The HMRC 6-month penalty (£300 or 5%): the August letter explained
If your Self Assessment return is 6 months late (31 July, for a return due 31 January) HMRC charges a further penalty of £300 or 5% of the tax due, whichever is greater, on top of the £100 and up to £900 in daily penalties. Notices typically land in August. It's appealable within 30 days on the same reasonable-excuse grounds, and the appeal should cover every penalty on the account.
Written by the team behind Agent Arthur, our HMRC and tax specialist. This guide is general information about UK law, reviewed for accuracy.Why this letter arrives in August
The Self Assessment deadline is 31 January. Six months later is 31 July, so each August, HMRC issues a wave of 6-month penalty notices to everyone whose return is still outstanding. If you've just received one, you're not alone: hundreds of thousands land in the same fortnight.
By this point the total stack is serious: £100, plus up to £900 daily, plus £300 (or 5% of tax due, if higher). That is £1,300 minimum for a still-unfiled return, before any tax or late-payment penalties.
What to do, in order
- File the return immediately: the 12-month penalty (another £300 or 5%) triggers at 31 January next, so stop the stack growing
- Appeal within 30 days of the notice, online or SA370, covering all penalties on the account in one letter
- Set out one evidenced timeline: what stopped you filing in January, why it continued, when you filed and why that was as soon as reasonably possible
- If the tax-geared 5% applies and you dispute the underlying tax, say so, because the penalty recalculates if the tax does
- If HMRC refuses: request a statutory review (independent officer), then the First-tier Tribunal if needed. Both are free
Reasonable excuse at 6 months late
The hurdle is naturally higher than for a few days' lateness, because HMRC will ask what prevented filing for half a year. Situations that genuinely persist succeed: serious or mental illness, the aftermath of a bereavement, caring responsibilities that overwhelmed everything else, insolvency chaos, or being genuinely unaware you were in Self Assessment at all (common for new side-hustle registrants, so say when you first learned, and what you did next). Evidence the duration, not just the start.
August letter in hand? Answer the whole stack at onceArthur drafts one appeal for every penalty on your account: the timeline, the reasonable-excuse framing, and the evidence list, ready to send within the 30-day window.£5 launch price (normally from £19.99)Start my appeal letterCommon questions
Is the 6-month penalty £300 or 5%?
Whichever is greater. If your tax due is under £6,000, it's the flat £300; above that, 5% of the tax due. If no tax was due at all, it's still £300, and still appealable.
Can one appeal cover the £100, daily and 6-month penalties together?
Yes, and it should: one letter, one timeline, all penalty reference numbers listed. HMRC decides each penalty on the same reasonable-excuse story.
I only found out I needed to file when the penalties arrived: do I have a case?
Possibly. If you genuinely didn't know you were in Self Assessment (e.g. HMRC opened a record you never saw, or a new income source you didn't realise was reportable), explain when you first became aware and how quickly you acted. Ignorance of a deadline is weak; never having been notified you were in the system is a real argument.
What if I can't pay the penalties while appealing?
Appeal first, because a successful appeal cancels the debt. If you need time regardless, HMRC's Time to Pay arrangements exist, and appealing doesn't prevent agreeing one.