Self-Assessment Penalties: How Much and How to Avoid Them

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The short version

Right now, if you miss the Self Assessment filing deadline, HMRC hit you with an automatic £100 late filing penalty, then layer on daily penalties and percentage-based fines the longer you leave it. Similar percentage penalties apply if you pay late, and from 2026 the system moves gradually towards a points-based setup alongside Making Tax Digital. You can appeal penalties if you had a reasonable excuse and you sorted things as soon as you realistically could.

Why it matters

For tradies and small builders, it's very common to be one or two years behind then panic-file; that's exactly how you rack up hundreds or thousands in fines on top of the tax. Understanding the scale and timing of penalties -- and how to get some cancelled when life's genuinely kicked off -- lets you plan an honest catch-up instead of sticking your head in the sand.

Self Assessment penalties for individuals sit in TMA 1970 and later schedules, and HMRC split them into:

• Late filing penalties -- for missing return deadlines under section 8 TMA 1970.
• Late payment penalties -- for not paying the tax by the due date.
• Penalties for errors -- if you submit incorrect returns through carelessness or worse.

You can also get separate penalties under newer MTD rules for late digital updates, but here we're focusing on classic Self Assessment -- the one most small builders know.

5.14.1 Late filing penalties -- missing the return deadline

Under the current regime, if you file your tax return after the deadline (normally 31 January for online returns):

• 1 day late -- automatic £100 fixed penalty, even if you owe no tax.
• 3 months late -- daily penalties of £10 per day for up to 90 days (max £900).
• 6 months late -- extra £300 or 5% of the tax due, whichever is higher.
• 12 months late -- another £300 or 5% of the tax due, whichever is higher, with higher penalties possible in deliberate cases.

So if you leave a return a year late and you owe tax, you can easily be into four layers of penalties on one year alone.

From April 2026 HMRC begin moving Self Assessment onto a points-based late-submission regime as part of MTD -- instead of an instant £100, you build points and get a financial penalty once you hit a threshold -- but the basic idea remains: miss deadlines enough and you pay.

5.14.2 Late payment penalties -- paying the tax late

On top of filing penalties, HMRC charge penalties for late payment of the tax itself, plus interest.

Current style is broadly:

• About 30 days late -- first late-payment penalty (around 5% of tax due under current rules).
• Further penalties if tax is still unpaid at 6 months and 12 months -- more 5% charges on the outstanding amount.

Under the newer structure HMRC are phasing in (aligned with MTD), commentary suggests something like:

• A first percentage penalty if tax is still unpaid 15-30 days after the due date.
• A further percentage penalty after 30 days.
• Ongoing interest on top, accruing until you pay.

The key point: you can get hit for late filing, late payment, and interest all on the same year -- they're separate.

5.14.3 Penalties for incorrect returns (errors)

If HMRC think your return is wrong, penalties sit under the general error regime (for returns under section 8 TMA, etc.):

They look at behaviour:

• "Careless" (you didn't take reasonable care).
• "Deliberate".
• "Deliberate and concealed".

They also look at whether you told them first (unprompted disclosure) or they found it.

Penalty percentages are then applied to the extra tax HMRC say is due. For honest mistakes you correct yourself, penalties can be reduced heavily or dropped; for deliberate under-reporting, they can run very high.

For most small builders, the key avoidance move is simple: don't guess; work from real numbers or get help. That gives you a fighting chance of being seen as having taken "reasonable care" even if you got something wrong.

5.14.4 How to appeal a Self Assessment penalty

You can appeal Self Assessment penalties if:

• The penalty was incorrectly issued (for example, you weren't required to file).
• You had a reasonable excuse for being late or making an error, and you put things right as soon as you could.

Process, in plain terms

Check the notice:

• Read the penalty letter: what year, what type (late filing, late payment, error), amounts and dates.

Appeal within the time limit:

• You normally have 30 days from the penalty notice to appeal.
• Use HMRC's online Self Assessment service or form SA370 to appeal a late filing penalty.

Explain your reasonable excuse:

• HMRC say a reasonable excuse is something that stopped you meeting an obligation, despite taking reasonable care.
• Examples they accept in guidance include: bereavement close to the deadline, serious illness or hospitalisation, major IT failures, or postal issues outside your control.
• The excuse must cover the period from the missed deadline up until shortly before you actually filed or paid.

Show you acted quickly once you could:

• HMRC expect you to put things right "without unreasonable delay" once the excuse ends.
• If you sat on it for months after things calmed down, they're less likely to agree.

If HMRC reject your appeal:

• You can ask for an independent review by another HMRC officer.
• If still unhappy, you can appeal to the First-tier Tax Tribunal.

Appealing doesn't make the underlying tax disappear -- you still have to pay any tax and interest; you're only trying to reduce or remove the penalty.

5.14.5 Practical damage-limitation if you're already late

If you're already behind:

• File as soon as you can -- every day you leave it, penalties and interest tick up or future point-based penalties loom once MTD kicks in.
• If you can't pay in full, still file the return and then set up a Time to Pay arrangement with HMRC -- they're more flexible with people who've filed and are upfront.
• Keep evidence of anything that might count as a reasonable excuse (hospital letters, death certificates, IT outage proof, etc.).
• Get an accountant or tax adviser involved early if penalties are large or you've got multiple years to clean up -- the way you present the story to HMRC matters.

Missed 31 January -- do this next

If the deadline's gone, do this in order:

Get the return in, even if you can't pay:

• File the Self Assessment as soon as you can -- it stops more late-filing penalties stacking up and gives you a clear tax figure.

Work out what you can pay now:

• Pay as much as you can straight away to cut late-payment penalties and interest.
• If you can't clear it, look at a Time to Pay plan with HMRC once the return is in.

Check the penalty notice carefully:

• When the penalty letter lands, check the year, type (late filing vs late payment) and amounts -- mistakes do happen.

Decide if you've got a "reasonable excuse":

• If serious illness, bereavement or another genuine blocker stopped you filing or paying, gather evidence and appeal within 30 days using HMRC's process (or form SA370).

Stop it happening again:

• Set a diary reminder for next January and talk to an accountant if you're always scrambling -- one tidy system is cheaper than repeat penalties.

What to do next

• If you have missed a deadline, file the return as soon as possible to stop further penalties stacking up.
• Pay as much as you can now, even if it is not the full amount -- it reduces late payment penalties and interest.
• Check the penalty notice carefully for the year, type and amounts before you decide whether to appeal.
• If you had a genuine reasonable excuse (serious illness, bereavement, major IT failure), gather evidence and appeal within 30 days.
• Set a calendar reminder for next January so you are never scrambling again.

Sources and legislation

• Taxes Management Act 1970 -- sections 8, 9 and 9A: return obligations, penalties and enquiry powers. legislation.gov.uk/ukpga/1970/9
• Finance Act (various) -- penalty regime updates and MTD points-based system. legislation.gov.uk/ukpga
• Income Tax (Earnings and Pensions) Act 2003 -- income and employment rules relevant to penalty calculations. legislation.gov.uk/ukpga/2003/1

Related guides on this site

• 5.7 HMRC investigation
• 5.15 Record keeping
• 5.8 Making Tax Digital
• 5.3 Filing Self Assessment -- construction-specific deductions
• 5.1 Registering as self-employed for construction

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