Tax returns 2025/26 – five mistakes that could cost you money

With the 31 January 2027 tax return deadline approaching, now is a good time to make sure your tax affairs are in order. Whether you're self employed, a landlord or have additional sources of income, completing your tax return accurately and on time can help you avoid unnecessary penalties and potentially reduce your tax bill.

Here are five common mistakes to watch out for.

1. Missing the filing deadline

One of the simplest and most expensive mistakes is submitting your tax return late.

The deadline for filing your 2025/26 tax return is 31 January 2027. Missing it can result in an immediate £100 penalty, even if you have no tax to pay.

Further penalties can apply if your return remains outstanding, so it's worth getting organised well in advance.

2. Forgetting to declare all your income

It's easy to overlook income that falls outside your regular employment or business activities.

Rental income, freelance work, dividends, savings interest and income from online selling may all need to be declared, depending on your circumstances.

HMRC increasingly receives information from banks and digital platforms, making accurate reporting particularly important.

3. Missing out on allowable expenses

Failing to claim legitimate business expenses could mean paying more tax than necessary.

Depending on your circumstances, allowable expenses might include office costs, business travel, professional subscriptions, equipment and certain home working expenses.

Keeping accurate records throughout the year makes it easier to identify what you can claim and ensures you have evidence if HMRC asks questions.

4. Poor record keeping

Incomplete or disorganised financial records can lead to mistakes, missed deductions and unnecessary stress.

Receipts, invoices, bank statements and other supporting documents should be retained and organised.

Using accounting software or maintaining a regular bookkeeping routine can make completing your tax return considerably easier.

5. Forgetting about payments on account

Many taxpayers are surprised to discover that their January tax bill may include more than the tax due for the previous year.

Depending on your circumstances, HMRC may also require a payment on account towards your 2026/27 tax liability.

These advance payments are normally due on 31 January and 31 July, so understanding your obligations can help you plan your cash flow and avoid unexpected bills.

Get ahead of the deadline

Completing your tax return doesn’t have to be stressful. Preparing early gives you time to check your figures, identify potential tax savings and resolve any issues before the deadline.

At Swindells we can help you prepare and submit your tax return accurately, claim the reliefs you're entitled to and ensure you meet your HMRC obligations.

Don't leave it until January. Getting organised now could save you time, money and unnecessary stress.