A brown envelope lands on the mat and the words "compliance check" jump off the page. Your stomach drops. Before you do anything, take a breath, because a compliance check is not an accusation, and panicking is the one thing that genuinely makes it worse.
A compliance check (HMRC's term for what most people call a tax investigation or enquiry) is simply HMRC checking that your tax position is right. Plenty of checks end with no change at all, and some even end with a refund. What matters is how you respond: calmly, on time, and with the right facts to hand.
This guide explains what an HMRC compliance check actually is, what HMRC can and can't do, how penalties are worked out, and the practical steps to take from the day the letter arrives. It's written for sole traders, landlords and limited company directors who'd rather get this right than guess.
What is an HMRC compliance check?
A compliance check is HMRC's way of making sure you're paying the right amount of tax at the right time, and getting the right allowances and reliefs. HMRC can check almost any tax: your Self Assessment return, a Company Tax Return, PAYE records, VAT, or the underlying accounts and tax calculations.
Checks come in different shapes. Some are a single, narrow query about one figure on your return ("aspect" checks). Others look at a whole return or a whole year ("full" enquiries). A few are part of HMRC's broader fraud work and run under a formal code of practice. The letter you receive will tell you which taxes and which years HMRC is looking at.
The golden rule throughout: keep meeting your normal obligations. If you've got a Self Assessment return or a tax payment due while the check is open, file and pay as usual. A compliance check doesn't pause your other deadlines.
Why has HMRC opened a check on me?
HMRC rarely tells you exactly why, and you're not entitled to know. In practice, checks are triggered by a mix of risk profiling and chance.
Common triggers we see in practice include:
- Figures that look out of step with similar businesses, or a sharp swing year on year.
- A large or unusual expense claim, or persistent losses on a rental property or side trade.
- Information from a third party that doesn't match your return (banks, letting agents, online marketplaces and other countries all share data with HMRC).
- Simple inconsistencies, such as a VAT return that doesn't tie up with the income on your accounts.
- A random selection. Yes, genuinely random checks happen, and they're nobody's fault.
Being checked does not mean HMRC thinks you've done something wrong. It means a computer, a tip-off or a roll of the dice flagged your record for a closer look.
What can HMRC ask for, and what are my rights?
HMRC can ask to see records and documents that are reasonably required to check your tax position: bank statements, invoices, receipts, mileage logs, rental statements, and your bookkeeping records. They can ask to visit your business premises or your home (if you work from home), or invite you to their office.
You have real protections, though, and it pays to know them:
- You don't have to meet HMRC in person. You can ask to deal with everything in writing.
- You can challenge a request you think is unreasonable or not relevant to the check.
- You can have an adviser with you at every stage, including any meeting. Your accountant or tax adviser can deal with HMRC on your behalf.
- You're entitled to a written explanation of what's being checked and why HMRC wants each piece of information.
- You can claim a "reasonable excuse" (such as serious illness or a bereavement) if you genuinely can't meet a deadline, and you can ask for more time.
If HMRC issues a formal information notice and you don't comply without a reasonable excuse, penalties can follow, so don't simply ignore it. The right move is to engage, ask questions, and push back through the proper channels where a request really is excessive.
What should I do when the letter arrives?
Here's a clear, calm sequence to work through.
- Read the letter properly. Note which taxes and years are in scope, the deadline to respond, and the name and contact details of the officer handling your case.
- Don't ignore it, and don't rush a reply. A late or panicked response helps no one. If the deadline is tight, ring the officer and ask for reasonable extra time.
- Tell your accountant straight away. The earlier a professional is involved, the more they can do to frame the facts and keep the check narrow.
- Gather the records HMRC has asked for, and only those. Resist the urge to send your entire filing cabinet; supply what's requested, neatly and clearly.
- Keep meeting your other deadlines. File any return that's due and make any payment on its normal date while the check runs.
- Keep a paper trail. Save copies of everything you send and receive, with dates. If you spot a genuine error in your own figures, tell HMRC, because an unprompted disclosure can sharply cut any penalty (more on that below).
If your affairs are at all involved, this is exactly the point to bring in Zmartly's tax advisory team rather than going it alone. Good representation often shortens the check and reduces the final bill.
How are compliance check penalties worked out?
If HMRC finds an inaccuracy that means you've underpaid tax, it can charge a penalty. The size of that penalty isn't arbitrary. It's a percentage of the extra tax (the "potential lost revenue"), and the percentage depends on two things: your behaviour, and the quality of your disclosure.
Behaviour falls into three bands:
- Careless: you didn't take reasonable care to get it right.
- Deliberate: you knew the return was wrong when you sent it.
- Deliberate and concealed: you knew it was wrong and took active steps to hide it.
Disclosure is either:
- Unprompted: you tell HMRC before you have any reason to think they've spotted it.
- Prompted: you tell them at any other time.
