Self Assessment 2025/26: why filing in September beats January

The short answer: does filing early cost you anything?

No. Filing your 2025/26 Self Assessment return in September or October 2026 does not bring your payment date forward by a single day, and it does not increase your bill. You still pay whatever is due on 31 January 2027. What early filing changes is how much time and information you have before that date arrives. This guide is written by LOYALS, a King's Cross firm of chartered accountants that files Self Assessment returns and runs personal tax for London sole traders, company directors and landlords.

Most people treat the return and the payment as one job with one deadline. They are two separate things. HMRC lets you file any time after the tax year ends on 5 April, but it does not ask for the money until 31 January the following year. Separating the two is the whole trick, and it is why the owners we work with who file in the autumn spend January doing nothing while everyone else scrambles.

The 2025/26 dates that actually matter

There are four dates worth knowing for the 2025/26 tax year, and only one of them is the famous January deadline. Here they are at a glance.

Source: HMRC Self Assessment deadlines, verified at gov.uk as at August 2026. The paper and online filing dates and the payment date are fixed statutory deadlines.

The online filing and the payment share the 31 January 2027 date, which is where the confusion starts. HMRC sets out all of these dates in its Self Assessment deadlines guidance. If 2025/26 was your first year needing a return, note the earlier 5 October 2026 registration deadline, because you cannot file until you have a Unique Taxpayer Reference, and that can take a couple of weeks to arrive by post.

One more date is worth respecting for the wrong reasons. Miss the 31 January 2027 online deadline and HMRC charges an automatic £100 penalty even if you owe nothing or are due a refund, then £10 a day after three months up to £900 more, per its Self Assessment penalties guidance. Filing in the autumn takes that risk off the table entirely.

Why September, not January: five things you gain

Early filing hands you five practical advantages, and every one of them is about control rather than compliance. Take them in turn.

1. Months to budget for the exact bill

Knowing your number in September gives you four or five months to save for it. A sole trader who learns in September that they owe £6,200 can set aside roughly £1,240 a month and meet 31 January comfortably. The same person finding out on 28 January has three days and a cold sweat. Cash-flow planning is impossible when you do not know the figure, and filing is the only way to know the figure.

2. Any refund comes back sooner

Anyone who overpaid, through payments on account or through PAYE, gets the difference back once the return is processed. File in September and that repayment is typically back in your account within a few weeks. Leave it to January and the same money sits with HMRC through the busiest processing window of the year, when repayments are slowest.

3. You can cut payments on account that are too high

Payments on account are advance instalments toward next year's bill, based on this year's. If your 2025/26 income was lower than 2024/25, the instalments HMRC set may be more than you will actually owe. Filing early lets you formally reduce them to the right level, which keeps your cash rather than parking it with HMRC until it is repaid.

4. You keep the option to spread the bill through your tax code

If you owe under £3,000 and have PAYE income, you can ask HMRC to collect the tax through next year's tax code instead of in one lump. That option only survives if you file online by 30 December 2026. Wait until January and it is gone, and the whole balance falls due at once.

5. Fewer errors and no helpline queue

Late January is when the HMRC helplines jam, the online system slows under load, and tired people make mistakes on rushed returns. A return prepared in the autumn, with time to find the missing dividend voucher or query an odd figure, is simply more accurate. Accuracy matters, because an error found now costs minutes and an error found at enquiry costs weeks.

The myth that files early means paying early

Filing early never brings the payment forward, and this single misunderstanding is why most people leave it to the last minute. The return is a declaration. The payment is a separate event with its own fixed date of 31 January 2027. You can submit the return in September and the money will still not leave your account until January, unless you choose to pay sooner.

Where does the belief come from? Partly from confusing filing with payment, and partly from an old worry that HMRC will somehow chase the money the moment it sees the figure. It will not. HMRC's own guidance on paying your Self Assessment bill sets the deadline at 31 January regardless of when the return went in. If anything, knowing the number early gives you the option to pay in instalments through a Time to Pay arrangement if the bill is uncomfortable, which is far easier to set up in the autumn than in the last week of January.

