Nobody asks this question idly. It usually surfaces after a letter arrives, or after someone realises an old return might contain a mistake, or simply after a late-night conversation about a friend-of-a-friend who “got done for something from years ago”.
The good news is that the answer is not “forever”. HMRC works to time limits set out in law, and those limits follow a logic that is worth understanding — because the biggest factor in how far back HMRC can go is not HMRC. It is you.
The 12-Month Window Most People Have Never Heard Of
Start with the routine case. Once you file a Self Assessment return on time, HMRC generally has 12 months from the date you filed to open an enquiry into it. No particular suspicion is needed — within that window, HMRC can simply ask questions about anything on the return.
Once those 12 months pass without an enquiry, the return is broadly settled. Most returns sail through this window untouched: the overwhelming majority are never looked at individually at all. To reopen a closed year, HMRC needs something more — which is where the well-known numbers come in.
The 4, 6 and 20-Year Rules
If HMRC later discovers that tax was underpaid in an earlier year, how far back it can reach depends on why the tax was underpaid:
- 4 years — if you made an innocent error despite taking reasonable care
- 6 years — if the underpayment came from carelessness, such as sloppy or missing records
- 20 years — if the behaviour was deliberate, or key obligations (like registering for tax) were ignored
There is also an extended window of up to 12 years for some cases involving offshore income or assets, even where nobody acted deliberately. But for most sole traders and landlords, the ladder above is the one that matters.
Notice What the Ladder Is Really Measuring
The time limits are not really about time. They are about behaviour. Take reasonable care and even a genuine mistake generally closes after four years. Be careless and the window stretches to six. Act deliberately and two decades of your affairs are potentially back on the table.
This is the modern tax system in miniature: it is not designed to punish honest people harder, it is designed to distinguish honest people from the rest. The same principle runs through HMRC’s penalty rules — the system is getting smarter about who it pursues, not harsher on everyone.
So How Long Should You Keep Records?
This is the practical end of the question. If you are self-employed, HMRC expects you to keep your records for roughly five years after the 31 January filing deadline for each return. Keep them that long and you can answer any routine question that arises — and being able to answer quickly is exactly what makes enquiries short.
Good records do something subtler too: they are the evidence that you took reasonable care. The person with organised records who made an honest slip sits at the four-year end of the ladder. The person with no records at all has a much harder time arguing they were careful.
The Bottom Line
HMRC can normally question a return for 12 months after you file it, go back 4 years for honest errors, 6 for carelessness, and 20 for deliberate behaviour. You cannot control whether HMRC ever asks a question — but you can control which of those numbers applies to you, and that is decided by care and records, not luck.
123Tax keeps that side effortless: income, receipts and mileage recorded over WhatsApp as they happen, stored digitally and ready to answer any question — this year’s or a question about a return from years ago.