The catch is that the burden is on you, not on HMRC, to show the assessment is wrong and by how much. The clock is short: you have 30 days to ask for a review or appeal. To take an appeal to the tribunal you normally have to pay the disputed VAT first, or apply for hardship. And because the penalty and interest are worked out on the tax, they fall as the assessment falls. So a big assessment is challengeable, but only if you move quickly and with the right evidence.
A VAT assessment can arrive as a single, alarming number, often after an inspection, and it is easy to assume it is fixed and simply has to be paid. It is not fixed. But the rules for getting it reduced are unforgiving on timing and on evidence, and the way you respond in the first few weeks tends to decide how it ends.
Common mistakes & confusionsWhere a VAT assessment trips businesses up
- Assuming the assessment is the final bill. An assessment is HMRC's best-judgement estimate, not a settled liability. It can be reduced or withdrawn on the right evidence, but treating it as fixed, and simply paying or ignoring it, throws away the chance to bring it down.
- Missing the 30-day clock. You have 30 days from the decision to ask for a review or appeal to the tribunal. Miss it and your options narrow sharply. The single most expensive mistake is letting the deadline pass while deciding what to do.
- Thinking you must prove HMRC acted unreasonably. You don't. The tribunal's job is to find the correct amount of tax, and the burden is on you to show what that is. You win by evidencing the right figure, not by attacking how HMRC reached theirs.
- Not realising you may have to pay first. To have an appeal heard by the tribunal, the disputed VAT normally has to be paid or deposited, unless you apply for hardship. Businesses that overlook this find the appeal stalls before it starts.
- Arguing the process instead of the numbers. Complaining that the officer was wrong to assess rarely helps. Even a flawed step does not automatically cancel the whole assessment. What moves the figure is better evidence of your actual sales, purchases and records.
- Going quiet and letting it harden. An unanswered assessment does not go away, it becomes enforceable and interest runs on it. Engaging early, with records and a clear counter-figure, is what keeps the number negotiable.
What a VAT assessment actually is
A VAT assessment is the tool HMRC use when they cannot simply take your returns at face value. Under section 73 of the VAT Act 1994, where returns are missing, late or appear incomplete or incorrect, HMRC may assess the amount of VAT they believe is due to the best of their judgement. The key words are best of their judgement: an assessment is an estimate, built from whatever information HMRC have, not a precise calculation of your actual liability.
In practice an assessment usually lands after a compliance check or inspection, and it is often on the high side, because HMRC are estimating in the gaps and will tend to resolve doubt in the revenue's favour. That is exactly why an assessment is a starting position to be tested, not a final bill to be paid without question.
How "best judgement" actually gets calculated
It helps to know how these numbers are built, because that is where they can be challenged. HMRC rarely have your full records, that is usually why they are assessing, so they estimate. A common method is to take a sample, a short period of observed takings, a few weeks of till rolls, or a mark-up applied to purchases, and scale it up across the whole assessment period. Small assumptions get multiplied across years, so a modest daily figure can become a very large assessment.
That is both the strength and the weakness of a best-judgement assessment. It gives HMRC a defensible number from thin information, but every assumption inside it is a place where it can be wrong: an unrepresentative sample period, a mark-up rate that does not fit your business, wastage or staff meals not allowed for, or a suppression rate assumed rather than shown. Understanding the method is the first step to taking the figure apart.
Show that the sample fortnight fell over a local festival, when takings were far above normal, and produce the till and bank records for the rest of the year, and the figure can fall sharply. The assessment was only ever an estimate built on a small, unrepresentative window.
Best judgement means it can be challenged, and reduced
Because an assessment is only a best-judgement estimate, it can be displaced. The leading case, Pegasus Birds [2004] EWCA Civ 1015, is clear about how a tribunal approaches one: its primary task is to find the correct amount of tax on the evidence available, and the burden of showing that figure rests on the taxpayer. So the door to reducing an assessment is open, but you are the one who has to walk through it, with the numbers.
