“I submitted my tax return. I’m safe… right?”
Every January, millions of people in the UK breathe a sigh of relief after filing their tax return just before the Self Assessment deadline. The confirmation email arrives. The screen says “Submitted.” You think it’s done and dusted.
But here’s the reality we see time and time again:
- Just because your tax return was submitted on time doesn’t mean it’s correct.
- And just because HMRC hasn’t contacted you yet doesn’t mean everything is fine.
In fact, many returns are riddled with errors — incorrect income, missed deductions, overstated profits, forgotten allowances — and no one realises until they’re being investigated, audited, or have overpaid tax for years.
This article will show you:
- What “hidden mistakes” could be in your already-submitted return
- Why HMRC still flags returns after they’re accepted
- How you can review and amend your return
- What to do right now — even if you filed with another accountant
The Truth About “On-Time” Tax Returns
Filing on time only means you met a deadline.
It does not mean:
- The return is accurate
- You’ve claimed everything you’re allowed to
- HMRC has accepted the figures as correct
- You won’t be audited later
Let’s look at what might be wrong under the surface.
Wrong Income or Expense Figures
The most common error we see? Basic numbers are wrong.
Examples:
- Bank income doesn’t match the return
- Client invoices were missed or duplicated
- Business expenses weren’t included — or were estimated
- Spreadsheets have formulas overwritten or totals miscalculated
Real-life case:
A self-employed designer submitted her return showing £42,000 profit. We reviewed it a month later and found:
- £2,800 in business insurance she forgot to claim
- £1,250 in software subscriptions she paid annually (but missed)
- Her profit was overstated by nearly £4,000 — which meant £800+ in unnecessary tax.
Missed Tax Reliefs or Allowances
Most business owners don’t know all the reliefs they’re entitled to, especially if they file their own return or use a “cheap and cheerful” accountant.
Often missed:
- Annual Investment Allowance (AIA) for business equipment
- Use of home as office (even a flat rate claim)
- Capital allowances on tools, laptops, vans
- Mileage instead of actual fuel costs
- Flat rate expenses for things like uniforms or utilities
- Marriage allowance transfer
- Loss relief from previous years
- Trading allowance (£1,000 tax-free if eligible)
Why this matters:
These reliefs can significantly reduce your tax bill — if you know about them. If you don’t? HMRC won’t tell you. They’ll take what you submit and move on.
Incorrect Director’s Loan or Dividends
If you’re a limited company director, your accountant should carefully report:
- Salary
- Dividends
- Director’s loan balance
Problems we see:
- Loans taken from the company not repaid within 9 months (Section 455 tax risk)
- Dividends declared without enough profit
- Mixing personal and business money, leading to messy reporting
HMRC checks for this. The Connect system automatically flags inconsistencies between:
- Company accounts
- Personal tax returns
- Previous filings
Expenses That Look “Off” to HMRC’s Algorithms
HMRC’s software compares your return with:
- Your past years
- Others in your industry
- Expected expense ratios
If your return shows:
- Round numbers (e.g. £1,000 every month)
- Unusually high travel, marketing, or entertainment costs
- Big swings in profit or costs with no notes
…you’re more likely to get flagged.
Even if it was submitted on time.
Poor or Missing Record Keeping
A tax return might be accepted by HMRC’s system…
…but if you can’t back it up later, it could be rejected during a compliance check or investigation.
Typical issues:
- No receipts for claimed expenses
- Mileage with no logbook
- Claims for assets (e.g. equipment) with no invoice
- Bank transfers that don’t match reported income
HMRC doesn’t need a reason to review past returns — and they can go back up to 6 years. If you can’t justify a figure, it can be disallowed.
“But It Was Submitted by an Accountant…”
Even worse, we often review tax returns filed by other accountants and find:
- No capital allowances claimed
- Vague expense totals with no notes
- No questions asked before submitting
- Rushed or copy-paste job from last year
Your accountant hitting “Submit” doesn’t mean they did a full review.
Especially if you were charged a low fee for “just the return.”
Can I Fix a Mistake in My Tax Return?
Yes. Here’s what the rules say:
- You can amend a Self Assessment return up to 12 months after the deadline (e.g. by 31 January 2026 for the 2024/25 tax year).
- You can also claim overpaid tax or correct missed reliefs going back up to 4 years.
- You can fix capital allowances, income errors, expense claims, and loan balances if there’s a clear correction.
- HMRC lets you amend online or by writing to them (though it’s best done through your accountant if it’s complex).
What You Should Do Next
Even if everything seems fine, it’s worth double-checking your return — especially if:
- You did it yourself
- You changed accountants recently
- You were rushed or unsure
- You weren’t asked many questions during the process
- You made guesses or estimates
Use Our Free Red Flag Checker Tool
We’ve built a simple tool that helps you spot common warning signs — even after you’ve filed your return.
It checks for:
✅ Missed tax reliefs
✅ Risky expense patterns
✅ Mileage issues
✅ Unclaimed allowances
✅ Red flags HMRC might notice
Submitting your return on time feels good. But if it was rushed, confusing, or full of guesswork, it’s worth asking:
“Is this actually correct — or did I just avoid a late fee?”
Filing is just step one. Making sure it’s accurate, optimised, and future-proof? That’s what protects your money — and your business.
