30/09/2026
If you started self-employment, became a landlord, or had other new income in the 2025/26 tax year, you need to register for Self Assessment with HMRC by 5 October.
Miss it and you can still sort it, but the process gets more awkward and the penalty clock can start ticking.
We've explained on our blog: who actually needs to register, how HMRC's faster online service works, and exactly what to do if you've already missed the date.