Common tax mistakes made by small UK businesses

A new client came to us a few weeks ago after receiving an HMRC enquiry letter. She’d been running her business for four years, doing her best to stay on top of her taxes. The problem wasn’t dishonesty; it was simply that nobody had ever told her the rules properly. A handful of avoidable errors had quietly been stacking up.

That story is far from unusual. We see it all too regularly. The good news is that most common tax mistakes made by small UK businesses are entirely preventable, once you know what to watch out for. Here are the ones we come across most often.

1. Missing tax deadlines

Late filing and late payment are among the most common tax mistakes small businesses make. Unfortunately, HMRC has become considerably less forgiving about them in recent years. A Self-Assessment return filed even one day late triggers an automatic £100 penalty, regardless of whether any tax is owed. Late payment attracts interest at Bank Of England base rate + 4%, (currently 7.75% per annum) and a 5% penalty on any tax still unpaid after 30 days, with further 5% charges at 6 and 12 months.”

The fix is straightforward: get a tax calendar in place and set reminders well ahead of each deadline. Aim to file your accounts as soon after your year-end as possible. Waiting until January to think about your January tax return is a recipe for stress and unnecessary cost.

2. Mixing personal and business finances

Using the same bank account for personal and business transactions is a deceptively easy habit to fall into, particularly for sole traders in the early days. The trouble is, it makes it very hard to identify what’s a legitimate business expense and what isn’t. HMRC’s digital compliance systems now use data-matching technology to compare expense patterns across businesses in the same sector. Anything that looks out of place can trigger a formal enquiry.

A dedicated business bank account is one of the simplest and most effective steps any small business owner can take. It creates a clean audit trail and makes your bookkeeping much easier.

3. Getting expenses wrong in both directions

This is one of the common tax mistakes we see most often, and it works both ways. Some business owners claim personal costs as business expenses – non-business travel, personal meals, or household bills without proper apportionment. HMRC only allows expenses that are ‘wholly and exclusively’ for business purposes, and it increasingly flags claims that look disproportionate to turnover.

At the same time, plenty of small businesses under-claim. Home working costs, mileage, professional subscriptions, bank charges, and use-of-home calculations are all regularly missed. Paying more tax than you need to is just as much of a mistake as paying too little. Our free expenses guide is a good starting point if you’re not sure what you can claim.

4. Missing the VAT registration threshold

Once your taxable turnover exceeds £90,000 in any rolling 12-month period, you must register for VAT within 30 days. The threshold is calculated on a rolling basis – it’s not tied to a tax year – so it’s entirely possible to creep past it without noticing. Failing to register on time means HMRC can claim the VAT you should have charged, going back to the date you first exceeded the threshold. Even if you didn’t collect it from your customers, you’ll still owe it. This is how some small businesses get into trouble with their VAT payments.

Keep an eye on your turnover monthly, not just at year-end. A simple rolling total in your accounting software should flag when you’re getting close.

5. Choosing the wrong business structure

Running as a sole trader when you’d benefit from being a limited company – or vice versa – is a common tax mistake that can cost thousands in unnecessary tax each year. The right structure depends on your level of profit, your personal circumstances, and your plans for the future.

We helped a freelance marketing consultant in Dartford save over £3,800 in tax by moving to a limited company at the right point in her business growth. The timing matters enormously. Read our blog on salary vs dividends for limited company directors for more on this.

6. Poor record keeping

HMRC can investigate your tax affairs going back up to four years for innocent errors, and considerably further if they suspect you’ve been careless or deliberately underreported income or overreported expenses. Without good records – receipts, invoices, bank statements, mileage logs – you have no defence if they come calling. Under Making Tax Digital, the expectation is that digital records are maintained throughout the year, not reconstructed at year-end from a carrier bag of receipts.

Cloud accounting software like Xero, QuickBooks, or FreeAgent makes record keeping significantly easier and is increasingly essential for small businesses. Our Making Tax Digital checklist has more information on getting your digital records in good shape.

Avoid common tax mistakes with the right support

If any of these common tax mistakes sound familiar, the best time to deal with them is now – not when HMRC comes calling. Contact Adams Accountancy for a free, no-obligation chat with our friendly Kent-based team. No question is too silly, and we’d rather help you get it right from the start.

Frequently asked questions about common tax mistakes

What happens if I’ve already made a tax mistake?

If you’ve realised you’ve made an error in a previous return, correct it as quickly as possible. You can amend a Self-Assessment return online within 12 months of the original filing deadline. For errors going back further, or for more complex situations, you may need to make a voluntary disclosure to HMRC. Acting promptly generally results in lower penalties than waiting for HMRC to find the mistake. See HMRC’s guidance on correcting tax returns for more detail or speak to your accountant.

How far back can HMRC investigate a small business?

For innocent errors, HMRC can typically go back four years. Where they believe there has been careless behaviour – for example, consistently poor record keeping – they can look back six years. In cases of deliberate or fraudulent behaviour, the window extends to 20 years. This is why accurate records and honest returns matter so much, even when a mistake was entirely unintentional.

Can I correct a tax mistake myself or do I need an accountant?

Simple corrections to recent tax returns can often be made yourself via HMRC’s online portal. However, if the error involves multiple years, significant sums, or anything that might attract HMRC attention, it’s worth getting professional advice before you act. The way a disclosure is handled can significantly affect the penalties you’re asked to pay. An accountant can also identify whether there are related errors elsewhere that need addressing at the same time.

How does HMRC find out about tax mistakes?

HMRC uses sophisticated data-matching technology to cross-reference information from banks, employers, Companies House, online selling platforms, and third-party software. It compares your reported income and expenses against others in the same industry and sector. Anything that deviates significantly from the norm – unusually high expense ratios, unexplained drops in turnover, or lifestyle indicators that don’t match reported income – can trigger a compliance check. The system is increasingly automated, which means even small errors are more likely to be spotted than they once were.

About the author

Michelle Adams is a qualified accountant and director of Adams Accountancy, a friendly, all-female accountancy practice based in Dartford, Kent. With over 15 years’ experience helping limited companies, sole traders, and landlords across Kent avoid the most common tax mistakes and stay on the right side of HMRC, Michelle and her team are here to make tax straightforward. Call us on 01322 250001 or visit adams-accountancy.co.uk for a free, no-obligation chat.