Top 10 Tax Mistakes UK Businesses Makeand How to Avoid Them in 2026/27

HMRC collected over £41 billion in tax penalties and interest in the last reported year. The majority of investigations are triggered not by fraud but by avoidable errors — wrong expense claims, missed deadlines, incorrect VAT treatment, and structures that don’t hold up to scrutiny. This guide identifies the ten most common mistakes we see and explains how to avoid them.

1. Claiming Personal Expenses Through the Business

The most common and most scrutinised area. HMRC’s “wholly and exclusively” rule is unforgiving — an expense is only allowable if it is incurred entirely for business purposes. Common mistakes include:

• Claiming home broadband and mobile phone bills in full (only the business proportion is allowable)
• Claiming meals as “business entertainment” — client entertainment is not allowable
• Claiming personal travel as business mileage
• Using company funds for personal items and not treating them as a benefit or director’s loan

2. Missing HMRC Deadlines

3. Getting VAT Wrong

VAT errors are one of the most common triggers for HMRC investigation. Key mistakes include:

• Failing to register once turnover exceeds £90,000 (rolling 12 months)
• Reclaiming VAT on cars (blocked in most circumstances)
• Treating exempt supplies as zero-rated — very different VAT treatments
• Not applying the CIS domestic reverse charge to construction services
• Missing the MTD VAT digital link requirement

4. Ignoring the Director’s Loan Account

Overdrawn director’s loan accounts are a persistent problem. If a director owes money to their company and it isn’t repaid within 9 months of the year end, the company faces a 33.75% Section 455 tax charge. Additionally, if the loan exceeds £10,000, a beneficial loan benefit in kind arises. Many directors don’t realise their drawings are creating a loan until the accountant produces the year-end accounts — too late to avoid the charge.

5. Wrong Business Structure

Operating through the wrong structure costs money every year. The most common scenario: a sole trader generating £60,000+ profit who hasn’t considered incorporation and is paying 40% income tax + Class 4 NIC on profits that could be taxed at 19% corporation tax instead. Equally common in reverse — a small company with minimal profits that incurs unnecessary compliance costs. Structure should be reviewed annually as circumstances change.

6. PAYE and Employment Status Errors

Misclassifying employees as self-employed is one of HMRC’s highest priority enforcement areas. If HMRC determines that workers are employees, the company faces retrospective PAYE, NIC, penalties and interest going back 6 years. The off-payroll working rules (IR35) add complexity for businesses engaging contractors through intermediaries.

7. Property Tax Errors

• Not claiming all allowable expenses on rental properties
• Misunderstanding Section 24 — still claiming full mortgage interest deduction
• Missing the 60-day CGT reporting deadline on property sales
• Claiming capital improvements as repairs (capital items are not deductible against rental income)
• Failing to declare rental income at all — HMRC’s Connect system cross-references Land Registry data

8. Poor Record-Keeping

HMRC can open an enquiry up to 4 years after the relevant tax return (20 years where fraud is alleged). Poor records make it impossible to defend legitimate claims. For MTD businesses, inadequate digital records risk penalties. Keep bank statements, invoices, receipts and mileage logs — digitally, in MTD-compatible software where required.

9. Missing R&D Tax Relief

R&D tax relief is one of the most under-claimed reliefs in the UK. Many businesses assume they don’t qualify — but the definition of qualifying R&D is broader than most realise. It includes software development, process improvement, product development and more. Under the merged RDEC scheme, a 20% credit is available for qualifying expenditure. Many businesses leave significant cash on the table by not claiming.

10. No Tax Planning — Just Compliance

The final and most expensive mistake is treating tax as a compliance exercise rather than a planning opportunity. Most tax-saving strategies — pension contributions, salary sacrifice, profit extraction structure, EIS/SEIS investments, group structures — must be in place before the year end. Planning after the event is usually too late. Annual tax planning reviews pay for themselves many times over.

