How to Fill In a Self Assessment Tax Return: Reliefs and Deadlines

To fill in a Self Assessment tax return, you register with HMRC to get a Unique Taxpayer Reference, gather your income and expense records for the tax year running 6 April to 5 April, complete the main SA100 form together with any supplementary pages that match your income sources, claim the reliefs you’re entitled to, and submit online through the Government Gateway by 31 January.¹ Paper returns have to be with HMRC by the earlier date of 31 October. The steps below walk through what each stage actually looks like.

Step 1: Register and Get Your UTR

You can’t file until HMRC has issued you a Unique Taxpayer Reference. If you’ve never filed before, the deadline to register is 5 October following the end of the tax year you need to report.¹ Registration happens online on GOV.UK, and the process differs slightly depending on whether you’re registering as self-employed or for another reason such as rental income.² Have your full name, address, date of birth, National Insurance number, and phone number ready before you start, because the form can’t be saved partway through.

HMRC then posts your UTR, usually within around 15 days (longer if you live overseas).³ It’s typically 10 digits, occasionally 13.⁴ You’ll need it every time you file or contact HMRC. If you’ve filed before, your UTR is on previous tax correspondence or in your Personal Tax Account.

Step 2: Pull Together Your Records

The filing itself goes far more smoothly if the numbers are already in front of you. For the tax year you’re reporting on, gather:

Your P60, showing total pay and tax deducted if you were employed at the end of the tax year.⁵
Any P45 from a job you left during the year.⁵
A P11D if your employer gave you taxable benefits like a company car or private medical insurance.⁶
Bank and building society statements covering untaxed savings interest.
Dividend vouchers for shares and investments.
Self-employment records: invoices, receipts, and a summary of income and expenses.
Rental income records: tenant payments, letting agent statements, and receipts for allowable expenses.

It also helps to know where you fall against the Personal Allowance, currently £12,570. If your income exceeds £100,000, the allowance shrinks by £1 for every £2 above the threshold and disappears entirely at £125,140.⁷

Step 3: Work Through the Main Return (SA100)

The SA100 is the core form everyone completes. It captures personal details, employment income, savings interest, dividends, and pension income. Filing online through the Government Gateway, the system walks you through each section and only shows fields relevant to your situation. Paper filers can download the SA100 from GOV.UK or request a copy by phone.⁸

Work through it methodically. Enter figures exactly as they appear on your P60, P45, and bank statements. The online system does basic calculations but won’t catch a figure typed into the wrong box. Where you have employment income alongside self-employment or rental income, the SA100 ties everything together while the details of each income type sit on separate supplementary pages.

Step 4: Add the Supplementary Pages That Apply

If your tax affairs go beyond straightforward employment income, you’ll add one or more supplementary forms alongside the SA100. Online, the system prompts you to add the right ones based on your answers.

Self-Employment (SA103)

Sole traders report business income and expenses on SA103. There are two versions: the short form (SA103S) for simpler businesses and the full form (SA103F) for those with turnover above the VAT threshold.⁹ Enter your total turnover, then deduct allowable expenses such as office costs, work travel, professional insurance, and stock. The taxable profit flows back into the SA100.

UK Property Income (SA105)

Rental income from residential or commercial property goes on SA105.¹⁰ List total rental receipts and deduct allowable expenses like repairs, letting agent fees, and insurance. One catch trips up many landlords: mortgage interest is no longer deductible as an expense for residential lettings. Instead you get a 20% tax credit on finance costs, which works out worse for higher-rate taxpayers. The resulting profit or loss carries over to the main return.

Capital Gains (SA108)

Sold shares, a second home, or another asset at a profit? The gain goes on SA108.¹¹ The form tracks what you paid, what you sold it for, and allowable costs such as legal fees. The annual tax-free allowance is currently £3,000 (£1,500 for trusts), so you only pay tax on gains above that.¹²

Foreign Income (SA106)

SA106 captures income earned abroad, such as foreign dividends or overseas rental income. You may be able to claim Foreign Tax Credit Relief to avoid being taxed twice on the same income. The dividend allowance is £500. For overseas property, there’s a £1,000 property income allowance, but total property income above £1,000 means completing the full foreign pages.¹³

Other Supplementary Pages

Additional forms cover employment income (SA102), partnership income (SA104), and non-UK residence (SA109).⁸ Only complete what applies. Make sure the totals on your supplementary pages match what flows into the SA100 summary.

