10 Common Small Business Tax Mistakes to Avoid in the UK

Tax is one of the most complex and high-stakes responsibilities facing UK small business owners. Whether you operate as a sole trader, limited company director, or partnership, the consequences of getting it wrong can range from financial penalties to formal HMRC investigations. At Accounting Wise, we work with small businesses across the UK every day, and the patterns are consistent: the same mistakes appear time and again, and the majority are entirely preventable.

This guide covers ten of the most common small business tax mistakes we see in practice, along with clear, actionable guidance on how to avoid them.

1. Missing Tax Deadlines

Late filing and late payment are among the most frequent and costly errors small businesses make. His Majesty’s Revenue and Customs (HMRC) operates a strict penalty regime, and even a single day past the deadline can trigger an automatic fine.

Key deadlines to keep in your diary include:

• Self Assessment tax return (online): 31 January following the end of the tax year. The same date applies for payment of any tax owed for that year, plus the first payment on account for the following year.
• Corporation Tax payment: Nine months and one day after the end of your company’s accounting period.
• Corporation Tax return (CT600): 12 months after the end of your accounting period.
• VAT returns: Usually one calendar month and seven days after the end of each VAT quarter.
• PAYE and payroll submissions: Monthly or quarterly, depending on the size of your payroll.

The penalty structure for a late Self Assessment return starts at £100 for missing the deadline by a single day, rising significantly if the return remains outstanding for three months or more. Late payment of tax also accrues interest from the due date.

Practical tip: Set calendar reminders at least four weeks before each deadline, not just on the day itself. Better still, work with an accountant who monitors your obligations throughout the year and ensures nothing is missed. Cloud-based accounting software such as Xero, QuickBooks, or FreeAgent can also provide automated alerts for key dates.

For a full list of HMRC filing and payment deadlines, visit the GOV.UK Self Assessment deadlines page.

2. Not Registering for VAT on Time

VAT registration becomes a legal requirement once your taxable turnover exceeds the VAT threshold within any rolling 12-month period. For the 2025/26 tax year, that threshold is £90,000. Many small business owners only check their annual totals and miss the point at which they have crossed the threshold on a rolling basis, leaving them exposed to backdated VAT liability, interest, and penalties.

Common scenarios that catch businesses out include:

• A period of rapid growth where monthly income increases quickly and cumulatively breaches the threshold before year end.
• Seasonal businesses with a strong peak period that pushes rolling 12-month turnover over the limit.
• Businesses that misunderstand which supplies count as taxable turnover, such as zero-rated goods, which do count towards the threshold even though no VAT is charged.

Once you are required to register, you must notify HMRC within 30 days. Failure to do so means HMRC can backdate your registration to the date you should have registered, leaving you liable for VAT on sales already made, which you may not have collected from customers.

Practical tip: Review your rolling 12-month turnover at the end of each month, not just at year end. If you are approaching the threshold, start planning for VAT registration in advance. You may also wish to consider whether voluntary registration is beneficial before you reach the mandatory limit, particularly if your customers are VAT-registered businesses.

You can register for VAT online via GOV.UK. If you would like guidance on which VAT scheme suits your business, our VAT return services page explains the options available.

3. Incorrect Expense Claims

Claiming expenses incorrectly is one of the most common issues we see, and it cuts both ways. Some businesses over-claim by including personal costs that do not qualify, while others under-claim by failing to record legitimate allowable expenses at all. Both create problems: the former risks an HMRC compliance check, the latter means you are paying more tax than necessary.

Frequently mishandled expense areas include:

• Home working costs: If you work from home, a proportion of household bills such as heating, electricity, and broadband may be allowable. HMRC provides a simplified flat rate, or you can calculate the actual cost based on the proportion of your home used for business.
• Mileage and vehicle costs: If you use a personal vehicle for business journeys, you can claim the approved mileage rate (currently 45p per mile for the first 10,000 miles in a tax year for cars). Many business owners forget to log business mileage consistently and lose out on a meaningful deduction.
• Subsistence and meals: Meals are only allowable as a business expense in specific circumstances, such as when you are travelling away from your usual place of work on a business trip. Routine lunches are not deductible.
• Client entertainment: The cost of entertaining clients is not tax-deductible, even though it is a genuine business cost. Staff entertainment, up to a limit of £150 per head per year, may qualify for tax relief under the annual staff function exemption.
• Equipment and technology: Computers, phones, software subscriptions, and office furniture used for business purposes are generally allowable, though mixed personal and business use requires a proportionate claim.

Practical tip: Keep a separate business bank account and card so that business expenditure is clearly distinct from personal spending. Retain receipts for all business purchases, whether physical or digital, and record them promptly. If you are unsure whether a cost qualifies, take advice before claiming rather than after.

For a full breakdown of allowable expenses, refer to the GOV.UK guidance on self-employed expenses. Our own expenses guides also cover many of the most common queries we receive from clients.

4. Misclassifying Workers

The distinction between an employee and a self-employed contractor is one of the most scrutinised areas of UK tax compliance. Getting this wrong can result in significant back-payments of PAYE income tax, National Insurance contributions, and penalties, with the liability falling on the engaging business in many cases.

HMRC applies a detailed employment status test when reviewing worker relationships. Relevant factors include:

• Whether the worker is required to carry out the work personally, or can send a substitute.
• The degree of control the business has over how, when, and where the work is done.
• Whether the worker bears any financial risk if the work is not delivered correctly.
• Whether the worker is integrated into the business in a way that resembles employment.

The IR35 rules (formally known as the off-payroll working rules) add a further layer of complexity for businesses engaging contractors through personal service companies. Since April 2021, medium and large private sector businesses have been responsible for determining whether IR35 applies to each contractor engagement, a responsibility that previously sat with the contractor.

Practical tip: Do not rely on what a contract says on paper alone. HMRC looks at the reality of the working arrangement, not just the written terms. Use HMRC’s free Check Employment Status for Tax (CEST) tool to assess the status of each engagement, and document the outcome in case of a future query.

5. Poor Record-Keeping

Inadequate record-keeping is both a tax risk and an operational one. HMRC requires businesses to retain financial records for a minimum of five years after the 31 January Self Assessment deadline (for sole traders and partnerships) or six years for limited companies. If records are incomplete or inaccurate, you may be unable to substantiate your figures in the event of an enquiry, and HMRC has the power to estimate your liability, often unfavourably.

The practical consequences of poor records include:

• Difficulty reconciling income and expenditure at year end, leading to errors on your return.
• Inability to support expense claims if HMRC requests evidence.
• Increased accountancy fees, as your accountant must spend additional time reconstructing information.
• Greater exposure to HMRC compliance checks, which are more likely where submitted figures appear inconsistent or incomplete.

Practical tip: Cloud-based accounting software such as Xero, QuickBooks, or FreeAgent makes record-keeping significantly more manageable. Many platforms allow you to photograph and upload receipts directly from your phone, categorise transactions automatically, and produce reports at the press of a button. Making Tax Digital (MTD) requirements are also expanding in scope, making digital record-keeping increasingly important for UK businesses regardless of size.

HMRC’s guidance on record-keeping requirements for both self-employed individuals and limited companies is available on GOV.UK.