How are HMRC penalties calculated?

HMRC does not charge a flat penalty. It works out the extra tax you owe, called the potential lost revenue, and applies a percentage to that figure.

The percentage depends on two things: how the inaccuracy came about, and how you behaved once HMRC or you found it. A genuine mistake made despite reasonable care can attract no penalty at all. A deliberate and concealed error attracts the highest percentage in the range.

Within each behaviour band, HMRC then reduces the percentage based on the quality of your disclosure. Telling HMRC everything, helping them establish the size of the problem, and giving them access to records all count in your favour.

The framework sits in Schedule 24 Finance Act 2007 for inaccurate returns and Schedule 41 Finance Act 2008 for failing to notify a tax liability, such as not registering for VAT or Self Assessment when required.

Both schedules use the same structure. There is a behaviour category, a disclosure type, and a percentage range within that combination. HMRC caseworkers must show their working, and the resulting figure should sit somewhere in the statutory range for your circumstances, not above it.

Where you have taken reasonable care and simply made an error, no penalty applies at all. The system is designed to be proportionate to fault, not to punish honest mistakes.

Penalties can fall on the company, on you personally, or on both, depending on which tax and which return is involved. A Corporation Tax inaccuracy penalty is usually charged to the company; a personal Self Assessment penalty is yours alone.

As director, you're often the person HMRC deals with during the enquiry, so your conduct in meetings and correspondence directly affects the disclosure reduction. Being slow to respond or withholding records can push the percentage upward even where the underlying error was innocent.

If your company has multiple errors across different years, HMRC calculates the potential lost revenue separately for each, then applies the relevant behaviour and disclosure reduction to each one. The total penalty can look large even when individual errors are modest.

Because the penalty is tax-geared, the cost scales directly with how much tax was underpaid, not with how the error happened. A small inaccuracy on a large tax bill can still produce a meaningful penalty.

On top of the penalty, HMRC charges interest on the unpaid tax from when it was originally due, so the total bill is usually tax plus interest plus penalty. Growth plan clients get free tax investigation insurance included, which covers professional fees if HMRC opens an enquiry — see /fees for details.

Getting professional representation early often reduces the final penalty percentage, because a specialist knows how to present disclosure in the way HMRC's guidance rewards.

Don't assume the penalty is fixed once HMRC states a figure. The percentage within the range is negotiable based on cooperation, and caseworkers sometimes start high.

Don't delay responding to HMRC's letters. Slow engagement is treated as poor cooperation and can cost you the maximum available reduction.

Don't confuse the penalty percentage with the tax bill itself. Directors sometimes panic at a headline percentage without checking it applies to the tax underpaid, not turnover or total income.