Most people think that once HMRC accepts a tax return, the matter is closed for good. That is not always true. According to HMRC’s own figures, £46.8 billion in tax went unpaid in the 2023 to 2024 tax year alone, and HMRC has the tools to chase that money, sometimes years later.
One of those tools is a discovery assessment, a formal tax charge that HMRC can raise even after the normal window for checking your return has passed. This post covers what a discovery assessment is, when HMRC can use it, how far back they can go, and what you can do if you receive one.
An HMRC discovery assessment is a formal tax charge that HMRC raises when it finds out that tax has been underpaid on a return that is no longer open for a standard check. Under Section 29 of the Taxes Management Act 1970, HMRC can raise an assessment outside the normal window if an officer finds that income or gains have not been properly taxed. This can affect Income Tax, Capital Gains Tax, Corporation Tax, and VAT. It can apply to individuals, company directors, property investors, business owners, and sole traders.
The word that matters most here is discovery. For HMRC to use this power, it must have found something genuinely new. It cannot reopen a return just because a new officer has looked at old information and formed a different view.
The discovery must be based on information that was not available, or not properly considered, when the original check was closed. This is one of the most important points to understand about how this power works.
This distinction also matters because it is one of the main reasons a discovery assessment can be challenged. If HMRC is relying on information it already had at the time, the assessment may not hold up. A specialist tax adviser can look at the basis of the assessment and tell you if it meets the legal test.
HMRC can raise a discovery assessment in several situations. The most common is when someone has not reported all of their income or gains on a Self Assessment tax return. HMRC may find this through its own data checks, a tip from a third party, or information shared by an overseas tax authority. It can also happen when a tax relief or deduction has been claimed incorrectly and HMRC was not able to spot this at the time.
Different types of taxpayers are affected in different ways. Here are some common examples:
HMRC may act if rental income has not been declared, or if a Capital Gains Tax bill from selling a property was not reported.
HMRC now gets data directly from crypto exchanges and uses it to find gains that were not reported through Self Assessment.
Issues can come up around undeclared dividends, director’s loan accounts, or income that was not properly reported.
A discovery assessment can also come up during a wider tax check. If HMRC is already looking into one part of your tax affairs and finds something off in a different year or a different tax type, it can use a discovery assessment to deal with that too. This is one reason why it is important to handle any HMRC contact carefully and with the right support.
How far back HMRC can go depends on why the tax was underpaid in the first place. There are three time limits to know about.
If you made an honest error and took reasonable care when filing your return, HMRC has four years from the end of that tax year to raise a discovery assessment.
If HMRC decides you did not take reasonable care, for example, by not keeping proper records or missing something you should have caught, it has six years to act.
If HMRC believes tax was hidden on purpose or fraud was involved, it can go back up to 20 years.
A lot of people think that once a few years have passed, HMRC can no longer look at their returns. That is not true. If HMRC can show that careless or deliberate behaviour caused a tax loss, the window stays open much longer than most people expect.
This is also why keeping good records matters. HMRC may check your records to make sure you are paying the right amount of tax, Low Incomes Tax Reform Group, and GOV.UK’s record-keeping guidance sets out how long you should hold on to them. If you are unsure whether your past returns are in order, a specialist tax adviser can help you review your position before HMRC does.
HMRC does not just wait for someone to come forward. It actively looks for tax that has not been paid, and it has some very powerful tools to help it do that.
The main tool HMRC uses is called Connect. In the 2024 to 2025 tax year, leads provided by Connect helped HMRC bring in an extra £4.6 billion in tax revenue. Many firms use artificial intelligence and advanced algorithms to sift through over a billion data items and compare them against what individuals and businesses report on their tax returns. The data it pulls from includes:
Connect is not the only route. HMRC also receives tip-offs from members of the public, former business partners, and even ex-spouses. It shares information with overseas tax authorities under international agreements, which helps it spot undisclosed foreign income and offshore accounts. For crypto investors, HMRC receives data directly from UK crypto exchanges to identify gains that have not been reported.
