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

Mileage: If you use simplified expenses for a vehicle, check the current rate on GOV.UK simplified vehicle expenses before you put a figure in your records.

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.

First, check the dates. GOV.UK: Self Assessment tax returns lists the current dates for registering, for paper returns, and for filing online and paying. Write the ones that apply to you in your diary.

The advice changes depending on how close you are to the online filing date. 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. Keep the evidence behind every expense together so you can find it quickly if anyone asks. Email folders, photo libraries, supplier portals, the tin on your kitchen counter.

Step five: do the mileage calculation. Total your business miles for the year and apply the current simplified expenses rates from GOV.UK simplified vehicle expenses. 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.

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 the deadline is close and figures are incomplete, do not deliberately omit information. Use the most accurate evidence available, follow HMRC guidance for provisional or estimated figures where necessary, and seek professional help if you are unsure.

Three priorities:

Do not arbitrarily round income up or down. Reconcile the most reliable evidence available, use HMRC's guidance for provisional or estimated figures where it applies, and correct the return when better figures become available (GOV.UK explains how to correct a return).

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. A deliberately incomplete return is not a safe alternative to proper provisional or estimated figures. If you cannot complete the return correctly, contact HMRC or a qualified adviser promptly.

Pay what you owe on time. If you can't pay in full, read GOV.UK: If you cannot pay your tax bill on time and contact HMRC early. 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.

LaunchKit Financial Forms can help organise source records for the business types listed in the live catalogue: Customisable £13.99 and Premium £19.99. They are admin templates, not filing software.

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