The Real Cost: What These Mistakes Add Up To

The mistakes above don’t usually fail alone — they cluster. A typical owner-managed business making three or four of these errors might be losing £8,000–£25,000 a year in unnecessary tax and penalties. Over a decade, that’s enough to fund a property deposit, retire a director’s mortgage, or capitalise a second business line. Here is what each mistake tends to cost in real numbers:

• Personal expenses through the company: a £4,000/year mix of personal expenses claimed costs the company corporation tax relief (£1,000), creates a benefit-in-kind income tax charge for the director (£800–£1,800), and triggers HMRC penalties up to 30% of unpaid tax if challenged. Real cost over five years: £15,000–£25,000.
• Late filing of CT600 and Self Assessment: £100 immediate penalty + £100 after 3 months + 10% of unpaid tax at 6 months + further 10% at 12 months. A single late £40,000 corporation tax payment can attract £8,000–£10,000 of penalties plus daily interest at 7.75%.
• Late VAT registration: if turnover crossed £90,000 in March but you only register in October, HMRC backdates the registration. You owe VAT on all sales from registration date — typically £15,000–£40,000 of unbudgeted VAT for a small business that has already spent the cash. Late-registration penalties add 5–15%.
• Overdrawn director’s loan account: if a £30,000 director’s loan isn’t cleared within 9 months of year-end, the company pays a 33.75% s455 corporation tax charge (£10,125). HMRC eventually refunds when the loan is repaid, but the cash sits with HMRC for years.
• Wrong business structure: staying as a sole trader at £80,000 of profit instead of incorporating typically costs £4,000–£7,000/year in extra income tax and Class 4 NIC. Multiplied across 5–10 years, it’s a significant compounding loss.
• PAYE/IR35 errors: a misclassified contractor at £60,000/year can cost the engager £15,000–£20,000 in back-NIC, income tax and penalties if HMRC reclassifies them as employed.
• SDLT and property tax errors: the 5% Additional Dwelling Supplement on a £400,000 property is £20,000. Mistakes here are individual six-figure mistakes, not running drains.
• Poor record-keeping: not directly costly, but it amplifies every other mistake on this list and makes HMRC enquiries vastly more painful. The hidden cost is in director time and accountant fees during enquiries.
• Unclaimed R&D relief: a tech business spending £200,000 on qualifying R&D in 2026/27 is entitled to roughly £18,000–£32,000 in cash credits or corporation tax savings. Many small companies never claim despite qualifying.
• No tax planning: the most expensive single line item. An owner-manager extracting £80,000 a year sub-optimally typically pays £6,000–£12,000 more tax than necessary.

How HMRC Discovers These Errors

Many business owners assume HMRC enquiries are random or rare. They aren’t. HMRC’s data and risk-assessment capabilities have changed substantially over the past five years, and most enquiries are now triggered by automated risk flags rather than random selection. Understanding the triggers helps explain why the mistakes above don’t stay hidden indefinitely.

• Cross-matching from third-party data: HMRC receives bank interest data, dividend data from listed companies and crypto exchange data automatically. Inconsistencies between Self Assessment returns and these data feeds trigger automated nudge letters.
• VAT registration triggers: Companies House filings and bank account flows are cross-matched against VAT registration status. A business with £150k of bank receipts but no VAT registration will be flagged within months.
• Director’s loan / dividend nudge campaigns: HMRC runs targeted campaigns examining director’s loans, dividends in excess of available retained earnings, and unusual extraction patterns. These often arrive as “we have information that suggests...” letters that prompt voluntary disclosure.
• R&D claims under scrutiny: the opposite problem — HMRC enquiries into over-claimed R&D have surged since 2023. If you claim R&D relief, ensure the technical narrative and cost apportionment are professionally prepared. HMRC rejection rates on poorly-prepared claims have climbed sharply.
• Sector-wide compliance exercises: certain industries (online sellers, property landlords, construction, dental practices) face periodic sector-wide reviews. If you’re in a target sector and your figures are out of line with norms, you may be selected.
• Discrepancies on incorporation, sale, or company strike-off: any major lifecycle event triggers a closer look at the surrounding tax history.

What To Do If You’ve Already Made One of These Mistakes

The instinct on discovering a tax error is often denial or paralysis — neither is helpful. HMRC treats voluntary disclosures dramatically better than discoveries they make themselves. Penalties range from 0% (unprompted disclosure, careful behaviour) to 100% (deliberate, concealed). The gap is enormous.

Step 1: Stop the bleeding

Whatever the mistake is, ensure it isn’t continuing. If you’ve been claiming personal expenses through the business, stop today. If you haven’t registered for VAT and you should have, register immediately. If you’re operating outside IR35 with a contractor who really should be inside, change the engagement now. Continuing errors compound penalties.

