Pension higher rate relief & simple assessments query

N1AK

I've just received my simple assessment and the figures appear exactly as I expect, except my SIPP contributions are ignored.

I'm a higher rate payer but in previous tax years I contributed to pensions through PAYE and received tax relief directly; this is the first year I have contributed directly to a SIPP. I received 20% tax relief on the contributions automatically, and knew I'd need to do something to get the higher rate relief, but I'm struggling to work out what I am supposed to do.

What I would really appreciate is guidance on whether:

1. I can contact HMRC to claim the higher rate tax relief for 20/21 and this would adjust the simple assessment, and if so would I also need to query the simple assessment separately?
2. I need to register for self-assessment in order to get the higher rate tax relief, and if so do I need to do something else to claim the higher rate relief as well?
3. The best approach if it isn't either of the above.
   
4. The boat has sailed and I need to take this as an expensive lesson.
   

Dazed_and_C0nfused

1 is the correct answer.  The pension contributions will increase your 20% band and can therefore reduce anything taxed at 40%.

This will revise the 2020:21 Simple Assessment and HMRC will also assume you will pay similar amounts in this tax year (2021:22) and provisionally adjust your tax code if necessary.  This 2021:22 tax code adjustment is never to allow relief for 2020:21.

You should let HMRC know your estimated relief at source contributions for 2021:22 to keep things as accurate as possible.