For many sole traders, Self Assessment is straightforward in principle: report the right income, claim the expenses you are entitled to, submit the return and pay the tax due. The difficulty is usually not the form itself. It is getting to January with incomplete records, a tax bill that has not been planned for, or a figure that does not quite tie back to the business.
The good news is that most Self Assessment problems are preventable. A simple routine during the year will make the return easier to prepare, reduce the chance of errors and give you a clearer view of what you are likely to owe.
If you would prefer support with the return itself, take a look at my Self Assessment tax return service. The points below are a useful checklist whether you prepare your own return or work with an accountant.
1. Missing income from the return
It is easy to focus on the invoices you remember sending and overlook other money that needs to be considered. Depending on your circumstances, that can include card payments, cash sales, work carried out through an online platform, interest, rental income, dividends or a side activity that started part-way through the tax year.
A good starting point is to reconcile your records to your business bank account and payment providers. If you use more than one account, make a list of them. This does not mean every receipt is taxable business income, but it gives you a sensible prompt to understand and document each item.
2. Claiming costs that are partly personal
Business expenses need to be considered carefully. A cost does not become allowable simply because it was paid from the business account. For a sole trader, the usual test is whether it was incurred wholly and exclusively for the trade. Where there is a mixed business and personal element, only the business part may be relevant.
Common examples include mobile phones, internet, motor costs, use of home and travel. Keep the invoice or receipt, note what the cost related to and apply a consistent method. That makes it much easier to explain the treatment later. If you are thinking about moving from sole trader to limited company, it is also worth reading when a sole trader may want to become a limited company, as the rules and admin change once you incorporate.
3. Waiting until January to organise the records
January is a poor time to discover that receipts are missing, invoices have not been recorded or a bank account has not been reconciled for ten months. The return may still be possible, but the process becomes slower and less reliable.
Set aside a short slot each month to:
- reconcile business bank and payment accounts;
- save sales invoices, supplier bills and receipts;
- review any cash payments or expenses paid personally;
- put aside money towards Income Tax and National Insurance; and
- note anything unusual, such as the purchase or sale of a large asset.
This habit also means you are better placed for Making Tax Digital for Income Tax where it applies. From 6 April 2026, it began applying to some people with qualifying self-employment and property income above the relevant threshold. Check HMRC’s current MTD for Income Tax guidance rather than relying on an old threshold quoted online.
4. Being caught out by payments on account
One of the biggest surprises for new Self Assessment taxpayers is that their first January bill may include both the balancing payment for the tax year just ended and the first payment on account towards the following year. It can feel like being asked to pay tax twice, but the payment on account is an advance payment against the next bill.
There are circumstances in which a payment on account can be reduced, but it should only be reduced when there is a genuine reason to expect your next tax bill to be lower. I explain the detail in my guide to HMRC payments on account.
5. Missing the registration, filing or payment deadline
Deadlines depend on your situation, but they are not something to leave until the final week. For example, people who need to complete a return for the first time usually need to tell HMRC by 5 October after the relevant tax year. The normal online filing deadline is 31 January following the end of the tax year, and the tax is normally due by the same date.
Always check the dates that apply to you on HMRC’s Self Assessment deadlines page. Filing early does not make the payment due earlier, but it does give you time to plan for it.
6. Treating the tax return as a one-off compliance task
A completed return should tell you something useful about your business: how much profit it made, what proportion has gone on costs, whether the tax bill is affordable and whether the business structure still works for you. If profit is growing steadily, it may be sensible to review whether remaining a sole trader is still the best route. It is a business decision as well as a tax one.
The return is also a useful trigger to improve your record-keeping for the next year. Small changes—using a separate business account, invoicing promptly, keeping digital receipts and reconciling regularly—can remove a lot of avoidable stress.
What if you have already spotted an error?
Do not ignore it. HMRC allows a Self Assessment return to be amended within a limited period after the filing deadline. The right approach depends on what was wrong and whether it changes the tax due, so keep a clear note of the issue and supporting evidence. You can read HMRC’s guidance on correcting a return for the current process.
A simple way to stay in control
Keep the records current, plan for the tax before it is due and ask questions early when something does not look right. That is usually enough to avoid the mistakes that turn an ordinary tax return into a January emergency.
If you are a sole trader, company director or landlord and would like help preparing your return, get in touch with SB Business Consulting. I provide clear, practical Self Assessment support from Cheltenham for clients locally and across the UK.
This article provides general information only. Tax treatment depends on individual circumstances and the rules in force at the time.



