How to Fill Out and Submit HMRC Form SA100: UTR, Filing, and Payment

HMRC Form SA100 is the main UK Self Assessment tax return, and you complete it to report income that was not already taxed at source through PAYE. The form covers the tax year from 6 April to 5 April, and you file it online by 31 January after the tax year ends, or on paper by 31 October.¹ The SA100 itself is a summary; the detail for each type of income sits on supplementary pages you attach to it.²

Do You Actually Need to File

Before touching the form, check that HMRC expects one from you. You need to send a Self Assessment return if any of these applied in the tax year:³

You were a sole trader with more than £1,000 in gross self-employment income before expenses or allowances.⁴
You had £2,500 or more of untaxed income from rentals, tips, commissions, or investment dividends.
Your total taxable income was £150,000 or more, even if PAYE already deducted tax.
You or your partner claimed Child Benefit and the higher earner had an adjusted net income over £60,000.⁵
You sold assets such as a second property or shares and need to report capital gains.
You had foreign income to declare.
You were a company director with income not fully taxed through payroll.

HMRC can also send you a notice to file when none of these apply. If they do, you still have to submit a return.

Register First to Get Your UTR

You cannot fill in an SA100 without a ten-digit Unique Taxpayer Reference. If you have never filed before, register on GOV.UK by 5 October following the end of the tax year the filing obligation arose in. Miss that date and you may face a late-notification penalty.⁶

Registration tells HMRC why you need to file — self-employment, rental income, or another reason — and they post your UTR to you. To file online you also need a Government Gateway account, which comes with a separate activation code sent by post. Each letter takes around 10 working days to arrive (longer if you live abroad), so start early. When you pay, your reference is your UTR followed by the letter “K.”⁷

Gather Your Documents

Having your paperwork ready before you open the form makes the process much faster and cuts down on errors. Depending on your income, you might need:

Your P60 from your employer for the tax year, or a P45 if you left a job during the year.⁸
Your P11D, which lists taxable benefits such as a company car or private medical cover.
Bank and building society interest statements, plus dividend vouchers for any shares.
Self-employment records: invoices, receipts, and a summary of income and expenses.
Rental accounts showing total rent received and allowable expenses.
Pension contribution statements, especially for personal pensions where you want to claim tax relief.
Gift Aid receipts for charitable donations.
Capital gains records showing purchase and sale details for anything you disposed of.

Your UTR and National Insurance number go at the top of the form, so keep those together with the rest of your records.²

Work Through the Main Return

The SA100 opens with your personal details: name, address, UTR, National Insurance number, and date of birth. From there, the form works through categories of income and deductions in numbered boxes. Employment income, self-employment profits, rental income, and foreign earnings each have their own section or their own supplementary page. You enter the gross figure and any tax already deducted where the form asks. Taxable state benefits, student loan repayments, and pension income all have dedicated boxes on the main return.

The tax reliefs section is where you record pension contributions, Gift Aid donations, blind person’s allowance, and married couple’s allowance. Watch the Gift Aid boxes carefully: if you made a one-off large donation and want to carry back part or all of it to the previous tax year, there is a specific box for that election.

On paper, you can either calculate the tax yourself using the SA110 working sheet or leave the calculation to HMRC. To have HMRC do it, your paper return must arrive by 31 October; miss that and you have to work out the figure yourself. Online, the system calculates automatically once the sections are complete.

Pick the Right Supplementary Pages

Attach only the supplementary pages that match your circumstances. A salaried employee with no other income might only need the SA102, while a self-employed landlord with overseas investments could need three or four sets.⁹ The most commonly used are:

SA102 (Employment): income from a job or directorship.
SA103S or SA103F (Self-Employment Short or Full): sole trader income. Use the short version if your turnover is below the VAT registration threshold; otherwise the full version.
SA104S or SA104F (Partnership Short or Full): your share of partnership income.
SA105 (UK Property): rental income from UK land or property.
SA106 (Foreign): income or gains from outside the UK.
SA108 (Capital Gains Summary): gains or losses from selling assets. The annual exempt amount for individuals in 2025–26 is £3,000.¹⁰

Less common pages include SA101 (additional information for unusual income or relief), SA107 (trust income), SA109 (residence and remittance basis for non-UK residents), and SA110 (tax calculation summary). Online, the system only shows the sections that match the income types you tick, so you do not have to hunt down paper forms.⁹

Submit Online or on Paper

Filing Online

Most people file through HMRC’s online service using their Government Gateway login. The system walks you through each section, lets you save progress, and calculates your bill in real time. After submission you get an on-screen confirmation and a receipt number, both worth saving.¹

You can also file through commercial software rather than the HMRC portal. HMRC publishes a list of approved products, including FreeAgent, Xero Tax, TaxCalc, and GoSimpleTax among many others. Some are free for basic returns; others charge a fee or are aimed at accountants filing for clients.¹¹

