The dates that matter
Self Assessment covers the UK tax year from 6 April to 5 April. For the year that ended 5 April 2025, the key dates are:
- 5 October 2025, the deadline to register with HMRC if this is your first return.
- 31 October 2025, the paper return deadline. Almost nobody uses this.
- 31 January 2026, the online return deadline, and the deadline to pay what you owe.
- 31 July 2026, the second payment on account, if HMRC asked you for one.
What missing the deadline costs you
Miss the online return deadline by one day and HMRC charges a fixed £100 penalty, even if your final tax bill is zero. The penalty does not go away just because you had nothing to pay.
After three months late, HMRC adds £10 per day for up to 90 days, so another £900. At six months late, you pay an extra 5% of the tax owed or £300, whichever is higher. At twelve months late, another 5% or £300.
The tax itself attracts interest from 1 February at the Bank of England base rate plus 2.5%. On top of that there are late-payment penalties: 5% of the unpaid tax after 30 days, another 5% after six months, another 5% after twelve months. Stacked together, late filing on a modest tax bill can easily double what you owe.
If cashflow is tight
Set up a Time to Pay arrangement with HMRC before the 31 January deadline, not after. HMRC will almost always agree to a monthly plan if you ask early. The arrangement does not stop interest, but it does stop the late-payment penalties, which is the expensive bit.
Reasonable excuse appeals
If you genuinely could not file on time (serious illness, bereavement, HMRC system failure on the day), you can appeal. Simple forgetfulness and "I was too busy" do not count. The burden of proof is on you.
The practical advice
Our standard turnaround once you have uploaded everything we need is seven to ten working days. If you are reading this in January, you are not too late, but every day you wait makes it harder to find an accountant with capacity. Start now, and let someone qualified handle the paperwork.