Self Assessment Deadlines You Need to Know in 2026/27
Key Self Assessment Dates for 2026/27
Staying on top of HMRC deadlines is crucial. Missing them doesn’t just mean penalties — it can also trigger unwanted scrutiny. Here’s what you need in your diary.
Registration Deadline
If you’re newly self-employed or have a new source of untaxed income, you must register for self assessment by 5 October following the end of the tax year.
Paper Return Deadline
If you still file a paper return, the deadline is 31 October. We strongly recommend switching to online filing to give yourself more time.
Online Filing Deadline
The headline date: 31 January. This is when your online return must be submitted AND your tax paid. Leave it to the last minute at your peril — the HMRC website regularly crashes under the load.
Payment on Account
Don’t forget that if your tax bill exceeds £1,000, you’ll likely need to make payments on account. The first is due 31 January (with your balancing payment) and the second on 31 July.
Penalties for Late Filing
- 1 day late: £100 fixed penalty
- 3 months late: £10/day for up to 90 days
- 6 months late: 5% of tax due or £300 (whichever is higher)
- 12 months late: further 5% or £300
How Thyme Can Help
We send reminders well in advance and handle the entire filing process for you. No more deadline stress.