The online deadline is 11:59 pm on 31 January following the end of the tax year. For 2025–26, that is 31 January 2027.¹

Filing on Paper

Download the SA100 and any supplementary pages from GOV.UK, or phone HMRC to have them posted.¹² Send the completed return to:

Self Assessment
HM Revenue and Customs
BX9 1AS
United Kingdom

If you live outside the UK, send it to:

HM Revenue and Customs
Benton Park View
Newcastle Upon Tyne
NE98 1ZZ
United Kingdom⁹

Paper returns must reach HMRC by 11:59 pm on 31 October, three months earlier than the online deadline. For 2025–26, that is 31 October 2026.¹

Pay What You Owe

Filing and paying are separate obligations with separate penalties. The tax you owe for the year is due by 31 January, the same date as the online filing deadline. For 2025–26, the balancing payment is due by 31 January 2027.¹

If your previous year’s Self Assessment bill was £1,000 or more, and less than 80% of your total tax was collected at source, HMRC requires two advance payments called payments on account. Each one is half the previous year’s bill. The first is due on 31 January (alongside the balancing payment for the prior year); the second is due on 31 July.¹³

If your income has dropped, you can ask HMRC to reduce your payments on account, but if you cut them too far and underpay, HMRC will charge interest on the shortfall. A Budget Payment Plan lets you spread costs through weekly or monthly Direct Debits toward your next bill, taking some of the sting out of the January lump sum.⁷

Fixing Mistakes After You File

You can correct a filed return within 12 months of the Self Assessment deadline. For a return due 31 January 2027, that means until 31 January 2028. Online filers amend through their Government Gateway account; paper filers send a corrected return by post.¹⁴

Miss that 12-month window, or need to fix an earlier year, and you have to write to HMRC directly. You can claim overpayment relief for up to four years after the end of the tax year the return relates to.¹⁴

What Late Filing Costs

Miss the deadline and penalties start immediately, even if you owe no tax. Schedule 55 of the Finance Act 2009 sets the structure:¹⁵

Day one: an automatic £100 fixed penalty.
After three months: £10 per day for up to 90 days, adding up to £900.
After six months: a further penalty of 5% of the tax due or £300, whichever is greater.
After twelve months: another 5% of the tax due or £300, whichever is greater. If HMRC decides you deliberately withheld information, this can rise to 70% (or 100% if deliberate and concealed).

Late payment carries separate charges: 5% of the unpaid tax at 30 days, another 5% at six months, another 5% at twelve months, and interest on the outstanding balance throughout.¹⁶ Filing on time with wrong figures carries its own inaccuracy penalties, scaled by how careless or deliberate HMRC judges the error to be.¹⁷ If you receive a penalty and believe you had a reasonable excuse, such as a serious illness, a bereavement close to the deadline, or an HMRC service outage, you can appeal.¹⁸

Keep Your Records

If you are not self-employed, hold on to your records for at least 22 months after the end of the tax year the return covers. Records for a 2025–26 return filed on time should sit in your files until at least the end of January 2028.¹⁹

Self-employed people and partners have a longer obligation: at least five years from the 31 January filing deadline. Records for a 2025–26 return filed by 31 January 2027 should be kept until at least 31 January 2032. Late filing or an HMRC compliance check extends that clock. If HMRC checks your records and they fall short, the penalty for a first offence is typically £500 (£250 in the first year of trading), and deliberately destroying records can result in a fine of up to £3,000.²⁰

1
GOV.UK. Self Assessment Tax Returns: Deadlines
2
HM Revenue and Customs. SA100 2026 Tax Return
3
GOV.UK. Self Assessment Tax Returns: Who Must Send a Tax Return
4
GOV.UK. Register as a Sole Trader
5
GOV.UK. Child Benefit Tax Calculator
6
GOV.UK. Check How to Register for Self Assessment
7
GOV.UK. Pay Your Self Assessment Tax Bill: Pay Weekly or Monthly
8
GOV.UK. Your P45, P60 and P11D Form
9
GOV.UK. Complete Your Self Assessment Tax Return
10
GOV.UK. Capital Gains Tax Rates and Allowances
11
GOV.UK. Commercial Software Suppliers for Self Assessment
12
GOV.UK. Self Assessment Tax Return Forms
13
GOV.UK. Understand Your Self Assessment Tax Bill: Payments on Account
14
GOV.UK. Self Assessment Tax Returns: If You Need to Change Your Return
15
Legislation.gov.uk. Finance Act 2009 – Schedule 55
16
GOV.UK. Self Assessment Tax Returns: Penalties
17
GOV.UK. Penalties: An Overview for Agents and Advisers
18
GOV.UK. Disagree With a Tax Decision or Penalty: Reasonable Excuses
19
GOV.UK. Keeping Your Pay and Tax Records: How Long to Keep Your Records
20
GOV.UK. Record Keeping Checks on Your Business