This changes how you should fight it. You do not have to prove HMRC behaved unreasonably, and you do not get the whole assessment cancelled just because one step in their method was flawed. What works is evidence: showing, figure by figure, what your real liability was. An assessment challenged with good records and a credible counter-calculation is very often reduced, sometimes substantially.
The 30-day clock: review or appeal
The most important thing to know on day one is that you have 30 days. When HMRC issue an assessment or a decision, you have 30 days to either accept their offer of a review, which puts the case in front of a different HMRC officer, or to appeal to the independent First-tier Tribunal. You can ask for the review first and still appeal afterwards if it does not go your way.
Thirty days is not long to gather records and frame a response, and the deadline is the trap. Businesses that spend three weeks deciding whether to challenge, then scramble, are the ones who lose options. The moment an assessment arrives, the clock is the first thing to manage, not the last.
HMRC Response Support puts a specialist on your assessment, to challenge the figure and protect your position while the 30-day clock is running.
You may have to pay before the tribunal will hear you, unless it causes hardship
There is a further catch that surprises people. To have an appeal against an assessment heard by the tribunal, the disputed VAT normally has to have been paid or deposited with HMRC first. For a large assessment that is a serious cash-flow problem in its own right.
The relief valve is hardship. If paying the VAT up front would cause you genuine hardship, you can apply, to HMRC or to the tribunal, to have the appeal heard without paying first, and the tribunal's decision on hardship is final. Getting that application right early keeps your appeal alive without draining the business, but it needs to be handled alongside the main challenge, not as an afterthought.
The penalty and interest ride on the assessment
An assessment rarely arrives alone. Interest runs on the tax from the date it should have been paid, and HMRC will often add an inaccuracy penalty, charged as a percentage of the tax under Schedule 24 to the Finance Act 2007. That percentage depends on your behaviour, higher for careless or deliberate errors, and it can be reduced for telling HMRC, helping them work out what is due, and giving access to your records.
This matters for two reasons. First, because the penalty is a percentage of the tax, every pound you take off the assessment takes a slice off the penalty as well, so challenging the figure pays twice. Second, how you engage, cooperatively and with records, directly affects the penalty rate. Going silent or combative can turn a careless-error penalty into something far worse. Handling the assessment and the penalty together, rather than as separate fights, is usually what produces the best overall result.
What actually gets an assessment reduced
Reducing an assessment is a matter of evidence and method, not indignation. In practice, the figure comes down when you can reconstruct the records HMRC did not have, produce bank statements, till data or supplier information that contradicts their estimate, or show that the assumptions behind their calculation, a mark-up rate, a sample period scaled up across years, a suppression percentage, do not hold for your business.
Much of this is done by engaging with the officer and putting a clear counter-figure on the table, backed by documents, often well before any tribunal hearing. It is detailed, deadline-driven work, and the quality of the response in the first few weeks tends to set the ceiling on the outcome. This is where a specialist who has answered HMRC assessments before earns their fee.
When you might need expert VAT advisory
A VAT assessment is one of the few VAT problems where the size of the bill and the shortness of the deadline collide, and where the response in the first few weeks tends to decide the outcome. In practice, the situations below are where a senior specialist's read meaningfully improves the outcome:
- A large or unexpected VAT assessment has landed and you need to know, quickly, whether and how to challenge it
- The 30-day deadline to request a review or appeal is running and you want the response framed properly
- You believe HMRC's figure is too high and need the right records and counter-calculation put together
- The assessment was built by scaling up a sample period or a mark-up, and you want the method itself challenged
- Paying the disputed VAT up front would cause hardship and you need that application made alongside the appeal
- The assessment came with a penalty and interest, and you want both brought down with the tax
- You are an accountant supporting a client through an assessment and want a second, specialist read before the deadline
Whether you're a business facing an assessment or an accountant supporting a client through one, we focus on the VAT questions where extra expertise pays off, and we work in plain English.