How to file your Self Assessment tax return online: 2026 guide

What is Self Assessment, and is it still how you file?

Self Assessment is the system HMRC uses to collect Income Tax and National Insurance from people whose tax is not fully deducted at source. You report a tax year's income and expenses on a return — the SA100 plus whichever supplementary pages apply to you — and pay what you owe by 31 January after the tax year ends. For 2025/26, that return is due by 31 January 2027.

But the system is now splitting in two. From 6 April 2026, sole traders and landlords with qualifying income above £50,000 moved to Making Tax Digital for Income Tax, which replaces the annual return with four quarterly updates and a year-end Final Declaration. Everyone else still files an SA100 exactly as before. This guide covers the whole process, and how to tell which side of that line you are on.

Who must file a Self Assessment return

There is no single income figure that puts you in Self Assessment. It is a list of triggers, and one is enough.

The one that has changed: high PAYE earners. The old rule that anyone earning over £100,000, later £150,000, through PAYE had to file no longer exists — HMRC removed the income threshold for people taxed solely through PAYE from the 2024/25 tax year. Income level alone does not trigger a return now, whatever the amount. A £200,000 salaried employee with no other trigger simply does not file. The personal allowance taper between £100,000 and £125,140 is handled through the tax code. What still brings a high earner in is everything else on the list: a rental flat, a share sale, the Child Benefit charge.

CIS subcontractors deserve a separate note. Contractors deduct 20% from your labour if you are registered for CIS, 30% if you are not, and pay it to HMRC on your behalf. That almost always exceeds your final liability once expenses come off, so subcontractors file to reclaim the difference. See our CIS deductions guide and the CIS calculator.

Self Assessment or MTD for Income Tax: which system you are in

This is the first thing to settle, because it changes everything downstream.

Qualifying income is gross income before expenses, added together across every trade and every property business you have. It is not profit. A designer turning over £34,000 who also collects £22,000 in rent has £56,000 of qualifying income and was mandated from April 2026, even though taxable profit is far lower. The £20,000 tier from April 2028 is confirmed policy, not a proposal. Partnerships and limited companies are not in this phase.

HMRC decides by looking backwards at your last filed return, so the figure on the return due 31 January 2027 is what determines whether you join in April 2027. Our MTD checker works it out from your gross figures.

What changes if you are in scope:

• Records must be digital from the first day of the mandated tax year, transaction by transaction, with date, amount and category.
• Four quarterly updates per income source, due 7 August, 7 November, 7 February and 7 May. A trade plus a property business means eight submissions a year, not four.
• One Final Declaration on 31 January replaces the SA100. It carries year-end adjustments, employment income, dividends, savings, reliefs and the Child Benefit charge — everything a quarterly update does not.
• There is no End of Period Statement. HMRC removed it and folded its function into the Final Declaration. Any guide still listing EOPS as a step is out of date.
• Payment dates do not move. Quarterly updates are not quarterly payments. You still pay on 31 January and 31 July.

Full mechanics are in our MTD for Income Tax guide, and the property version in MTD for landlords. Everything below applies to an ordinary Self Assessment return — and most of it, expenses and allowances and payments on account included, applies identically under MTD.

Registering, your UTR, and what to gather before you start

If this is your first return you must tell HMRC by 5 October after the tax year ends — 5 October 2026 for 2025/26. Register online as self-employed, as a partner, or as "not self-employed" for property and other untaxed income; the route differs, the deadline does not.

Registration produces a Unique Taxpayer Reference, a ten-digit number posted to you, typically within about ten working days. You then need a Government Gateway account, and HMRC posts an activation code for that separately. Two letters, each with post in the middle, is why leaving registration until January is a genuinely bad idea: people miss the deadline waiting for a code, not because they had not done the sums.

Before you open the return, collect:

• Business records — invoices raised, income received, every allowable expense with its receipt
• Bank statements for the full tax year, business and any personal account used for business
• P60 and P45s if you were also employed, and a P11D for benefits in kind
• CIS payment and deduction statements from every contractor, if you are a subcontractor
• Interest certificates and dividend vouchers — our dividend tax calculator helps check the numbers
• Pension contributions and Gift Aid donations, both of which extend your basic rate band
• Rental records — rent received, agent statements, mortgage interest kept separate from other costs
• Last year's return and any payments on account already made

Filing the SA100: the supplementary pages and the order to do it in

The SA100 is the core return: personal details, employment, interest, dividends, pensions, reliefs and the Child Benefit charge. Everything else arrives as a supplementary page you tick to add.

A sensible order of work: log in, add the supplementary pages you need before entering anything, then take income sources one at a time and leave expenses to last. Put CIS deductions in the box for tax already taken off — that is what turns the return into a refund claim. Check HMRC's calculation before you submit, looking specifically for a misplaced decimal point, which is far and away the most common error, then keep the receipt and reference number.

