Most of the stress in Self Assessment does not come from the tax return itself. It comes from arriving in January with a shoebox of receipts, a bank feed you half remember, and no record of which trips were business ones. HMRC's expectations are actually quite simple, and if you keep the right records as you go, the return becomes an hour of copying numbers rather than a weekend of archaeology.
The records HMRC expects
If you are self employed, HMRC expects you to keep records of all your business income and expenses, and to hold on to the evidence behind them: invoices, receipts, bank statements and till rolls or platform statements if you sell through a marketplace. You do not send any of this in with your return. You keep it so you can back up the figures if HMRC ever asks, and you should keep it for at least 5 years after the 31 January deadline of the return it relates to.
In practice that means four running lists through the year:
- Income: every payment received, when, and from whom.
- Expenses: every allowable business cost, categorised the way HMRC groups them.
- Mileage: business journeys if you use your own vehicle.
- Anything else you will declare: rental income, dividends, bank interest.
A spreadsheet built around those categories does the job for most sole traders. The TidyDocs Self Assessment tax spreadsheet is set up exactly this way, with the income and expense tabs already matched to HMRC's categories and a summary that totals the year for you.
The dates that actually matter
- 5 October: register for Self Assessment if the tax year that just ended was your first with untaxed income.
- 31 October: paper return deadline.
- 31 January: online return deadline and payment deadline for the year, plus the first payment on account if you owe one.
- 31 July: second payment on account.
Payments on account are the one that catches people. If your last bill was 1,000 pounds or more and less than 80 percent of your tax is collected at source, HMRC asks for two advance instalments towards the next year, each normally half of your last bill. Your first January can therefore be one and a half times the bill you expected. It is not extra tax, but it is real cash flow, so budget for it through the year.
Mileage: the record people regret not keeping
If you use your own car or van for business journeys, the simplified mileage method is usually the easiest route: you claim a fixed amount per business mile instead of tracking every vehicle cost. For the 2026/27 tax year the approved rate for cars and vans is 55p per mile for the first 10,000 business miles, then 25p after that; 2025/26 journeys were at 45p. What HMRC wants to see is a contemporaneous log: date, from, to, purpose and miles.
Reconstructing that in January from a diary and fuel receipts is miserable. Logging it weekly takes minutes, and a dedicated tracker such as the TidyDocs mileage log and claim calculator does the 55p and 25p split for you and totals the claim by tax year.
Landlords: the same discipline, different categories
Rental income goes on the property pages of the return, and the expense categories differ from self employment: repairs and maintenance, agent fees, insurance, services, and legal and professional costs, with residential mortgage interest handled separately as a basic rate tax credit rather than a normal expense. Keeping property records in the same shape as those categories makes the return straightforward, which is what the TidyDocs landlord spreadsheet is built around, month by month and property by property.
Landlords should also keep an eye on Making Tax Digital for Income Tax, which began rolling out from April 2026 for higher-income landlords and sole traders and moves record keeping to quarterly digital updates. Good digital records now mean no scramble later.
A simple year-round routine
Once a week, ten minutes: log the income that arrived, the costs you paid, and the business miles you drove. Once a month, half an hour: reconcile against the bank statement and file the receipts. Come January, the return is a copying exercise, your payments on account hold no surprises, and if HMRC ever asks a question you can answer it the same day.
For the wider picture on side income and when it needs declaring at all, our guide to the 1,000 pound trading allowance covers where casual earnings stop being hobby money and start being taxable.
This article is general guidance, not tax advice. For anything beyond the basics, or if your affairs are complicated, speak to a qualified accountant.