How to Complete and Submit Your Self Assessment Return Online in 2026 (Part 6)
Step-by-step walkthrough of completing the online self assessment SA100 form in 2026: every section, which supplementary pages you need, and how to check before submitting.
What completing the return actually involves
The HMRC online self assessment system is a stepped questionnaire that adapts to your circumstances. You do not see 60 pages of form at once. Instead, you work through a sequence of screens, answering questions about which income types apply to you. The system then presents only the supplementary pages you need.
The complete process takes most people between 30 minutes and two hours, depending on the complexity of their affairs. Having all the documents ready before you start dramatically reduces both the time and the chance of errors.
This is Part 6 of the UK Self Assessment From Scratch series. Parts 1โ5 covered whether to file, how to register, records to keep, key deadlines, and how your tax is calculated. Here we walk through the submission process screen by screen.
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Attempting to complete your return without the right documents leads to interruptions, estimates, and mistakes. Clear the desk and collect the following before logging in:
| Document | Why you need it |
|---|---|
| UTR (Unique Taxpayer Reference) | 10-digit number to access your return โ printed on all HMRC letters |
| Government Gateway user ID and password | Your login credentials for HMRC Online Services |
| P60 from each employer | Total pay and tax deducted for the full tax year |
| P11D (if applicable) | Benefits in kind โ company car, medical insurance, interest-free loans |
| Self-employment income and expense records | Total turnover, total allowable expenses, capital allowances claimed |
| Bank statements or interest certificates | Total interest received from savings accounts (banks report gross figures) |
| Dividend statements or broker summaries | Total UK and overseas dividends received |
| Rental income and expense records | Rent received, allowable property expenses, mortgage interest paid |
| Share or property disposal records | Date of sale, proceeds, original cost, any costs of acquisition/disposal |
| Overseas income documents | Foreign pension letters, overseas bank interest, foreign employment |
| Gift Aid donation receipts | Total charitable donations under Gift Aid in the year |
| Pension contribution records | Personal pension contributions not made via salary sacrifice |
If you are self-employed, your accounts โ whether prepared by you on a spreadsheet or by an accountant โ should show turnover, expenses broken down by category, net profit, and any capital allowances claimed.
Logging in
Go to gov.uk/log-in-file-self-assessment-tax-return and sign in with your Government Gateway user ID and password. If you have HMRC's Authenticator app or use another two-factor authentication method, you will need that too.
Once logged in, navigate to Self Assessment from the main menu. Your tax return for 2025/26 (the year ending 5 April 2026) will be available from 6 April 2026. Select "Start your 2025 to 2026 tax return" (HMRC labels tax years by their start year in the online system).
Section by section: completing the SA100
1. Tailor Your Return
The first substantive section asks you to confirm which types of income and claims apply to you. This step is critical โ every box you tick adds a supplementary page to your return. Every box you leave blank removes one.
Tick the categories that apply in 2025/26:
- Employment โ if you were employed by one or more employers and received a P60
- Self-employment โ if you ran a business as a sole trader at any point in the year
- UK property โ if you received rental income from UK property
- Foreign โ if you received any income from outside the UK
- Trusts etc. โ if you are a beneficiary of a trust or estate
- Capital gains โ if you disposed of assets during the year
- Residence, remittance basis etc. โ if you have a non-UK domicile or split-year residence claim
- Additional information โ for pension contributions, charitable giving, and other reliefs
If you tick Self-Employment, the system asks whether you have a single self-employment business or multiple. Tick accordingly โ each separate business (different trade descriptions or activities) gets its own SA103 page.
Do not rush Tailor Your Return. Missing a tick means a whole category of income goes unreported; ticking too many adds unnecessary pages and prompts.
2. Employment (SA102)
If you were employed at any point during 2025/26, you will complete at least one SA102 page โ one per employer.
What to enter:
- Pay from this employment: total pay received in the year from your P60 โ the figure in the "Total pay" box, not the monthly salary. This includes all wages, bonuses, commission and tips paid via payroll.
- Tax taken off pay: total income tax deducted by your employer, from your P60.
- PAYE reference: your employer's PAYE reference, printed on your P60 and payslips. Format is typically three digits / five alphanumeric characters.
- Benefits (P11D): if you received benefits in kind (company car, private medical insurance, employer-provided accommodation, or an interest-free loan over ยฃ10,000), the taxable values appear on your P11D. Enter each benefit in the relevant box.