How to reduce payments on account when your income has fallen

You can apply to reduce your payments on account whenever you expect this year's bill to be lower than last year's, and filing early is what makes the case clear. Payments on account are two advance instalments, due 31 January and 31 July, each worth half of your previous year's tax bill, and they apply once that bill is over £1,000. HMRC explains the mechanism in its payments on account guidance.

Here is the trap. If your 2024/25 income was strong and your 2025/26 income dropped, HMRC still set your 2025/26 instalments from the higher year. You may have already paid too much in July 2026. Filing the 2025/26 return now reconciles the position: it either releases a refund or lets you formally reduce the January 2027 instalment to the real figure. Leave it and you effectively give HMRC an interest-limited loan until you get round to filing.

One caution worth stating plainly. Reduce the payments only to a figure you can defend. If you cut them too far and the eventual bill is higher, HMRC charges interest on the shortfall from the original due date. The point of filing early is that you set the reduction from real numbers, not a guess.

Spread the bill through your tax code by 30 December

Owe less than £3,000 and have PAYE income? You can have the tax collected through your tax code instead of paying it in one go, but only if you file online by 30 December 2026. This is called coding out, and it is one of the most useful and least known reasons to file in the autumn rather than January.

It works like this. When you file by 30 December and meet the conditions, HMRC adjusts your 2027/28 tax code so the Self Assessment tax is deducted in equal amounts from your salary or pension across the year, from April 2027. A £1,200 bill is then collected in twelve small instalments through your wages, rather than £1,200 leaving your bank in one go on 31 January. HMRC sets out the three conditions in its guidance on paying through your tax code: the bill is under £3,000, you already pay tax through PAYE, and you filed online by 30 December.

Miss 30 December by even a day and the option vanishes, and the full balance is due on 31 January. That single deadline is a strong argument on its own for treating December, not January, as your real filing target.

What to gather now: the September checklist

Gather your records first and the return itself takes very little time. For the year to 5 April 2026, most people need the following in one place:

• Self-employment figures. Your income and allowable expenses for the year. If you are unsure what counts, our guide to self-employed allowable expenses lists what HMRC accepts and what it quietly rejects.
• Employment documents. Any P60, P45 or P11D if you also had a salary or benefits during the year.
• Savings and investment income. Bank and building society interest, and dividend vouchers from any shares or your own company.
• Property income. Rent received and the costs against it, if you let out property.
• Pension and Gift Aid. Personal pension contributions and any Gift Aid donations, both of which can extend your basic rate band.
• Payments already made. The payments on account you made in January and July 2026, so they are credited correctly.

Once these are together, the numbers do the talking. If your position is straightforward, the self-employment and personal tax return is a short job. If it is not, for example you have mixed income, a company, property and dividends, that is exactly where an accountant earns the fee, by getting the reliefs right and the figures reconciled the first time.

A note on the year ahead. For anyone with gross self-employment or rental income above £50,000, Making Tax Digital for Income Tax became mandatory from 6 April 2026, so 2026/27 is the first year of quarterly digital updates rather than a single annual return. The 2025/26 return you file this autumn is still the familiar once-a-year form, which makes it the right moment to get your bookkeeping and software ready. Our Making Tax Digital guide for the self-employed covers what changes and when.

What this means for you

File the 2025/26 return in the next few weeks and January becomes a non-event. You will know your number, you will have set the money aside, any refund will already be back, and if the bill is small and you are on PAYE you can spread it through your code. LOYALS is a King's Cross firm of chartered accountants that prepares and files Self Assessment returns for London sole traders, directors and landlords, and reconciles payments on account so clients are never overpaying or caught short. If you would rather hand it over, you can check your position in a free call and we will tell you the number and the deadlines that apply to you.