If you owe less than £3,000 and already pay tax through PAYE, HMRC can collect the bill through your tax code over the following year rather than demanding it on 31 January — but only if you file online by 30 December. That is a month earlier than the filing deadline and easy to miss.

Allowable expenses, and the claims HMRC rejects

You are taxed on profit, so expenses matter. The test is that a cost must be incurred wholly and exclusively for the business.

Clearly allowable: stock and raw materials; office costs, stationery and software subscriptions; premises rent, utilities and rates; staff wages and subcontractor payments; business travel, parking and accommodation; professional fees and insurance; advertising; bank charges and interest on business borrowing.

For vehicles you can claim a share of actual running costs, or use HMRC's simplified mileage rate. That rate changed this year:

So the return you file this January uses 45p; the year you are trading in now uses 55p. Once you use the flat rate for a vehicle you must keep using it for that vehicle.

The claims HMRC rejects are consistent and worth knowing:

• Client entertaining. Never allowable, however business-critical the lunch was.
• Everyday clothing. Uniforms, branded kit and protective gear are fine; a suit is not, even if you only wear it to see clients.
• Commuting from home to a regular place of work. Travel between client sites is fine.
• Fines and parking penalties.
• The whole phone or broadband bill where there is private use. Apportion it, and be able to explain how.
• Buying equipment outright. That is capital, and goes through capital allowances or the Annual Investment Allowance instead.
• Residential mortgage interest as a rental expense. Landlords cannot deduct it. It is a basic-rate tax reducer applied to the bill, not to taxable profit — which is why higher-rate landlords fare worse than under the old rules. The furnished holiday lettings regime, once the exception, was abolished from 6 April 2025, so holiday lets now follow the same restriction as any other let.

Keep the evidence: HMRC can ask you to substantiate any figure, and records should be held for six years.

The trading, property and Rent a Room allowances

Three flat allowances remove a lot of small-scale reporting.

• Trading allowance: £1,000 of gross trading income. Below it, no need to register or report. Above it, you may deduct the £1,000 instead of your actual expenses — "partial relief" — but never as well as.
• Property allowance: £1,000 of gross property income, on exactly the same terms.
• Rent a Room relief: £7,500 of gross receipts from letting furnished accommodation in your own home, halved to £3,750 where someone else also receives letting income from the same property.

They are alternatives to expenses, not additions, and the arithmetic is simple: claim the allowance if your real expenses are under £1,000, claim expenses if they are over. You can hold a trading allowance and a property allowance at once if you have both kinds of income. Note the test is on gross income, so a side business turning over £4,000 with £3,800 of costs still has to be reported, trivial profit or not.

Payments on account: the January that catches people out

This is where first-time filers get hurt, and it is not a penalty or an extra tax. It is the same tax, collected earlier.

You must make payments on account unless either your last Self Assessment bill was under £1,000, or more than 80% of the tax you owed was already collected at source through PAYE or CIS. If neither exemption applies, HMRC asks for two advance instalments toward the following year, each half of the previous year's bill, due 31 January and 31 July.

The trap is that the first instalment falls due on the same day as the balancing payment for the year that has just been assessed. Here is what that looks like with real numbers.

Daniel's first full year of self-employment, 2025/26. Turnover £52,000, allowable expenses £12,000, so taxable profit £40,000. No employment income, nothing deducted at source.

His bill is over £1,000 and none of it was collected at source, so payments on account are triggered. Each is half of £7,131.80, or £3,565.90.

Daniel budgeted for a £7,131.80 tax bill. He is asked for £10,697.70 — exactly 150% of it — and another £3,565.90 six months later. Nothing has gone wrong. He is paying one year in arrears and one year in advance on the same day, and this happens once, in the first January that payments on account apply.

Year two is much gentler, and that is the point. Say 2026/27 profit rises to £46,000. Income Tax is £6,686.00 and Class 4 is £2,005.80, a liability of £8,691.80. He has already paid £7,131.80 on account, so on 31 January 2028 he owes a balancing payment of just £1,560.00, plus a first payment on account for 2027/28 of £4,345.90 — £5,905.90 in total, against £10,697.70 the year before.

If your income falls, reduce them. Suppose instead that 2026/27 profit dropped to £24,000. The real liability would be £2,971.80, against £7,131.80 demanded on account — an overpayment of £4,160, eventually refunded. Daniel can apply to reduce the instalments, online through his HMRC account or on form SA303, to whatever he genuinely expects to owe. The catch is real: reduce them too far and HMRC charges interest on the shortfall from each original due date, currently at Bank Rate plus 4 percentage points. Reduce to a considered estimate, not to a hopeful one.

CIS subcontractors usually escape this entirely. Take a subcontractor with £45,000 of gross labour receipts, 20% deducted at source, and £6,000 of expenses. Profit is £39,000, so Income Tax is £5,286.00 and Class 4 is £1,585.80 — a liability of £6,871.80 against £9,000 already deducted. That is a £2,128.20 refund, and because far more than 80% of the tax was collected at source, no payments on account arise. The same logic protects an employee with a modest side income: if PAYE is doing most of the work, the 80% test keeps you out.

