Most Self-Assessment mistakes that lead to an enquiry aren’t fraud. They’re small inconsistencies, an income figure that doesn’t match what a bank already reported, an expense claim that looks unusual for the trade, a return that reads differently to the one filed the year before. HMRC’s Connect system cross-references returns against bank data, Land Registry records, employer payroll, and digital platform reports automatically, so a mismatch doesn’t need a human to notice it first.Â
The standard enquiry window is 12 months from the date a return is filed, but discovery assessments let HMRC go back 4 years for innocent errors, 6 for careless ones, and up to 20 where deliberate evasion is suspected. This piece runs through the tax return errors that most reliably invite a closer look, and what actually keeps a return, and the client behind it, off HMRC’s list.Â
1. Missing the filing or registration deadline
An unfiled or late-registered return is itself a flag, and it’s the simplest one to avoid. Clients who register after 5 October, or file after 31 January, don’t automatically get investigated, but persistent lateness across multiple years signals disorganised record-keeping, which is exactly what an enquiry is designed to test.Â
2. Filing with estimated figures and not saying so
Using provisional figures is allowed when genuine records aren’t yet available, but only if the return says so explicitly and the actual figures follow later. A return quietly built on guesswork, with no flag and no follow-up, reads very differently to HMRC than one that’s transparent about its own limitations.Â
3. Leaving out income HMRC already knows about
This is where Connect does most of its work. Bank interest, dividends, rental income reported by letting agents, and payments through digital platforms are often already sitting in HMRC’s systems before the return is even filed. An incorrect tax return that omits any of these isn’t hiding anything, it’s just handing HMRC an automatic mismatch to query.
4. Suspiciously round numbers across every line
A return where every expense category ends in a neat £00 rarely reflects how a real business actually spends money. It’s not proof of anything on its own, but it’s exactly the kind of pattern that nudges a return from “unremarkable” to “worth a second look.”Â
5. Expense claims that don't match the trade
Home office costs for someone with no home-based work, or vehicle expenses for a business that doesn’t obviously need one, stand out precisely because HMRC has a rough sense of what expense ratios look like for most trades. A claim well outside that range needs a genuine explanation on file, not just a number in a box.Â
6. Getting property income relief wrong
Mortgage interest relief for landlords has been restricted to a basic-rate tax credit rather than a full deduction for several years now, and it’s still one of the most common errors on rental income pages. Claiming full interest relief, the old way is an easy, mechanical mistake, and an easy one for HMRC’s checks to catch.Â
7. Missing the new Making Tax Digital obligations
Sole traders and landlords with qualifying income over £50,000 are now required to keep digital records and submit quarterly updates rather than relying solely on the annual return. Clients who ignore this because “the January deadline is what matters” are building a compliance gap that shows up the moment HMRC checks whether the required updates were actually filed.Â
8. Records that can't back up the figures submitted
Even an honestly prepared return falls apart under an enquiry if the client can’t produce the receipts, invoices, or bank records behind it. HMRC record keeping rules require the self-employed and landlords to keep business records for at least 5 years after the 31 January submission deadline, and HMRC can charge a penalty of up to £3,000 for failing to maintain adequate records, separate from any tax owed.Â
9. Figures that don't line up with last year's return
A significant, unexplained swing in income or expenses from one year to the next is one of the more common triggers for a closer look, particularly when the client’s circumstances haven’t obviously changed. It’s not that the swing is wrong. It’s that nothing on the return explains it.Â
10. Not reporting capital gains, including crypto
Property disposals, share sales, and crypto asset transactions all need reporting, and crypto specifically has become a growing area of HMRC’s attention, given how exchanges now share user data directly with tax authorities. A gain that never appears on a return, but does appear in an exchange’s data feed, is close to a guaranteed enquiry trigger.Â
Why this matters more than the fine itself?
The real cost of any of these mistakes usually isn’t the initial correction. It’s what happens once a Self-Assessment audit actually opens. A narrow, single-issue enquiry might resolve in three to six months. A full enquiry into the whole return, or several years of returns, can run past a year, particularly if the records requested aren’t readily available. Â
Tax compliance UK rules don’t punish an honest error the same way they punish carelessness or concealment, but an enquiry doesn’t distinguish between the two until the client has already been through the process of proving which one it was.Â
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Most of what lands a return in front of an HMRC officer might look like fraud, but it is essentially a mismatch. Â
Datamatics Business Solutions works with UK firms on exactly this kind of review, catching the mismatches and record gaps before a return goes anywhere near HMRC’s Connect system. Want a second opinion on where your own review process might have blind spots? Book time with our team and start Â
What triggers an HMRC enquiry into a Self-Assessment return?
Common triggers include income mismatches against bank or employer data, unusual expense claims for the trade, inconsistent figures year to year, and unreported capital gains or crypto disposals.Â
How far back can HMRC investigate a tax return?
The standard enquiry window is 12 months from filing, but discovery assessments allow HMRC to go back 4 years for innocent errors, 6 for careless ones, and 20 for deliberate evasion.Â
How long do I need to keep Self-Assessment records?
Self-employed individuals and landlords must keep business records for at least 5 years after the 31 January submission deadline for that tax year, per gov.uk.Â
What happens if my records aren't adequate during an HMRC check?
HMRC can charge a penalty of up to £3,000 for failing to keep or preserve adequate records, separate from any additional tax and interest owed on the return itself.Â
