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What are the common mistakes with Self Assessment tax returns and how do you avoid them?

By FastBuySale Newsroom · 5 Nov 2026 · 26m ago

Mistake 1: not registering in time

Common mistakes with Self Assessment tax returns usually come down to dates: registering late, filing after the deadline and paying late. For the 2025 to 2026 tax year, HMRC must receive a paper return by 11:59pm on 31 October 2026 and an online return, plus any tax owed, by 11:59pm on 31 January 2027.

Miss the filing deadline and HM Revenue and Customs charges an initial £100 penalty straight away, even if you owe no tax, with more charges after 3, 6 and 12 months. Paying late adds 5% surcharges and interest. The good news is that every one of these mistakes is avoidable with a simple calendar and a little preparation.

If you have to complete a return for the first time, or you were registered before but did not need to file for 2024 to 2025, you must tell HMRC by 5 October. Common triggers include starting self-employment, renting out property or receiving untaxed income.

Register after 5 October and fail to pay all your tax by 31 January, and you may get a 'failure to notify' penalty, based on the amount still unpaid. HMRC says you will receive it within 12 months of it receiving your return.

If you register after 5 October 2026, HMRC will give you a different filing deadline, three months from the date on its letter or email, but the tax itself is still due by 11:59pm on 31 January 2027. Do not assume a later filing date also means a later payment date.

Readers evaluating HMRC Self Assessment should separate marketing claims from the actual rule text that governs their account or product. When two summaries disagree, the governing document or statute wins. If a rule depends on your age, filing status, or income band, re-check it when any of those facts change. Life events quietly move people across thresholds they last reviewed years earlier.

Compare fees the same way you would compare two similar listings before you buy: look at what you pay each year, what you get, and whether a simpler option meets the same need. Readers evaluating HMRC Self Assessment should separate marketing claims from the actual rule text that governs their account or product. When two summaries disagree, the governing document or statute wins.

Mistake 2: missing the filing deadline

There are two filing deadlines, and people often mix them up. Paper returns must arrive by 11:59pm on 31 October 2026; online returns are due by 11:59pm on 31 January 2027. You can file any time from 6 April 2026, so there is no advantage in waiting.

HMRC's late filing penalties stack up: £100 as soon as you are late; after 3 months, £10 a day up to a maximum of £900; after 6 months, a further 5% of the tax due or £300, whichever is greater; and after 12 months, another 5% or £300, whichever is greater. A return that is a year late can therefore cost at least £1,600 in penalties before interest.

Partnerships face an extra risk: if a partnership return is late, every partner is charged a penalty. One person's delay can cost everyone.

A useful habit is to write down your goal, time horizon, and cash buffer before you change any election related to HMRC Self Assessment. Decisions made during a stressful week are the ones people reverse later at a cost. Educational explainers like this one cannot replace personalized advice for complex situations such as divorce orders, inherited accounts, or cross-border tax residency. Escalate those cases to a qualified professional.

Mistake 3: paying late or ignoring a penalty notice

Filing on time does not stop payment penalties. If the tax is not paid, HMRC adds 5% of the unpaid tax at 30 days, at 6 months and again at 12 months, and charges interest on the amount owed.

Many first-time filers are surprised by payments on account. If you make them, there is a second payment deadline of 31 July towards the next bill. Budget for both 31 January and 31 July so the second instalment is not a shock.

If you are employed and want a small amount collected through your tax code instead, you must submit online by 11:59pm on 30 December 2026. Miss that and you must pay another way.

Penalties must be paid within 30 days of the date on the notice. Ignoring one simply adds interest and, for filing, more penalties.

If you had a reasonable excuse, such as a serious illness or a system failure on HMRC's side, you can appeal. Do it promptly and keep evidence. You can estimate any penalty with HMRC's online tool before deciding what to do.

Keep digital and paper copies of confirmations after every change. Screenshots of election pages, email receipts, and year-end statements make tax season and job changes far less chaotic. Compare fees the same way you would compare two similar listings before you buy: look at what you pay each year, what you get, and whether a simpler option meets the same need.

Mistake 4: waiting for perfect figures

HMRC is explicit that if you do not know your profit for the whole tax year, you still need to send a return. Waiting for perfect numbers is a classic way to miss the deadline.

Use HMRC's estimator to get an idea of your bill early, so you can budget and pay on time. Keep invoices, receipts and bank statements together through the year; a simple spreadsheet or the budgeting tools at can help self-employed sellers track income as it arrives.

If you sell items regularly online, for example on marketplaces, check HMRC guidance on whether that counts as trading income. Our seller guides at cover record-keeping basics.

If a rule depends on your age, filing status, or income band, re-check it when any of those facts change. Life events quietly move people across thresholds they last reviewed years earlier. Automation helps. Calendar reminders for annual reviews, automatic transfers, and beneficiary checkups reduce the chance that a good plan dies from neglect.

Printable checklist for your next return

April to June: gather your P60 or other income records, rental statements, invoices and expense receipts for the tax year that ended on 5 April. Starting early gives you months of buffer before any deadline.

By 5 October: if this is your first return, or you did not file last year, register for Self Assessment. Write down your Unique Taxpayer Reference when it arrives and store it somewhere safe.

By 31 October: send a paper return if you prefer paper. Otherwise, aim to file online in the autumn so there is time to fix problems such as login issues or missing figures.

By 30 December: file online if you want a small balance collected through your PAYE tax code.

By 31 January: make sure both the online return and the balancing payment have gone through, plus your first payment on account if one applies. Save the confirmation.

By 31 July: pay your second payment on account. Then start the cycle again for the next tax year, and consider setting aside a fixed share of every self-employed payment in a separate savings pot.

Found this HMRC Self Assessment explainer useful? Share it with a friend, tell us what we missed in the comments, and get the FastBuySale app on Google Play () for more guides. Have something to sell? Post a free listing at in a couple of minutes.

Educational explainers like this one cannot replace personalized advice for complex situations such as divorce orders, inherited accounts, or cross-border tax residency. Escalate those cases to a qualified professional. When in doubt about a number that changes every year, describe your plan qualitatively first, then look up the current official figure for the tax year you care about before you act.

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