Important: Enter the figures from your P60 exactly. Do not adjust or estimate. Banks, employers and government bodies all report to HMRC directly โ discrepancies between what your employer reported and what you enter are automatically flagged.
If you left employment mid-year, use the P45 your employer gave you โ it shows the same cumulative figures.
3. Self-Employment (SA103S or SA103F)
Most self-employed sole traders use the SA103S (short) version, which covers the vast majority of simple trading situations. The full SA103F is required if your annual turnover exceeded ยฃ85,000 (the VAT registration threshold) or if you have complex adjustments.
Key fields on SA103S:
- Description of business: a brief label (e.g. "Freelance graphic designer" or "Market trader โ clothing"). This is not used for tax calculations but helps identify the return.
- Accounting period dates: usually 6 April 2025 to 5 April 2026. If you use a different accounting year-end, Part 4 of this series explains the basis period rules.
- Total turnover from this business: your gross receipts for the year โ all invoiced amounts or cash received, before any deductions. If you are VAT registered, exclude VAT from this figure.
- Allowable business expenses: enter either:
- The total of all allowable expenses (travel, equipment, subscriptions, professional fees, home working, etc.), or
- Simply the ยฃ1,000 Trading Allowance as a flat deduction if your total expenses are below ยฃ1,000 and your turnover is also below ยฃ1,000 (though in this case you may not need to file at all โ see Part 1)
- Capital allowances: deductions for business equipment you purchased. The Annual Investment Allowance allows 100% deduction for qualifying plant and machinery up to ยฃ1,000,000. Items that do not qualify for AIA use the Writing Down Allowance (18% main rate or 6% special rate).
- Net profit: turnover minus expenses minus capital allowances. This figure feeds directly into the Tax Calculation page.
If you use cash basis accounting (permitted for businesses with receipts below ยฃ300,000), you record income when received and expenses when paid, rather than on an invoice/accruals basis. Most small sole traders find cash basis simpler.
4. UK Property (SA105)
If you let residential or commercial property in the UK, complete SA105.
Key fields:
- Total rents and other income from property: all rent received during the year. For residential lettings, include service charges you collect and pass on.
- Allowable expenses: repairs and maintenance, letting agent fees, buildings insurance, ground rent, utility costs you bear, travel to inspect or manage the property, accountancy fees attributable to the rental business.
- Finance costs (formerly mortgage interest): since April 2017, you can no longer deduct mortgage interest directly as an expense. Instead, enter the total mortgage interest and finance charges in the "Finance costs" box. The system converts this to a 20% tax credit which reduces your final tax bill โ but does not reduce your rental profit. Higher-rate taxpayers receive only 20% relief on mortgage interest, not 40%.
- Rent a Room relief: if you let a furnished room within your own main residence and your receipts are below ยฃ7,500, you can claim Rent a Room relief to make the income tax-free. Tick the relevant box. You cannot claim both Rent a Room relief and allowable expenses for the same property โ choose the more beneficial option.
- Furnished Holiday Lettings: note that FHL status was abolished from April 2025. Any property that previously qualified as an FHL is now treated as ordinary residential lettings. The separate FHL boxes no longer appear in the 2025/26 return.
5. Dividends (SA101 โ Additional Information)
Dividend income is entered in the Additional Information section (SA101), not a separate supplementary page.
- UK company dividends: total dividends received from UK companies. Find these on dividend statements, broker annual summaries (Hargreaves Lansdown, AJ Bell, etc.), or company dividend vouchers. Enter the total before the ยฃ500 Dividend Allowance is applied โ the system deducts the allowance automatically.
- Foreign dividends: declared on SA106 (Foreign) if you ticked that category in Tailor Your Return. Enter the sterling equivalent using the exchange rate at the date of payment or HMRC's published average rate.
- Tax credits on dividends: the system calculates dividend tax automatically based on the rates (8.75% basic, 33.75% higher, 39.35% additional) โ you do not need to calculate it yourself.
6. Savings interest (SA101)
- Total UK savings interest: the sum of all interest received from savings accounts, current accounts, fixed-term bonds, peer-to-peer lending and similar products. Banks report gross interest to HMRC, so enter the gross figure. Do not subtract the Personal Savings Allowance โ the system does this.
- ISA interest: never included. Interest earned inside a Cash ISA, Stocks & Shares ISA or any other type of ISA is always tax-free and not declared.
