Every year, thousands of UK small businesses get things wrong with their taxes. In fact, HMRC received over 11 million Self Assessment returns in the 2022 to 2023 tax year, and errors in those returns are one of the most common triggers for penalties and unexpected bills. Some mistakes are small, like a missed deadline. Others, like miscalculating VAT or forgetting to keep proper records, can cost a business far more than expected.
The truth is, most of these mistakes are not down to dishonesty. They happen because tax rules can be confusing, and running a business leaves little time to stay on top of every detail. Missing a deadline, claiming the wrong expenses, or getting payroll wrong are all easy traps to fall into. The good news is that once you know what to look out for, most of these HMRC tax errors are simple to avoid.
When you run a small business, there is always something that needs your attention. You are chasing invoices, looking after customers, and managing day-to-day tasks all at once. It is no surprise that taxes often get pushed to the bottom of the list.
HMRC updates its guidance regularly. What worked two or three years ago might not be correct today. Many business owners rely on what they already know, without realising the rules have moved on. Keeping up with those changes takes time that most small business owners simply do not have.
UK tax rules treat different businesses differently. A landlord reporting rental income has different responsibilities from a contractor working through a limited company. A sole trader filing Self Assessment has a different process from a limited company paying Corporation Tax. When you apply the wrong rules to your situation, even with good intentions, errors can creep in.
Not everyone knows they need to file a Self Assessment tax return. HMRC does not always tell you directly, so it is easy to miss. As a general rule, you need to file one if you earn money that has not already been taxed through PAYE.
If you need to file and you do not, HMRC will still charge you a penalty even if you owe no tax. The £100 fine lands automatically on 1 February, the day after the deadline. It is one of those situations where not knowing the rule does not protect you from the consequences.
That is completely normal. A lot of people fall into self-assessment without realising it, especially when their income changes, they start renting out a property, or they take on freelance work alongside a regular job.
The easiest first step is to use the HMRC online checker to see if you need to register. If you would rather talk it through with someone, our tax compliance services can help you figure out exactly where you stand and make sure nothing gets missed.
VAT trips up a lot of small businesses, and it is usually not because they are being careless. The rules are just genuinely confusing.
As of June 2026, the UK VAT registration threshold stands at £90,000 of taxable turnover within any rolling 12-month period. Once a business exceeds this figure, it is legally required to register for VAT with HMRC.
The VAT deregistration threshold currently stands at £88,000 (June 2026). Accordingly, a business that is already registered may only apply to cancel its registration once its turnover falls below this figure. Both thresholds have remained in place since 1 April 2024 and are unchanged.
Even after you are registered, mistakes still happen. Not all goods and services are charged at the same rate. Some are exempt, some are zero-rated, and some carry the standard 20% rate. Charging the wrong rate or missing out on VAT you could reclaim on purchases means your records will not add up, which can trigger a closer look from HMRC.
There is also Making Tax Digital for VAT to think about. Most VAT-registered businesses now need to keep digital records and submit returns using approved software. Getting the numbers right is not enough if you are not submitting them the right way. You can find full guidance on HMRC VAT.
If you have staff, payroll is one of the easiest places for errors to build up without you noticing.
Under PAYE, you are responsible for taking Income Tax and National Insurance out of your employees' wages every time you pay them. You then pass that money to HMRC, usually every month. If the calculations are off, your employees could end up paying too much or too little tax, and HMRC will eventually want it sorted.
Common payroll mistakes include:
That last one catches a lot of employers off guard. Even an honest mistake can lead to a penalty. Our payroll services for businesses are designed to take this pressure off your plate entirely.
This is one of the few tax errors that actually costs you more money than you should be paying. HMRC lets businesses deduct a range of expenses from their taxable income, which brings down the amount of tax owed. Many business owners either do not know what they can claim or they do not keep the records needed to back up a claim.
Expenses you may be able to claim include:
One area that causes problems is mixed use. If you use a car or a laptop for both work and personal reasons, you can only claim the business portion. Getting that split wrong is a mistake HMRC looks out for.
Bad records lead to bad tax returns. It really is that simple. HMRC requires self-employed individuals to keep financial records for at least five years after the 31 January deadline of the relevant tax year. Limited companies must keep records for at least six years.
Without proper records, you cannot accurately report what you have earned or spent. Many small businesses rely on paper receipts and spreadsheets, which makes it easy to lose important information. If HMRC opens an enquiry into your business and your records are incomplete, it becomes very difficult to defend your position.
Good bookkeeping software makes this much easier. Our small business bookkeeping services help you keep everything organised throughout the year, not just at tax time.
Limited companies pay Corporation Tax on their profits. The current rate is 25% for profits over £250,000, and 19% for profits up to £50,000. If your profits fall in between, marginal relief applies.
Two deadlines catch directors out regularly. Corporation Tax must be paid within nine months and one day after the end of your accounting period. The return itself is due 12 months after that period ends. Many people assume both deadlines are the same, and that mistake leads to interest charges from HMRC.
Director's loan accounts are another common problem. If a director borrows money from the company and does not pay it back within nine months of the company's year-end, the company can face an extra tax charge. Full guidance is available on HMRC Corporation Tax. Our tax compliance services can help you stay on top of all of this.
A lot of small businesses are leaving money on the table by not looking into Research and Development Tax Credits. HMRC offers this relief to companies that spend money on developing or improving products, processes, or services. Many owners assume it is only for big tech firms, but that is not the case.
A manufacturer trying out a new production method or a software company building something bespoke could both qualify. The relief can reduce your Corporation Tax bill significantly, or in some cases, HMRC will pay you directly.
If your business has spent money on something new or experimental, it is worth finding out whether you qualify. Our R&D tax credits services can help you work out what counts and put together a proper claim.
The good news is that most HMRC tax errors are preventable. You do not need to be a tax expert to get things right. You just need a few good habits and the right support around you.
Trying to pull everything together at the last minute is where most mistakes happen. If you keep your records tidy throughout the year, you always know where you stand. Small business bookkeeping services can help you stay organised without it taking over your week.
HMRC expects most businesses to keep digital records now, especially for VAT. Using HMRC recognised software means your records are stored properly, and your returns are submitted the right way. It also makes it much easier to spot anything that looks off before it becomes a problem.
A good accountant does more than fill in forms. They spot problems early, make sure you are claiming everything you are entitled to, and can speak to HMRC on your behalf if needed. You can also browse the UK Government Tax Guidance for general information on your obligations.
Tax mistakes can happen to any business, but they do not have to happen to yours. The earlier you get the right support, the easier it is to stay on top of things and avoid unexpected bills from HMRC.
Red Fish Accountancy helps UK small businesses, sole traders, company directors, and landlords keep their taxes in order all year round. Whether you are worried about a specific issue or just want peace of mind that everything is correct, the team is ready to help.
You can explore services including tax compliance, small business bookkeeping, and payroll support, all in one place. Get in touch with Red Fish Accountancy today and take the guesswork out of your tax obligations.
