Preparing for Self Assessment: How to Get Your 2025-26 Tax Return Ready

If you need to send a tax return for the 2025-26 tax year, September is exactly the right moment to start preparing for Self Assessment. The year in question ran from 6 April 2025 to 5 April 2026, the online filing and payment deadline is 31 January 2027, and anyone who has not filed before must tell HMRC they need to complete a return by 5 October 2026. Getting the groundwork done this autumn gives you time to chase missing paperwork, budget for the bill and file at a moment that suits you, rather than fighting the January rush with everyone else.

This guide walks through who actually needs to file, the registration deadline that first timers keep missing, the records worth pulling together now, and the payments on account system that surprises so many people with their first bill. It is general guidance rather than advice on your own affairs, so speak to us for advice tailored to your situation.

Who needs to send a return for 2025-26?

More people than you might think. The clearest case is anyone who was self employed as a sole trader and earned more than £1,000 in the year before deducting expenses. Partners in a business partnership must file too, as must anyone who had Capital Gains Tax to pay on something they sold, anyone liable for the High Income Child Benefit Charge who does not pay it through PAYE, and off-payroll workers repaying a student or postgraduate loan. The full list is set out in the GOV.UK guidance on who must send a tax return.

Untaxed income is the category that catches people who do not think of themselves as being in business at all. Rent from a property, dividends from shares, interest on savings, tips and commission, and income from abroad can all pull you into Self Assessment even if you are employed and taxed through PAYE for everything else. If any money arrived in 2025-26 that nobody taxed on the way in, it is worth checking your position now rather than in January.

First return? Register by 5 October 2026

You must tell HMRC by 5 October 2026 that you need to complete a return for 2025-26 if you have never sent one before, or if you registered in the past but did not need to file for 2024-25. You do this by registering for Self Assessment, and it is not something to leave until the last week, because HMRC has to set you up on its systems before you can file anything at all.

Miss the date and the world does not end, but the process gets tighter. HMRC will send you a letter or email with its own deadline for your return, three months from the date on that letter, and the tax itself is still due by 31 January 2027 regardless. Register late and leave tax unpaid at that point and you can face a failure to notify penalty on top. Our taxation service in Widnes handles registration and the return itself, which takes the whole question off your desk.

How to prepare for Self Assessment this autumn

Preparation is mostly a matter of collecting things while they are still easy to find. Here is the order we suggest to clients.

1Round up every source of income

Sales invoices and takings if you trade, rental statements if you let property, dividend vouchers, bank interest summaries, and your P60 or P45 if you were also employed during the year. The return covers everything, not just the self employed part.

2Pull the expense evidence together

Receipts, purchase invoices, mileage logs and the basis of any home working claim. Gaps are far easier to fill in September, while suppliers can still reissue paperwork and you can still remember what a bank line was for. If your records are in a carrier bag rather than a system, our bookkeeping service can turn them into something a return can actually be built from.

3Check you can actually get in

Dig out your Unique Taxpayer Reference and make sure your Government Gateway sign in still works. Recovering lost credentials takes time you will not want to spend in January.

4Reconcile the bank account

Work through the year’s statements against your records and query anything that does not match. Unexplained deposits and forgotten expenses both surface here, and both are better found before the return is drafted than after.

5Estimate the bill and start setting money aside

Once the income and expense picture is roughly complete, a draft calculation tells you what January will cost. Four months of putting money aside is a far gentler slope than finding the whole amount in one go.

6Decide who is filing it

If an accountant is preparing the return, brief them now rather than in the new year. Early instruction means your return is prepared with time to check queries properly, and we set out the case for that in our guide to why filing your Self Assessment early pays off.

Payments on account, the part that catches people out

Payments on account are advance instalments towards your next tax bill, and they are the single biggest source of first year shock. Unless your last Self Assessment bill was under £1,000, or more than 80 per cent of the tax you owed was collected at source, HMRC asks for two payments towards the following year, each half of the bill you have just settled, due by 31 January and 31 July. The mechanics are set out in the GOV.UK guidance on payments on account.

The sting is in the first year. Using the example from that guidance, a £3,000 bill for the year just filed becomes £4,500 to pay by 31 January, because the first £1,500 payment on account for the following year is added on top, with another £1,500 due by 31 July. Nobody enjoys discovering that in the last week of January, which is precisely why the estimate in step five matters. If you know your income is going to be lower this year than last, you can apply to reduce your payments on account, though reducing them too far attracts interest on the shortfall.

Making Tax Digital is changing the timetable

There is one more reason to treat this year’s return with care. Making Tax Digital for Income Tax is HMRC’s new way of reporting for sole traders and landlords, with digital records and quarterly updates in place of a single annual return, and the figures on your Self Assessment returns decide when it reaches you. Qualifying income from self employment and property over £50,000 in 2024-25 meant joining from 6 April 2026. If your 2025-26 return shows qualifying income over £30,000, you will be brought in from 6 April 2027, and the threshold falls to £20,000 from 6 April 2028. The return you are preparing this autumn is the one HMRC will read to decide whether the £30,000 test catches you, which is one more argument for getting it right rather than rushing it.

What happens if you leave it too late?

Missing 31 January costs £100 immediately, even if you owe no tax at all. Leave the return three months late and daily penalties of £10 begin, up to £900, with further penalties of 5 per cent of the tax due or £300, whichever is greater, at six months and again at twelve. Paying late runs on its own separate track, adding 5 per cent of the unpaid tax at 30 days, six months and twelve months, plus interest all the while. The two sets stack, which is how a modest bill left unattended turns into an expensive one. We have covered the full arithmetic in our article on the real cost of late tax returns.

The dates themselves, and the small print around them, are on the GOV.UK Self Assessment deadlines page, which is always worth checking against your own circumstances because trusts, partnerships with corporate partners and a handful of other cases work to different rules.

Frequently Asked Questions

Make January the quiet month

Every January, accountants meet two kinds of taxpayer: the ones who prepared in the autumn and are filing calmly, and the ones reconstructing a year of paperwork against a countdown. The difference between them is rarely ability and almost always timing. A few hours spent now on registration, records and a rough estimate buys you a bill you saw coming and a deadline that holds no fear. If you would rather hand the whole process over, get in touch with FD Accountants and we will take it from here.