- NS&I Premium Bond prizes: not taxable, not declared.
Most online banking platforms show a "tax year interest" summary โ often available under account statements or the "tax" section of the app, especially in April to June following the year end.
7. Capital Gains (SA108)
If you disposed of investments, property, cryptocurrency or other assets during 2025/26 and your total gains exceeded the ยฃ3,000 Annual Exempt Amount, complete the Capital Gains section.
For each disposal, enter:
- Date of disposal
- Description of asset (e.g. "FTSE 100 tracker fund โ HL" or "2 Acacia Avenue, Bristol")
- Total proceeds (what you received)
- Allowable costs (original purchase price, plus costs of acquisition such as stamp duty and legal fees, plus costs of disposal such as estate agent fees for property)
- Net gain or loss
The system totals all gains and losses, deducts the ยฃ3,000 AEA, and calculates CGT. For residential property gains, there is a distinction between basic rate (18%) and higher rate (24%) depending on the taxpayer's income band.
Special fields for residential property: if you sold a UK residential property and already reported and paid CGT within 60 days of completion via HMRC's standalone CGT service, you will enter those figures here to avoid double-counting. The 60-day report references are matched automatically.
Shares and funds: if you bought the same shares or fund units on multiple occasions, share pooling rules apply โ your cost basis is the average cost of the pool, not the specific lot you sold. Most brokers provide a CGT report that calculates this for you.
8. Tax Calculation page
After you have completed all relevant income sections, the system generates a Tax Calculation. This page does not require you to enter anything โ it is generated from what you have already entered.
Review carefully:
- Personal Allowance used (should be ยฃ12,570 unless your income exceeds ยฃ100,000)
- Total income from all sources (check it matches your records)
- Income tax by band โ does the 40% band start at the right point?
- Class 4 NI calculation
- Student loan repayment, if applicable (Plan 1, 2, 4 or 5 โ each has different thresholds)
- Total tax and NI payable
- Less tax already paid (PAYE from employment, tax credits on savings interest)
- Net amount due or overpaid
If the net amount due surprises you significantly, go back through each section and check the figures. Common reasons for a larger-than-expected bill: entering gross interest when only the PSA excess should have been entered (the system handles the PSA automatically โ enter the full gross figure and let it do the maths); accidentally entering monthly rather than annual figures; missing a deductible expense.
9. Additional Information (pensions, Gift Aid, marriage allowance)
Before submitting, check this section for reliefs you may be entitled to:
- Pension contributions: if you made personal pension contributions to a SIPP or other personal pension (not salary sacrifice), enter the gross contribution here to claim higher-rate relief. Basic-rate relief is added automatically by the pension provider; this section claims back the additional 20% or 25% for higher and additional rate taxpayers.
- Charitable donations (Gift Aid): total Gift Aid donations in the year. The charity claims the basic-rate relief; this section allows higher-rate taxpayers to reclaim the difference. Also allows you to carry back Gift Aid donations to the previous tax year if beneficial.
- Marriage Allowance: if your income is below the Personal Allowance and your spouse or civil partner is a basic-rate taxpayer, you can transfer ยฃ1,260 of unused allowance, saving up to ยฃ252 in tax.
Submitting and getting your receipt
Once you have completed all sections and reviewed the Tax Calculation, the final screen presents a summary. Read it, then click Submit. HMRC will display a confirmation screen containing:
- A unique transaction reference number
- Date and time of submission
- The calculated amount owed (or refund due)
Save or print this page immediately. It is your evidence of on-time filing. HMRC will also send a confirmation email to the address registered on your Government Gateway account โ check it arrives and keep it. If the email does not arrive within an hour, check your account's "Messages" inbox within HMRC Online Services for a "Return received" notification.
Paper returns: why they are not recommended
You can still file a paper SA100 return, but the deadline is 31 October โ three months earlier than the online deadline. HMRC processes paper returns more slowly, and complex supplementary pages (especially capital gains and foreign income) are easier to complete online where the system handles calculations automatically.
Paper returns are appropriate if you have no internet access or have a genuine need to file on paper (for example, certain complex trust or non-residence situations require paper supplementary pages). For most filers, online is faster, more accurate, and gives you more time.
