline.
Do I need an accountant to fill in my self assessment tax return?
No, straightforward returns can be filed yourself through GOV.UK. But if you have multiple income sources, property, capital gains, or a limited company, an accountant often saves more than they cost, in both tax and time.
What happens if I make a mistake on my self assessment tax return?
You can amend your return after submitting, normally up to 12 months after the filing deadline. Amendments are made through your Government Gateway account or by writing to HMRC.
Can HMRC collect the tax I owe through my tax code?
Yes, if you owe less than £3,000 and file online by 30 December (or on paper by 31 October), HMRC can usually collect it through your PAYE tax code across the following tax year, spreading the cost instead of one January payment.