Step 2: Quantify the exposure

Before approaching HMRC, you need to know what’s at stake. Calculate the unpaid tax over each relevant year, the interest that has accrued (currently 7.75% on unpaid tax), and the likely penalty range under HMRC’s framework. Get an accountant or tax adviser to do this with you — DIY calculations on complex situations almost always understate the position.

Step 3: Make a voluntary disclosure

HMRC’s Digital Disclosure Service handles most voluntary disclosures. For complex cases, the Contractual Disclosure Facility (Code of Practice 9) applies. Unprompted disclosures attract substantially lower penalties — often 0–15% versus 30–70% for prompted disclosures. The disclosure must be complete and accurate; partial disclosure is treated like concealment if discovered.

Step 4: Settle and remediate

HMRC will usually agree a time-to-pay arrangement for significant settlements, especially if the business shows reasonable behaviour. Remediation means putting the process in place so the mistake won’t recur — typically improved record-keeping, professional payroll, monthly bookkeeping reviews, and an annual tax planning meeting.

Frequently Asked Questions

How far back can HMRC go on tax mistakes?

The standard period is 4 years for innocent errors, 6 years for careless errors, and 20 years for deliberate or concealed errors. So if you under-paid corporation tax in 2018 due to a careless calculation, HMRC could still raise an assessment in 2026. The longer the period of error, the more it is in your interest to disclose voluntarily.

What’s the penalty for late VAT registration?

The penalty depends on how late and HMRC’s view of behaviour. Currently, late-registration penalties range from 5% (registration within 9 months of the date you should have registered, careful behaviour) to 15% (more than 18 months late, careless behaviour). Plus interest at 7.75% on the unpaid VAT. Deliberate failure to register can attract penalties of 70%+ of the tax owed.

Can I reverse a wrong incorporation decision?

Yes, but it’s painful. Disincorporating (transferring trade and assets back to a sole trader) triggers tax events on every asset transferred, including any goodwill, plant, and any property held by the company. Disincorporation relief was abolished in March 2018, so there is no special relief any more. The economics rarely justify reversing the decision; it’s usually better to optimise within the company structure or wind it up and start afresh.

If I claim R&D and HMRC challenges it, what happens?

HMRC will request the technical narrative, project records, cost apportionment, and evidence that the work qualified under BEIS guidelines. If the claim is poorly supported or doesn’t qualify, HMRC will deny it and recover the tax credit or relief. For “carelessly inflated” claims, penalties can be 0–30% of the tax. For deliberately inflated claims, 70–100%. Always use a specialist R&D adviser, not a general accountant, for claims above £20k of qualifying spend.

Should I disclose if I’m not sure I’ve made a mistake?

The safe answer is: get professional advice first. Premature disclosure of something that turns out not to be an error can create unnecessary problems. But if professional review confirms a mistake, disclose promptly — penalties for prompted disclosures (after HMRC contact) are typically twice as high as unprompted ones. Sitting on a known error and hoping it isn’t found is the most expensive option.

How often should I review my tax position?

For most owner-managed businesses, an annual tax planning meeting in autumn (before the calendar year-end and the January Self Assessment deadline) and a year-end review in the final month of the company’s accounting period are the minimum. For higher-stakes businesses — multi-property landlords, technology companies claiming R&D, businesses with international elements, or anyone with combined personal-and-business turnover above £500,000 — quarterly check-ins make more sense. The biggest tax savings nearly always come from decisions made before a transaction or year-end, not after. A standing relationship with a Chartered Tax Adviser is the single highest-leverage investment most business owners can make against the mistakes on this list. The cost of an annual planning meeting (typically £400–£1,200) is usually recovered many times over by a single well-timed pension contribution, expense reclassification, or dividend-timing decision.

Is HMRC enquiry insurance worth buying?

Tax investigation insurance — often offered by accountants as a fee-protection product — covers the professional fees of defending an HMRC enquiry. For most owner-managed businesses, it is worth having. A serious enquiry can run to £5,000–£20,000 of accountant time, even when the business ultimately owes nothing. The insurance premium (typically £150–£400 per year) buys peace of mind and removes the financial conflict between “fight properly” and “settle to limit costs.” It does not cover the tax or penalties owed, only the professional defence fees. Most reputable accountancy firms include it or offer it as a low-cost add-on.

Shamim Bhuiyan FCCA CTA BSc

Founder & Managing Director, The Tax Lead  ·  Tax Compliance Specialist

CTA-qualified specialist. Regulated by ACCA. Full biography →