Published 20 May 2026. The 2025/26 Self Assessment tax year ended on 5 April 2026. You have until 31 January 2027 to file the return online and pay any balancing tax — that is the legal deadline and the date almost every UK sole trader actually files on. I want to make the case for filing six months earlier, by 31 July 2026, which is the same day your second payment on account falls due. Filing in July is not a clever tax wheeze. It is just the sensible operating choice, and the cost of not doing it is bigger than the calm of January would suggest.
This is written from the perspective of someone who has watched a lot of January 30th calendar entries quietly become February 1st penalty letters. The mechanics are simple, the maths is honest, and the reasoning gets stronger the closer you look.
The actual deadlines for 2025/26
Before the argument, the facts. For the 2025/26 tax year (income earned between 6 April 2025 and 5 April 2026), the statutory deadlines are:
- 5 October 2026 — register for Self Assessment if you have a new source of untaxed income.
- 31 October 2026 — paper Self Assessment return deadline. Almost nobody files on paper.
- 31 January 2027 — online Self Assessment return deadline. Balancing payment of any 2025/26 tax owed is due the same day, alongside the first payment on account for 2026/27.
- 31 July 2027 — second payment on account for 2026/27.
The legal deadline to file the 2025/26 return is therefore 31 January 2027. HMRC's Self Assessment deadlines guidance is the authoritative source. The return itself becomes available to file the day after the tax year ends — so you can file your 2025/26 return any time from 6 April 2026 onwards. The deadline you need to think about is not the legal one but the operational one: 31 July 2026, the same day your 2025/26 second payment on account falls due.
Reason 1 — refund speed
If you have made payments on account during 2025/26 that exceed your final tax bill — common for sole traders whose income dropped year on year, or who incurred unusual deductible expenses — HMRC owes you money. That refund only starts to move once your return is processed.
File in late January, and the country is filing with you. HMRC processes around 11 million Self Assessment returns a year, with the vast majority arriving in the final fortnight. Refunds at that point sit in queues. Genuine processing time can stretch to 4–6 weeks for any return that needs a manual check, plus security verification on the bank details. Filed in July, processed in July, refunded in August. That is real working capital you have given HMRC interest-free.
HMRC's refund timing guidance describes the mechanics. Filing early does not change your tax position. It changes when the money sits in your account versus HMRC's.
Reason 2 — payment-on-account clarity
This is the reason most sole traders should care most about. If you owed more than £1,000 in tax for 2024/25, you are already in the payments on account system for 2025/26. You paid:
- 31 January 2026 — balancing payment for 2024/25, plus the first payment on account for 2025/26 (50% of your 2024/25 tax bill).
- 31 July 2026 — the second payment on account for 2025/26 (another 50% of your 2024/25 tax bill).
If you file your 2025/26 return in May or June 2026, you walk into 31 July knowing exactly three things: what tax you actually owe for the year that just ended, how that compares to the payments on account you have already made, and what the resulting cash position looks like. You can plan that 31 July payment from a position of knowledge.
If you wait until January 2027, the second payment on account hits on 31 July 2026 as a fixed amount with no context, and you then discover seven months later that you were either ahead or behind — by which point the 2026/27 first payment on account is also bearing down on you. HMRC's payments on account guidance explains the mechanism in detail. Filing early gives you a current view of three overlapping tax years. Filing late gives you a confused view of one.
Reason 3 — the SA303 window for reducing payments on account
If your income has fallen — sales down, a contract lost, a property sold mid-year, a year off — you are entitled to apply to reduce your 2025/26 payments on account, because they are calculated from 2024/25 when you earned more. The mechanism is form SA303 (Claim to reduce payments on account) or the equivalent reduction on your online return.
The SA303 must be filed before the payment falls due. File it before 31 July 2026 and the second payment on account is recalculated. Leave it until January 2027 and the second payment has already been taken in full at the higher level; you can claim a refund afterwards, but you have lent HMRC the difference for six months. The window for using SA303 effectively closes on 31 July, the very day filing the return forces you to confront the year's actual numbers.
Filing in July gives you the data to know whether SA303 is the right call. Filing in January closes that door before you have looked through it.
Reason 4 — avoiding the January panic
This is less about tax and more about how a small business operates. January is the worst month of the year to do detailed financial work:
- Holiday spend has just come through the books — every transaction needs categorising in the same window you are trying to look at the whole year.
- VAT returns for the December quarter, payroll year-end planning and the start of the next year's budgeting are all stacked on top.
- If your bookkeeper or accountant is the bottleneck, January is the month their entire client base needs them simultaneously. Fees go up, response times go down.
- Late-January submissions are statistically the most likely to contain errors — rushed returns get amended, and amendments draw HMRC enquiries more often than first-time clean returns.
July is the opposite. Your books to 5 April are recent enough to remember and old enough to be settled. There is no parallel deadline screaming for attention. Your accountant has bandwidth — and accountants who file early-bird clients in July often hold pricing for those clients in January, because they are the easy ones. None of this is romantic. It is just better operating practice.
If your books are not in a state to support a July filing, that is itself a signal. Our bookkeeping cleanup work exists precisely for sole traders whose 2025/26 records are not file-ready. Getting current in May or June is straightforward; doing it in late January under deadline pressure is not.
How to actually file in July
The mechanics are unchanged from a January filing. The 2025/26 online return became available on 6 April 2026. To file it cleanly in July you need three things in place by mid-June:
- Books closed and reconciled to 5 April 2026. Bank, card and any payment processors reconciled. Every transaction categorised. No "uncategorised" balance. If you are also in MTD ITSA (£50K+ from 6 April 2026), your software should already be holding the year-to-date figures.
- Non-trading income documents to hand. P60s, P11Ds, interest certificates, dividend vouchers, foreign income statements, pension contribution records, gift aid receipts. These are the items that hold returns up for weeks at a time in January because they get hunted down at the last minute.
- Your Government Gateway access working. Recover the user ID, reset the password, confirm the two-factor method. Discovering an access problem in July gives you weeks to fix it. Discovering it in January gives you days.
For sole traders heading into MTD ITSA on the £50K threshold from 6 April 2026, filing 2025/26 early also gives you a clean line under the pre-MTD year before the quarterly cadence takes over. Our MTD for Income Tax service handles both — the final pre-MTD Self Assessment and the quarterly rhythm that replaces it from April 2026 onwards. For sole traders with a limited company on the side, the limited company accountant work and the personal return need to be coordinated, ideally in the same window.
FAQs
The 31 January 2027 deadline is a safety net, not an operating plan. Treating it as the target keeps an entire industry busy in the wrong month and gives HMRC interest-free working capital from refunds and over-paid payments on account. Filing 2025/26 by 31 July 2026 is the same work, done earlier, with more useful outcomes.
Get your 2025/26 return filed before the second payment on account
If you want your 2025/26 return done and dusted before the 31 July 2026 payment-on-account deadline — and your books are not yet in shape to support it — let me see where you are. A written assessment is enough to tell you whether a July filing is realistic and what it would take.
Start with a free written assessment — Stuart will review your position and reply with a clear plan. No calls, no pressure.