Letters and requests
HMRC Discovery Assessment
What an HMRC discovery assessment means, the time limits involved, and how to challenge one.
At a glance
- Discovery assessments can only be used where specific legal conditions are met, not simply whenever HMRC wants to assess more tax
- Time limits for discovery vary significantly depending on whether careless or deliberate behaviour is alleged
- Discovery cases often relate to older years, making evidence harder to gather
- Challenges often focus on whether the legal conditions for discovery were met, not just the figures
- Interest and any penalty should be checked separately from the tax figure itself, since either can contain its own errors
- Where multiple years are covered, each year's discovery conditions and time limit should be checked individually
A discovery assessment is HMRC's mechanism for assessing additional tax after the normal enquiry window for a return has already closed, and it can only be used where specific legal conditions are met — generally that HMRC has discovered a loss of tax, and that the loss wasn't something HMRC could reasonably have been expected to know about based on information already available at the time the enquiry window closed. Discovery assessments often surface years after the return in question, which can make gathering evidence more difficult.
The time limits for a discovery assessment vary significantly depending on the behaviour involved — a longer period applies where careless or deliberate conduct is alleged than for an innocent error. Challenging a discovery assessment often focuses on whether the legal conditions for using discovery were actually met, not just on the tax figures themselves, since a discovery assessment issued outside its proper conditions can potentially be successfully appealed on that basis alone.
Establishing what information HMRC held, and when, is often central to challenging a discovery assessment, since the legal test partly turns on whether HMRC could reasonably have identified the issue earlier from information already available to it. This can mean reviewing what was included in the original return, any related correspondence, and what information HMRC would have had access to through third-party data at the relevant time, to assess whether the conditions for discovery were genuinely met.
Interest and any penalty on a discovery assessment are calculated separately from the tax itself, and it's worth checking both independently, since an error in either can sometimes be identified even where the underlying discovery assessment is otherwise valid. Where multiple tax years are covered by a single discovery assessment, each year should also be checked on its own facts, since the conditions for discovery, and the applicable time limit, can genuinely differ between years.
How to prepare
Taking a structured, evidence-led approach from the outset — rather than reacting to each request as it arrives — tends to produce a faster and less stressful outcome.
- 1
Read the discovery assessment notice carefully to identify the stated reasons and period
- 2
Gather the original return and any related correspondence from that time
- 3
Assess what information HMRC held or could reasonably have accessed at the relevant time
- 4
Calculate whether the figures in the assessment are accurate, independently of the legal challenge
- 5
Note the appeal deadline and decide on your approach before it passes
- 6
Check the interest and any penalty calculation separately from the core tax figure
- 7
Where multiple years are covered, review the discovery conditions for each year individually
What documentation to gather
- The discovery assessment notice, including the stated reasons and time period
- The original return and any correspondence from the relevant enquiry window
- Evidence relevant to the specific loss of tax alleged
- Records showing what information was available to HMRC at the relevant time
- A note of the appeal deadline stated on the assessment
- A breakdown of interest and any penalty calculated separately from the core tax figure
- Details of each tax year covered, if the assessment spans more than one
Common mistakes to avoid
These are the errors we see most often in practice — being aware of them in advance can help you avoid an entirely preventable setback.
Focusing only on disputing the figures without considering whether discovery was validly used
Assuming an old tax year is automatically out of reach, without checking the actual time limit
Not gathering evidence about what HMRC knew at the relevant time
Missing the appeal deadline while assessing the strength of the case
Underestimating how much a successful legal challenge can turn on procedural conditions alone
Assuming interest and penalty figures are automatically correct because the tax assessment looks right
Treating a multi-year discovery assessment as a single block rather than checking each year separately
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What happens next
Knowing the typical sequence of events helps you feel more in control and know roughly what to expect at each stage.
You review the assessment and identify both the figures and the legal basis for discovery
You gather evidence relevant to both the amount and whether the conditions were met
You appeal within the stated deadline if either the figures or the legal basis are disputed
HMRC considers the appeal, or it proceeds to independent review or tribunal
The final decision either confirms, adjusts, or cancels the discovery assessment
Interest and penalty calculations are reviewed alongside the core tax figure for accuracy
Where multiple years are involved, each is assessed on whether the discovery conditions were genuinely met
Frequently asked questions
The questions we're asked most often about this specific situation, answered in plain English.
Can HMRC always issue a discovery assessment?
No — specific legal conditions must be met, generally including that HMRC has discovered a loss of tax that it couldn't reasonably have identified earlier from information already available.
How far back can a discovery assessment go?
The time limit depends on the behaviour involved, with longer periods applying where careless or deliberate conduct is alleged compared to an innocent mistake.
Can I appeal a discovery assessment?
Yes, and appeals often succeed by challenging whether the legal conditions for discovery were actually satisfied, in addition to disputing the tax figures themselves.
Can a discovery assessment be challenged on a technicality?
Yes — successfully showing that the legal conditions for discovery weren't met can result in the assessment being cancelled, regardless of the underlying tax figures.
Does HMRC need new evidence to issue a discovery assessment?
Generally yes, in the sense that the loss of tax must not have been something HMRC could reasonably have identified earlier from information already available to it.
How is interest calculated on a discovery assessment?
Interest typically runs from the original due date for the tax year in question, regardless of when the discovery assessment is actually issued, which can result in a significant additional amount for older years.
Can I dispute the interest on a discovery assessment separately from the tax?
Yes, interest calculations can contain their own errors, such as an incorrect start date, and can be queried independently of the underlying tax figure.
Does a discovery assessment automatically include a penalty?
Not necessarily — a penalty depends on the behaviour involved, and its own conditions and calculation should be checked separately from the assessment itself.
What if only some of the years in a discovery assessment are valid?
Each year needs to meet the discovery conditions independently, so it's possible for parts of a multi-year assessment to be successfully challenged while others stand.
Can I get professional help reviewing a discovery assessment?
Given how much can turn on technical legal conditions as well as the figures themselves, professional review is often worthwhile for a discovery assessment specifically.
What's the difference between careless and deliberate behaviour for time limit purposes?
Careless behaviour generally means a lack of reasonable care was taken, while deliberate behaviour involves a knowing inaccuracy, and each carries a different, longer time limit for HMRC to raise a discovery assessment.
Can I settle a discovery assessment without going to tribunal?
Yes, many discovery assessments are resolved through negotiation and agreement with HMRC directly, with tribunal reserved for cases where the disagreement can't be resolved that way.
Read the full guide
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