How to Fill In Self Assessment: 12 Common Mistakes to Avoid (UK 2026/27)

Quick answer

Self Assessment is rarely as hard as people fear, but it is unforgiving of small omissions. The errors that cost the most money are not arithmetic slips — HMRC's system does the sums — but things you forget to include or forget to claim. This guide walks through the twelve most common mistakes for the 2026/27 tax year and shows you exactly how to avoid each one, so your return is both accurate and as low as it can legitimately be.

1. Leaving higher-rate pension relief unclaimed

If you pay into a relief-at-source personal pension or SIPP and earn above the £50,270 higher-rate threshold, your provider only adds the 20% basic-rate top-up. The extra 20% (or 25% for additional-rate taxpayers over £125,140) has to be claimed by you — and Self Assessment is where you do it. Enter your gross contributions in the pensions section and HMRC widens your basic-rate band accordingly.

This relief is one of the most widely missed in the UK. A higher-rate taxpayer paying £6,000 net into a SIPP is owed roughly £1,500 a year on top of what the provider claims. Miss it for four years and you have left around £6,000 on the table — relief you can still backdate, but only if you act. Model the saving with the

.

2. Forgetting savings and dividend income

Banks and platforms report your interest and dividends to HMRC, so this is a leading cause of correction notices. For 2026/27:

• The Personal Savings Allowance is £1,000 (basic rate), £500 (higher rate) and £0 (additional rate).
• The dividend allowance is just £500, with dividends above it taxed at 10.75%, 35.75% or 39.35%.

If your savings interest or dividends exceed these allowances, declare them. With higher savings rates over the past couple of years, many ordinary savers have tipped over the PSA for the first time without realising it.

3. Misunderstanding payments on account

This is the single biggest cash-flow shock for first-time filers. If your tax bill is over £1,000 and less than 80% is collected at source, HMRC requires two payments on account toward next year's bill — each equal to half of this year's liability, due 31 January and 31 July.

So in your first January you can owe: the balancing payment for last year plus the first payment on account for this year — up to 150% of your headline bill in one go. The fix is simply to expect it and budget accordingly. If you genuinely expect lower income next year you can apply to reduce the payments, but reducing them too aggressively triggers interest. Our guide on

walks through the timing in full.

4. Mixing up the tax year

Self Assessment runs to 5 April, not 31 December or your business's accounting date. Income earned on 6 April falls into the new tax year. From 2024/25 onward, sole traders and partnerships are taxed on profits arising in the tax year itself (the "tax year basis"), which removed the old overlap-relief complications but means you must apportion profits if your accounting date is not 31 March or 5 April.

5. Claiming expenses that aren't allowable

The rule is that an expense must be incurred wholly and exclusively for the business. Common over-claims that get challenged include ordinary commuting, everyday clothing, client entertaining (never allowable), and the full cost of a phone or car used partly for personal life. The safe approach is to apportion mixed-use costs honestly and keep the records. Conversely, many people under-claim — forgetting the simplified home-working flat rate, mileage at 45p/25p, or a share of broadband. See our

for the full list.

6. Missing the marriage allowance

If one spouse earns under the £12,570 personal allowance and the other is a basic-rate taxpayer, the lower earner can transfer £1,260 of allowance, worth up to £252 a year. It is claimed separately rather than on the return itself, but filers frequently overlook it — and it can be backdated four years.

7. Overlooking the High Income Child Benefit Charge

If you or your partner earns over £60,000 and someone in the household claims Child Benefit, the High Income Child Benefit Charge applies, tapering fully away by £80,000. The charge is collected through Self Assessment, and forgetting it is a common reason for an unexpected bill. A pension contribution that brings your adjusted net income below £60,000 can eliminate it entirely — a neat planning move. Check the numbers with the

.

8. Entering net instead of gross figures (or vice versa)

A perennial slip. Pension contributions go in as gross (your payment plus the 20% top-up). Gift Aid donations go in as the amount you actually gave (HMRC grosses them up). Employment income comes from your P60 as the taxable figure before tax but after pension if you are in a net-pay scheme. Mixing these up either inflates or understates your relief. When in doubt, read the help text beside each box — it states net or gross explicitly.

9. Ignoring the £1,000 trading and property allowances

If you have small amounts of side income, the £1,000 trading allowance and the separate £1,000 property allowance may mean you owe nothing and need not register at all. But if you have already registered, you must still report — and you choose between deducting the £1,000 allowance or your actual expenses, whichever is higher, not both. People routinely deduct both, which is incorrect.

10. Forgetting capital gains

From the 2023/24 year onward the annual CGT exemption fell to £3,000 (2026/27). With such a low exemption, modest share or crypto disposals now create reportable gains that previously fell within the allowance. Residential property gains have their own 60-day reporting rule on top of the annual return. Omitting gains is increasingly common precisely because people remember the old £12,300 allowance.