A discovery can also come out of a wider tax check already in progress. If HMRC is looking at one part of your tax affairs and spots something in a different year or tax area, it can use a discovery assessment to deal with that too. This is why getting proper support from a tax investigation specialist matters from the very start.
Yes, you can. Not every discovery assessment HMRC sends out is legally correct, and it is worth knowing your rights before you assume you have to pay.
For a discovery assessment to be valid, it must meet the conditions set out in Section 29 of the Taxes Management Act 1970. In simple terms, this means two things must be true:
If HMRC is basing its assessment on information it already had when your return was originally checked, that could be a valid reason to challenge it. The same applies if HMRC has used the wrong time limit, for example, treating an honest mistake as a careless one to get extra time to act.
Once you receive a discovery assessment, you have 30 days to decide what to do. If you miss that window, the assessment becomes final, and you will owe the tax, interest, and any penalties in full.
This is a short amount of time, so getting advice quickly is important. The team at ETC Tax specialises in HMRC tax investigations and discovery assessment challenges. They can check whether the assessment is valid, advise on whether an appeal is worth making, and represent you throughout the process.
If a discovery assessment is confirmed, HMRC will work out how much extra tax you owe and add interest on top, going back to the date it was originally due. In most cases, penalties will apply too.
HMRC’s factsheet CC/FS7A sets out the penalties it may charge for inaccuracies in a return or document. The amount depends on why the error happened:
The good news is that penalties can be brought down. If you tell HMRC about an inaccuracy before you have any reason to believe they are about to find it, this is called an unprompted disclosure, and the minimum penalty is lower than for a prompted one. Being open, cooperative, and providing everything HMRC asks for, accurately and on time, all help reduce what you pay.
Making a voluntary disclosure before HMRC contacts you is consistently the most effective way to limit the financial impact. If your returns involve property income or unreported crypto gains, getting specialist advice early can help you put things right on the best possible terms.
Getting a discovery assessment in the post can feel alarming. The most important thing is not to ignore it. Here is what to do.
Start by reading the notice and noting three things: which tax year or years it covers, how much HMRC says you owe, and the reason they have given for raising it. This will help you and any adviser understand what you are dealing with.
You have 30 days from the date on the notice to respond. There are two options:
Thirty days is not long. Getting advice as soon as possible gives you the best chance of making the right decision. The team at ETC Tax can review the legal basis of the assessment, tell you if an appeal is worth making, and manage the whole process for you. We also help private clients and business owners make voluntary disclosures where past errors need to be corrected before HMRC acts.
HMRC has more than one way to check your tax. Knowing the difference between a standard enquiry and a discovery assessment can help you understand what you are dealing with and what rights you have.
| Standard Enquiry | Discovery Assessment | |
| When can it start? | Within 12 months of filing your return | After the 12-month window has closed |
| What triggers it? | A routine check or HMRC query | New information HMRC has found |
| What does it do? | HMRC may amend your existing return | HMRC raises a brand new tax charge |
| Time limits | Fixed 12-month window | 4, 6, or 20 years depending on the error |
| Right to appeal? | Yes, with its own rules | Yes, but under different rules |
The two processes follow different rules, have different time limits, and give you different rights. A standard enquiry results in HMRC updating your return. A discovery assessment is a separate, formal charge that sits on top of that.
Knowing which one HMRC is using matters because it affects what you can do next, how much time you have, and what the penalties could look like. This is exactly why getting professional advice early is so important. The team at ETC Tax can help you understand what HMRC is doing and what your options are, whether you are facing a tax investigation or dealing with a private client tax matter.
Receiving a discovery assessment can feel overwhelming, but you do not have to deal with it alone. Whether HMRC has already been in touch or you are worried that a past return may have errors, getting advice quickly gives you the best chance of a good outcome.
ETC Tax helps people across the UK with HMRC tax investigations, discovery assessment challenges, and voluntary disclosures. The team works with private clients, property investors, crypto investors, and business owners across the UK. Contact ETC Tax today to find out where you stand and what your options are.
Optimized by Seraphinite Accelerator