Disposal of mixed use property reporting requirements

[Needhelpcgt]

Hello, I'd be grateful for any help with the following:

We are selling our mixed use property (residential flat and shop) for the price we paid for it, so presumably no CGT is due.
However, my issue is on what I have to report for CGT and self assessment.

We have lived in the residential flat for the whole time we have owned it, so I presume it is 100% covered by private residence relief.
There will no capital gain on the commercial aspect, the shop that we have used.
So, do I have to do a capital gains report for either of these?

Secondly, what are the requirements for self assessment reporting?
I have read that for self assessment, I only have to report the sale if I already do a tax return (which I do) and the disposal value is over £50,000.
However, I am wondering which figure the £50,000 refers to-
-the figure for the whole building which includes the residential flat and the shop or
-just the shop which through apportionment would be less than £50,000.

Thank you.