Put together, that gives six penalty ranges. These are the figures published by HMRC in compliance check factsheet CC/FS7A:
| Behaviour | Unprompted disclosure | Prompted disclosure |
|---|---|---|
| Careless | 0% to 30% | 15% to 30% |
| Deliberate | 20% to 70% | 35% to 70% |
| Deliberate and concealed | 30% to 100% | 50% to 100% |
Where you land within a range depends on how much you help HMRC, summarised as "telling, helping and giving access". The more openly and quickly you cooperate, the closer you get to the bottom of the range. A genuine mistake where you took reasonable care can carry no penalty at all.
On top of any penalty, you'll owe the underpaid tax plus interest. If a check finds you've underpaid, you normally have 30 days to pay the extra tax due, with interest running from the original due date.
Illustrative example: how a careless error is penalised
Illustrative example. Daniel is a sole trader. A compliance check finds he wrongly claimed £4,000 of private costs as business expenses across one tax year. As a higher-rate taxpayer for 2025/26, the higher rate of Income Tax is 40%, so the extra Income Tax (the potential lost revenue) is:
£4,000 x 40% = £1,600 of additional tax.
HMRC treats this as careless rather than deliberate. Daniel cooperates fully and his accountant helps HMRC reach the answer quickly, but because HMRC opened the check first, it's a prompted disclosure. The careless, prompted range runs from 15% to 30%. Given his full cooperation, HMRC settles on the bottom of that range, 15%:
£1,600 x 15% = £240 penalty.
So Daniel pays the £1,600 underpaid tax, plus the £240 penalty, plus interest on the late-paid tax. Total tax and penalty: £1,840, before interest.
Now compare the alternative. If Daniel had spotted the error himself and made an unprompted disclosure, the careless range would have started at 0%. With full cooperation, the penalty could have been nil, saving the entire £240 and very likely keeping the whole episode shorter and calmer.
The lesson is blunt: finding and fixing your own mistakes before HMRC does is worth real money. (Figures are illustrative and rounded for clarity; your own position will depend on your full circumstances.)
What if I disagree with HMRC's decision?
You don't have to accept the outcome. If HMRC makes a decision you can appeal, the letter will say so and explain your options. You normally have 30 days to act, so diary it the moment the decision arrives.
You usually have these routes, and you can combine some of them:
- Send new information. Within 30 days, you can give the officer fresh facts and ask them to reconsider.
- Ask for a statutory review. A different HMRC officer, not involved in your case, reviews the decision afresh. It's free and often quicker than a tribunal.
- Try Alternative Dispute Resolution (ADR). An impartial HMRC mediator helps both sides reach agreement. ADR can run alongside a review or appeal.
- Appeal to the tax tribunal. The First-tier Tribunal is independent of HMRC and hears the case if you can't agree.
You may also be able to delay paying the disputed amount until the matter is resolved, depending on the type of decision. Don't let the 30-day clock run out while you decide, as missing it can cost you the right to challenge.
How do I reduce the chance of a check?
You can't make yourself check-proof, but you can make yourself a low-risk, easy-to-defend taxpayer.
- Keep clean, contemporaneous records. Reconciled bank feeds, digital receipts and a clear audit trail answer most queries before they grow. This is the heart of good bookkeeping.
- File and pay on time. For Self Assessment, the online filing and balancing payment deadline is 31 January following the tax year, with payments on account due 31 January and 31 July.
- Be consistent. If your figures swing dramatically, add a short note in the white space of your return explaining why.
- Claim only what's allowable, and keep the evidence for every claim.
- Get a professional eye on anything unusual before you file, not after a letter arrives.
Sole traders, landlords and limited company directors all face slightly different risk profiles, but the principle is the same: accurate records and timely filing are the best insurance you can buy.
Want help dealing with an HMRC compliance check?
If you've had a letter, or you simply want your returns checked over before HMRC ever does, talk to us. Book a free 20-minute call with a Zmartly accountant and we'll explain exactly where you stand and what to do next, in plain English.
Frequently asked questions
How long does an HMRC compliance check take?
It varies a lot. A narrow aspect query about a single figure can wrap up in a few weeks, while a full enquiry into complex affairs can run for many months. Responding promptly and fully is the single biggest thing you can do to keep it short.
Do I have to attend a meeting with HMRC?
No. You're entitled to deal with the check in writing if you prefer, and you can have your accountant or adviser present at any meeting that does take place. Many checks are resolved entirely by correspondence.
Can HMRC charge a penalty if I made an honest mistake?
If you took reasonable care and still got something wrong, HMRC may charge no penalty at all. Penalties are based on behaviour, and a genuine, well-documented honest mistake sits at the most lenient end of the scale.
What happens if HMRC finds I've underpaid tax?
You'll normally have 30 days to pay the additional tax, plus interest from the original due date, and possibly a penalty. The penalty percentage depends on your behaviour and how openly you disclosed the issue.
Should I tell HMRC if I find a mistake before they do?
Usually yes. Telling HMRC before they have reason to suspect a problem counts as an unprompted disclosure, which can reduce a careless-error penalty to as little as 0%. It's almost always better to come forward than to wait.
Can I appeal an HMRC compliance check decision?
Yes. You normally have 30 days to challenge a decision. You can send new information, ask for a free statutory review by a different officer, try Alternative Dispute Resolution, or appeal to the independent tax tribunal.