Both dates are in our UK tax deadline calendar.

Every deadline in the Self Assessment year

If you are in MTD for Income Tax, add 7 August, 7 November, 7 February and 7 May for quarterly updates, per income source, and read 31 January as the Final Declaration date rather than the SA100 date.

Penalties: filing and payment are two separate ladders

People assume that paying on time protects them from a late filing penalty. It does not. The two run independently, and you can collect both.

Late filing — charged even where no tax is owed, and even where the tax was paid on time.

Late payment — charged on the tax itself.

Interest runs throughout, from the day after the due date until the balance clears, at Bank Rate plus 4 percentage points — 7.75% as at January 2026. A return filed six months late with the tax still outstanding therefore attracts £100, up to £900 of daily penalties, a 5% filing penalty, two 5% payment penalties and interest on top. Filing on time and paying late is a far cheaper mistake than the reverse, so if you cannot pay, file anyway.

Anyone who has moved to MTD for Income Tax falls under a different regime: penalty points for late quarterly updates, and percentage-based late payment penalties at 3% and 3% plus 10% per year.

If you cannot pay, and if you got something wrong

Time to Pay. HMRC normally prefers an instalment plan to enforcement, and for Self Assessment you can set one up online without speaking to anyone if you owe £30,000 or less, have no outstanding returns, no other tax debts and no existing plan, and arrange it within 60 days of the due date. It runs up to 12 monthly instalments by Direct Debit. Interest still accrues, but the arrangement stops further late payment penalties. Above £30,000, or for longer, you apply by phone. Filing the return is a precondition either way.

Amending a return. You have 12 months from the filing deadline, so the 2025/26 return can be amended until 31 January 2028, in your HMRC account or through your software, with the tax recalculated automatically. Past that window you cannot amend: you write to HMRC to disclose underpaid tax, or claim overpayment relief if you paid too much, available for up to four years after the end of the tax year. HMRC can also open an enquiry within 12 months of filing — the practical reason to keep your records rather than clear the desk in February.

Choosing Self Assessment software: what actually matters for you

Filing directly on HMRC's website is free and perfectly workable for a simple return. Software earns its cost by removing the reconstruction job — the fortnight in January spent working out what a year of bank transactions were for. What to look for depends on which of these you are.

Sole traders. You want expense categories that map to the SA103 boxes, a bank feed so categorisation happens weekly rather than annually, mileage and home-office handling, and an invoicing tool that records income as it is raised rather than when you remember. Check the software covers the supplementary pages you actually need before you commit.

CIS subcontractors. Your requirement is specific: record gross pay and the deduction separately on every payment, hold the monthly statements from each contractor, and carry the total into the "tax taken off" box so the return produces the refund. Software that treats a CIS payment as a single net figure loses the number your whole refund depends on.

Landlords. Keep each property business separate, keep residential finance costs in their own field rather than lumped with repairs, and record your share of any jointly owned property rather than the whole.

On a tight budget. Two cautions. Cheap annual filing tools often exclude exactly the supplementary page you need — foreign income and capital gains are the usual exclusions — so check before you pay. And if your gross income is heading past £30,000 or £20,000, an annual-return-only tool is a dead end: you will need MTD-compatible software with digital record-keeping, and it is much easier to start in the right place than to migrate mid-year.

Where Marchant fits in

Marchant is a UK accounting platform launching in November 2026, built so the tax return is a summary of bookkeeping you already did rather than a January reconstruction. Bank transactions come in through a feed and are categorised as they land, receipts are captured against them, and income, expenses and your rough tax position stay current through the year. Self-employment, UK property and foreign property are kept as separate income sources, which is how both the SA100 supplementary pages and MTD updates are organised. CIS payments are recorded gross with the deduction held separately, so the reclaim figure is there rather than reconstructed.

Filing is built on HMRC's Making Tax Digital APIs; our listing on HMRC's recognised software list is in progress ahead of the November 2026 launch. Every plan includes a free accountant seat, so whoever finalises your return works in the same data you do. See what each plan includes, or how Marchant compares to FreeAgent if you're weighing it against what you use now.

The filing checklist

1. 1Confirm which system you are in — SA100, or quarterly updates plus a Final Declaration.
2. 2Register by 5 October if this is your first return, and allow for two items of post.
3. 3Add the right supplementary pages before you start entering figures.
4. 4Reconcile your income to your bank statements, and your CIS deductions to the contractors' statements.
5. 5Categorise expenses, apportion anything with private use, and choose between the flat allowance and real costs.
6. 6Check the calculation, especially decimal points, then submit and keep the receipt.
7. 7Work out your payments on account before January so the 150% bill is expected rather than discovered.
8. 8File even if you cannot pay, then set up Time to Pay within 60 days.
9. 9Keep the records for six years.