Step 5: Claim the Reliefs You’re Owed

Self Assessment isn’t only about reporting income. It’s also where you claim back money that’s rightfully yours, and people leave hundreds of pounds on the table each year by skipping this part.

Pension Contributions

If you pay into a private pension through a “relief at source” scheme, your provider automatically claims basic-rate (20%) tax relief. Higher-rate and additional-rate taxpayers have to claim the extra relief themselves through the return. A 40% taxpayer can claim back an additional 20% on contributions; a 45% taxpayer can claim 25%.¹⁴ Scottish taxpayers have different rates reflecting Scotland’s income tax bands. If your pension scheme doesn’t claim basic-rate relief automatically, you claim the full amount here.

Gift Aid Donations

When you donate through Gift Aid, the charity claims 25% on top of your donation. Higher-rate taxpayers can claim back the difference between the higher rate and basic rate through the return. Donate £100 with Gift Aid making it £125 to the charity, and a 40% taxpayer personally reclaims £25.¹⁵

Marriage Allowance

If you’re married or in a civil partnership and one of you earns less than the Personal Allowance, that person can transfer £1,260 of their unused allowance to the other, reducing the recipient’s bill by up to £252 a year.¹⁶ You can backdate the claim by up to four years if you were eligible but didn’t apply.

Step 6: Handle the Things That Catch People Out

The High Income Child Benefit Charge

This is one of the most common reasons people get pulled into Self Assessment unexpectedly. If you or your partner receive Child Benefit and either has an adjusted net income over £60,000, the higher earner has to file a return and pay back some or all of the benefit.¹⁷

For 2024–25 onwards, you pay back 1% of your Child Benefit for every £200 your income exceeds £60,000, and once income reaches £80,000 you repay all of it.¹⁷ Even if you decide to stop receiving payments to avoid the charge, still register for Child Benefit (and opt out of payments) because that protects National Insurance credits for the stay-at-home parent.

Student Loan Repayments

If you have a student loan, HMRC calculates your repayment based on total income for the year. You need to know your plan, because the threshold and rate differ. On Plans 1, 2, 4, and 5 you repay 9% of income above your plan’s threshold. On a Postgraduate Loan, it’s 6%. If you’re both employed and self-employed, HMRC looks at combined income, which can mean owing more than your employer already deducted through PAYE.¹⁸ The SA100 has a section where you tick which plan applies; the Student Loans Company can confirm your plan if you’re unsure.

Step 7: Submit and Save the Confirmation

Most people file online through the Government Gateway, which needs a verified user ID and password. The system runs basic error checks and calculates the tax bill automatically. After a final review screen, submit and you’ll receive a confirmation reference number immediately. Keep that reference — it’s your proof you filed on time.

Paper returns should go by tracked or registered post. HMRC won’t treat the return as filed until it’s physically received, and illegible or incomplete forms can be rejected, pushing you past the deadline.¹

One practical tip: don’t wait until January. You can submit any time after the tax year ends on 5 April. Filing early doesn’t mean paying early — the payment deadline stays the same — but it gives you months to fix any problems and know exactly what you owe.

Step 8: Pay the Bill

Once your return is processed, HMRC generates a calculation showing what you owe, including income tax, Class 2 and Class 4 National Insurance for the self-employed, and any student loan repayments. The payment deadline is 31 January after the end of the tax year.¹

Payments on Account

If your tax bill was £1,000 or more last year (after tax taken at source), HMRC requires advance payments toward next year’s bill, called payments on account. Each payment is half of what you owed the previous year. The first is due on 31 January alongside the balance of your previous year’s bill, and the second on 31 July.¹⁹ This catches many first-time filers off guard. Your first January payment can effectively be 150% of a normal year, because you’re paying what you owe plus the first advance payment.