11. Filing late — the most expensive mistake of all

Miss the 31 January online deadline and you face an automatic £100 penalty even if no tax is due. After three months, daily penalties of £10 (up to £900) begin; after six and twelve months, further percentage-based penalties apply, plus interest on unpaid tax. None of this relates to the accuracy of your return — it is purely about timing. Filing in the autumn, not the last week of January, gives you room to find documents and fix errors calmly.

12. Not keeping records — or the SA302

Finally, keep your supporting records: P60s, P11Ds, interest certificates, pension statements, expense receipts. The self-employed must keep records for at least five years after the 31 January deadline. After filing, download your tax calculation (SA302) and tax-year overview — these are what mortgage lenders ask for and what you need if HMRC opens an enquiry.

A worked sanity-check before you submit

Before you press submit, run your own rough estimate and compare it to HMRC's calculation. Take a higher-rate employee with £70,000 salary, £2,000 of dividends and £6,000 net into a SIPP:

• Salary tax and NI are largely handled by PAYE, so the return should show little extra due on employment.
• Dividends: £500 allowance, then £1,500 taxed at 35.75% ≈ £536.
• Pension: the £7,500 gross contribution extends the basic-rate band, generating roughly £1,500 of higher-rate relief — which should reduce the bill.

If HMRC's calculation shows tax due that you can't reconcile to a back-of-envelope estimate like this, something has been entered in the wrong box. The

and let you reproduce each component independently.

Mistake 13 (bonus): registering too late

Before you can file at all, you must be registered — and the deadline catches people out. If you have new untaxed income (self-employment, rental, large dividends, capital gains) in a tax year, you must tell HMRC by 5 October following the end of that tax year. Register late and the return itself may still be on time, but you risk a "failure to notify" penalty based on the tax you owe. The registration process issues your Unique Taxpayer Reference (UTR), which can take a couple of weeks to arrive by post, so leaving registration until January is a recipe for missing the filing deadline through no fault of the return itself. If you became self-employed or started letting a property in 2025/26, your registration deadline was 5 October 2026 — don't assume you have until January.

Why the "wholly and exclusively" test trips people up

The expenses rules deserve a second look because they cause more enquiries than almost anything else. HMRC's test is that a cost must be incurred wholly and exclusively for the business — and the word exclusively is doing a lot of work. A suit you wear to client meetings has a "dual purpose" (it also keeps you warm and decent), so it fails the test and isn't allowable, however much it feels like a work cost. The same logic catches everyday meals, gym memberships, and the journey from home to a regular workplace.

The flip side is that apportionment is allowed where a cost genuinely has an identifiable business proportion. A phone used 60% for business can have 60% of the bill claimed; a spare room used as an office for a set number of hours supports the simplified flat-rate home-working deduction. The mistake is binary thinking — either claiming the whole of a mixed cost (over-claiming) or claiming none of it (under-claiming). Record the business proportion at the time and you'll have a defensible figure if asked. Our

helps you see how expenses flow through to the final bill.

The records that protect you in an enquiry

If HMRC opens an enquiry — most are routine, not accusations — the difference between a quick resolution and a painful one is your records. Keep, for at least the required period:

• Income evidence: invoices, bank statements, rental statements, dividend vouchers, interest certificates.
• Expense evidence: receipts, mileage logs, the basis for any apportionment.
• Relief evidence: pension contribution statements, Gift Aid confirmations, EIS/SEIS certificates.

The self-employed must keep records for at least five years after the 31 January filing deadline; others should keep them for at least 22 months after the end of the tax year. Digital copies are fine, and keeping everything in one folder per tax year as you go is far less painful than reconstructing it under enquiry pressure. As Making Tax Digital expands, this habit becomes mandatory rather than merely sensible.

How underpayments get collected — and the cash-flow sting

A subtle source of confusion is how a small underpayment is collected. If you file by 30 December and owe less than £3,000, HMRC can collect it through next year's tax code rather than demanding a lump sum — convenient, but it quietly raises your tax for a year and surprises people who don't read their coding notice. File later than 30 December and that option disappears, so the whole balance is due as a lump sum on 31 January. This is another reason to file in the autumn: it preserves the gentler collection route for any modest balance owed. Check how a coding adjustment affects your monthly pay with the

.

Putting it all together

The pattern across all twelve mistakes is the same: Self Assessment punishes omission, not effort. The taxpayers who get it wrong are almost never bad at arithmetic — they simply forgot to enter a source of income, forgot to claim a relief they were owed, or budgeted only for the headline bill and were ambushed by payments on account. File early, enter every source of income and every relief, read whether each box wants a net or gross figure, and reconcile HMRC's calculation against your own rough estimate. Do that and your return will be both accurate and as low as it legitimately can be.

This article is general information, not tax advice. Figures use 2026/27 UK rates and apply to England, Wales and Northern Ireland; Scottish income tax bands differ. Consider professional advice for complex affairs.