Using an accountant or tax agent
If an accountant or tax adviser files on your behalf, they access your return through HMRC's Agent Services Account rather than your personal login. To authorise them, either:
- Complete form 64-8 (available on gov.uk) and post it to HMRC, or
- Authorise them online via HMRC Online Services โ "Authorise a tax agent"
Once authorised, your agent can view, complete, and submit your return without you needing to log in. You will still receive copies of all correspondence at your own address. Authorisation does not transfer liability โ you remain responsible for the accuracy of the return, even if your agent completes it.
Common entry errors to avoid
| Error | What happens | How to avoid |
|---|---|---|
| Entering monthly rather than annual figures | Tax calculated on a fraction of actual income | Always use annual totals โ check P60, not a payslip |
| Using net interest instead of gross | Under-reporting savings income | Banks report gross โ enter gross, let system apply PSA |
| Forgetting a P60 from a short-term employer | Employment income understated | Check all employment in the year, including those under a month |
| Claiming mortgage interest as an expense | System rejects or applies incorrect calculation | Use the "Finance costs" box โ not the expenses box |
| Entering disposal proceeds but not acquisition costs | Gain overstated โ too much CGT | Always enter both sides of each disposal |
| Not ticking Gift Aid | Higher-rate relief lost | Review all charitable donations in the year |
What comes next
Once you have submitted, Part 7 covers what happens next: how refunds are processed, how to amend a submitted return, and what to do if HMRC writes to you.
Read Part 7: After You File โ Refunds, Amendments and HMRC Queries โ
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Can you change a self assessment return after you have submitted it?
Yes. You have a 12-month window from the 31 January filing deadline to amend a submitted return. For the 2025/26 return, filed by 31 January 2027, you can amend online via HMRC Self Assessment until 31 January 2028. Log in to HMRC Online Services, go to Self Assessment, select 'Amend return' and make your changes. HMRC recalculates the tax due automatically. For returns older than 12 months past the deadline you must contact HMRC directly, usually in writing.
How long does HMRC take to process a self assessment return?
For returns submitted online, HMRC aims to process and update your account within 10 working days. You will see a 'calculation received' status in your HMRC Online Services account. If you are owed a refund and have your bank details registered in your Personal Tax Account, BACS payment typically arrives within 5 working days of processing. Cheque refunds take up to 5 weeks. Paper returns take considerably longer โ up to 10 weeks in peak periods.
What if you do not have all your figures ready by the January deadline?
File with your best estimates rather than missing the deadline. You can amend the return within 12 months of the deadline once you have the correct figures. The ยฃ100 late filing penalty applies from 31 January regardless of whether you have everything perfectly accurate. An estimated but filed return on time is always better than a perfectly accurate return filed late. Note on HMRC's system: the online return does not permit you to save partially completed entries without finalising โ you must complete all mandatory fields for each supplementary page you have triggered.
Should I use an accountant or file self assessment myself?
For straightforward returns โ one employment source, small rental income or basic self-employment โ many people file confidently themselves using HMRC's online system. HMRC also provides a free webchat service. Where the return involves multiple income streams, capital gains disposals, foreign income, or tax planning around pensions and allowances, an accountant's fee (typically ยฃ300โยฃ800 for a sole trader's return) generally pays for itself through legitimate savings and correct reporting. If your circumstances changed significantly during the year, professional advice for at least the first return is worth considering.
What if the HMRC online system is down near the 31 January deadline?
HMRC's online service typically experiences heavy load in the final days before the 31 January deadline. If the system is genuinely unavailable due to HMRC's own technical failure (not your internet or device), this can constitute a 'reasonable excuse' for late filing. However, HMRC applies this narrowly. The safest approach is to file before 28 January to avoid the risk entirely. HMRC posts service status updates at gov.uk/government/organisations/hm-revenue-customs/contact โ check this if you encounter problems.
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Related reading
UK Self Assessment From Scratch โ Part 1: Do You Even Need to File?
Most UK workers never need to do a Self Assessment. But about 12 million do. Here's the precise list of trigger conditions for 2024/25 and 2025/26 โ and how to register if it turns out you do.
Self Assessment: Do You Actually Need to File? The Complete UK Checklist (Part 1)
The complete checklist for whether you need to file a self assessment tax return in the UK: employment income, rental, freelance, savings interest, CGT, dividends and more.
UK Self Assessment From Scratch โ Part 2: UTR and Government Gateway Setup
Step-by-step guide to registering for Self Assessment, getting your UTR (Unique Taxpayer Reference) number, setting up your HMRC Government Gateway account and what to do if things go wrong.