Every year, plenty of people confuse two different Self Assessment dates.

31 January is when most people file online and pay what they owe.

5 October is when you must tell HMRC you need to file — if you haven’t already.

According to GOV.UK, you must tell HMRC by 5 October 2026 if you need to complete a tax return for the previous tax year (6 April 2025 to 5 April 2026) and you have either:

• not sent a tax return before, or
• registered before but did not need to send a tax return for 2024/25

You tell HMRC by registering for Self Assessment (or reactivating your account if you’ve been registered before).

Why registration comes first

Filing isn’t available until HMRC has set you up. That usually means receiving a Unique Taxpayer Reference (UTR) and, for online filing, an activation code — both typically sent by post. Leaving registration until the last minute can squeeze the time you need to file calmly in January.

GOV.UK also notes that if you tell HMRC after 5 October 2026, you could get a penalty.

The rest of the timetable

Once you’re registered, keep these dates in mind for the 2025/26 return:

• 31 October 2026 — paper tax return deadline
• 30 December 2026 — deadline to file online if you want HMRC to collect a qualifying tax bill through your PAYE code
• 31 January 2027 — online return and payment deadline

Who should pay attention

This often catches:

• new sole traders and freelancers
• people with rental income for the first time
• company directors with untaxed income to report
• anyone who thought “I’ll sort Self Assessment in January” without registering first

How Bassline can help

We’ll check whether you need to register, help you get set up properly, and keep the process clear from day one through to filing.

Questions? Call 0113 347 0019 or email contact@bass-line.co.uk — or request a free consultation at bass-line.co.uk.