Falling behind on Self Assessment: what to do in October versus January

If you're a UK sole trader and your Self Assessment for the previous tax year is still untouched, the right next step depends on what month it is when you read this.

The deadlines that matter:

5 October following the end of the tax year: registration deadline if you've never filed Self Assessment before. After this date, registration is late, but you can still register and file.

31 October following the tax year: deadline for paper Self Assessment returns. Most sole traders file online so this rarely matters.

31 January following the tax year: deadline for online Self Assessment filing AND for paying any tax owed.

So a sole trader filing for the tax year that ended 5 April 2026 needs to register (if first time) by 5 October 2026, and file and pay by 31 January 2027.

The advice changes depending on how close you are to the 31 January deadline. If you're reading this in October, you have time to do this properly. If you're reading it in January, you need a different strategy.

If it's October and you've not started, you have three to four months. That's plenty of time to build records that will hold up.

Step one: open your business bank statements for the full tax year (6 April to 5 April). If you don't have a separate business account, open your personal account statements and prepare to do extra sorting.

Step two: identify every business income transaction. Bank deposits, cash takings, card payments through your terminal, online platform payouts. Total them up. This is your gross self-employment income for the year.

Step three: identify every business expense transaction. Each one should fall into one of your trade's standard expense categories. For a cleaning company that's typically: cleaning products, equipment, fuel, vehicle costs, insurance, professional fees, advertising, training, phone, software. If a transaction is genuinely business-related, it goes in. If it's personal that ended up on the business card, it doesn't.

Step four: gather receipts. HMRC can request supporting documentation for any expense claim. You don't need to send them with the return, but you need to be able to produce them if asked. Email folders, photo libraries, supplier portals, the tin on your kitchen counter.

Step five: do the mileage calculation. Total business miles in the year × 45p (first 10,000 miles) + remaining business miles × 25p. If you don't have a mileage log, reconstruct as honestly as you can from your calendar and customer addresses. Note that reconstructed mileage is usually 30 per cent lower than actual; tracking at the time would have been better.

Step six: file. The HMRC Self Assessment portal walks you through the form. If your numbers are clean, the form takes about an hour for a typical sole trader. Pay any tax owed by 31 January.

If this feels like a lot, that's because it is. October is the time to do it properly because there's room for it. January is the time to do it however you can.

If it's the second half of January and your Self Assessment is still untouched, you're not going to do this properly. The right move is to do it well enough to file on time and clean up later.

Three priorities:

Get the income figure as accurate as you can. Underreporting income is the worst kind of error to make on Self Assessment because it's the kind HMRC actively investigates. Total your bank deposits, your card terminal totals, your platform payouts. Round up rather than down if you're uncertain. The cost of slight overreporting is small (you pay slightly more tax than you owed); the cost of underreporting is much higher.

Get the major expense categories in. Don't try to itemise every £8 receipt from May. Get the big ones: vehicle costs, insurance, large equipment, professional fees, software subscriptions. Use your bank statement as the primary source. The smaller, fiddlier expenses you'd normally claim can be left out for this year, and you can do better next year.

File on time. Late filing penalties start at £100 for missing 31 January and escalate. Even a basic, slightly incomplete return filed by the deadline is much better than a perfect return filed two weeks late.

Pay the tax owed by 31 January. If you can't pay in full, HMRC's Time to Pay arrangement lets you spread payments over up to 12 months. Apply through your HMRC online account. Don't ignore it.

The honest version of January-mode Self Assessment is that you will probably pay slightly more tax than you owed because you'll miss some legitimate expenses. That's the cost of leaving it late. Better to know that and accept it than to miss the deadline trying to be perfect.

The point of the January experience is not to repeat it. Before the new tax year starts on 6 April: open a separate business bank account, decide on one record-keeping system (spreadsheet, accounting software, or hand it to your accountant) and commit to it, and set a recurring 15-minute weekly admin slot. Most UK banks offer business accounts free for the first 12–18 months. Friday afternoon works for most people. Done consistently, every business transaction is captured in a structured form from day one of the new tax year, and by the following January your records are complete and your Self Assessment is straightforward.

This is the same advice every business advisor gives every sole trader every year. It's also the advice most sole traders take seriously only after one or two unpleasant January experiences. Worth getting ahead of.

A separate note for anyone approaching the Making Tax Digital threshold: from April 2026, MTD ITSA becomes mandatory for self-employed income above £50,000, dropping to £30,000 from April 2027 and £20,000 from April 2028. Once MTD applies, the annual reconstruction approach stops working entirely (you submit quarterly summary updates plus a final declaration). Building the weekly admin habit now is significantly easier than retrofitting under MTD deadline pressure.

LaunchKit makes a niche-specific MTD Compliance Kit (£16.99) for sole traders approaching or in MTD scope, and a niche-specific Financial Forms Bundle (Standard £11.99 / Custom £13.99 / Premium £19.99) for the day-to-day records that feed it. Both available on Etsy and yourlaunchkit.co.uk. Built for over 140 UK trades and service businesses. One-time purchase.

If you'd rather build your own spreadsheet, the structure in this article gives you the template. The kit saves the setup time.

This article is general guidance, not tax advice. For your specific tax position, consult a qualified accountant.

For cleaning companies, turn the catch-up work into a weekly job, payment and expense routine.

• Cleaning Company templates and tools
• Cleaning Company MTD compliance kit