If income drops or tax relief increases, you can apply to reduce your payments on account to avoid overpaying.²⁰ Be careful though: if you reduce too much and underpay, HMRC charges interest on the shortfall.

Deadlines and What Late Costs

The online filing deadline is 31 January after the tax year ends. Paper returns must arrive by the previous 31 October. Miss either date and there’s an automatic £100 penalty even if you owe no tax, with further penalties escalating at 3, 6, and 12 months late.²¹

Late payment carries separate charges: a 5% surcharge on unpaid tax at 30 days, another 5% at 6 months, and a further 5% at 12 months, on top of daily interest that as of early 2026 sits at 7.75%.²² Filing on time even if you can’t pay is always the better move: it avoids the filing penalties while you sort out the money.

Fixing Mistakes After You File

Spotted an error after submitting? You can correct it within 12 months of the filing deadline. For a 2024–25 return, that means making changes by 31 January 2027.²³ Online amendments go through the Government Gateway. Miss the 12-month window, or need to fix an earlier year, and you write to HMRC directly. Correcting genuine mistakes promptly doesn’t usually attract penalties. Leaving errors for HMRC to discover during a compliance check is what causes problems.

How Long to Keep Your Records

If you’re self-employed, keep financial records for at least five years after the 31 January submission deadline for the tax year concerned.²⁴ Filed your 2024–25 return by 31 January 2026? Hold onto those records until at least the end of January 2031.

If you file for another reason such as rental income or the Child Benefit charge, keep records for at least 22 months after the end of the tax year the return covers.²⁵ Either way, keep bank statements, invoices, receipts, and any documents used to fill in the return.

Making Tax Digital Changes This from April 2026

If you’re self-employed or a landlord with total annual income from self-employment and property above £50,000, the process itself changes from 6 April 2026. Making Tax Digital for Income Tax requires compatible software to keep digital records of income and expenses, plus quarterly updates to HMRC through the year instead of a single annual return.²⁶ A final return and payment are still due by 31 January. Below the £50,000 threshold, you continue filing through Self Assessment as before, though HMRC has signalled the threshold will eventually come down. Start looking into MTD-compatible software well before April 2026 if it applies to you.

1
GOV.UK. Self Assessment Tax Returns: Deadlines
2
GOV.UK. Register for Self Assessment if You Are Not Self-Employed
3
GOV.UK. Find Your UTR Number
4
HM Revenue & Customs. Unique Taxpayer Reference – HMRC Patterns for Services
5
GOV.UK. Your P45, P60 and P11D Form: Why You Get Each Form
6
GOV.UK. Your P45, P60 and P11D Form: P11D
7
GOV.UK. Income Tax Rates and Personal Allowances
8
GOV.UK. Self Assessment Tax Return Forms
9
GOV.UK. Self Assessment: Self-employment (Full) (SA103F)
10
GOV.UK. Self Assessment: UK Property (SA105)
11
GOV.UK. Self Assessment: Capital Gains Summary (SA108)
12
GOV.UK. Capital Gains Tax: What You Pay It On, Rates and Allowances
13
GOV.UK. Foreign Notes (2024-2025)
14
GOV.UK. Tax on Your Private Pension Contributions: Tax Relief
15
GOV.UK. Tax Relief When You Donate to a Charity: Gift Aid
16
GOV.UK. Marriage Allowance: How It Works
17
GOV.UK. High Income Child Benefit Charge: Overview
18
GOV.UK. Repaying Your Student Loan: How Much You Repay
19
GOV.UK. Understand Your Self Assessment Tax Bill: Payments on Account
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GOV.UK. Claim to Reduce Payments on Account
21
GOV.UK. Self Assessment Tax Returns: Penalties
22
GOV.UK. HMRC Interest Rates for Late and Early Payments
23
GOV.UK. Self Assessment Tax Returns: If You Need to Change Your Return
24
GOV.UK. Business Records if You’re Self-Employed: How Long to Keep Your Records
25
GOV.UK. Keeping Your Pay and Tax Records: How Long to Keep Your Records
26
GOV.UK. Making Tax Digital for Income Tax for Sole Traders and Landlords