How far back HMRC can go depends on the behaviour behind the error. The standard assessment time limits are 4 years where reasonable care was taken, 6 years where the error was careless, and up to 20 years where it was deliberate or where you failed to notify HMRC that tax was due at all. Those limits sit alongside a much shorter window for opening an enquiry into a return that was actually filed.
The two clocks that matter
People usually mean one of two different questions. The first is how long HMRC has to open an enquiry into a filed return: under section 9A of the Taxes Management Act 1970 that is broadly 12 months from the date the return was delivered, longer for late-filed returns. The second is how many past years HMRC can assess once it believes tax was underpaid, and that is where the 4, 6 and 20 year limits apply.
When can HMRC go back 4 years?
Four years is the baseline. Even where you took reasonable care and made an innocent error, HMRC can assess tax for up to 4 years after the end of the tax year concerned. Beyond that, an honest mistake is out of reach.
When can HMRC go back 6 years?
Where the loss of tax was brought about carelessly, the limit extends to 6 years. Carelessness means failing to take reasonable care: poor records, guessed figures, expenses claimed without checking the rules.
When can HMRC go back 20 years?
Where the behaviour was deliberate, or where you failed to notify chargeability (income HMRC was never told about at all, a common issue for landlords with undeclared rents), the limit stretches to 20 years. Behaviour categorisation is therefore worth arguing about: the difference between careless and deliberate can be fourteen years of assessments plus a very different penalty position.
How does HMRC reopen a closed year?
Once the enquiry window on a return has passed, HMRC generally needs to make a discovery assessment, which has its own conditions: broadly, HMRC must discover something it could not reasonably have known from the return, or show careless or deliberate behaviour. Discovery disputes are one of the most commonly contested areas in tax investigations.
Should I just wait out the time limits?
No. Sitting on a known error converts an innocent position into a deliberate one, which extends the clock to 20 years and worsens penalties. Coming forward voluntarily, through a disclosure facility where one applies, consistently produces better outcomes than being found.
If HMRC is already asking questions
Time limit arguments are technical and worth making properly, which means professional representation, which means fees. Tax investigation insurance exists so that defending the right position never depends on the cost of advice: see who pays the accountant in an HMRC enquiry, or get a quote.