A straightforward sale of part of your “principal private residence” should be tax-free as the sale of the whole property and grounds would be.
Anything more complicated might give rise to some tax.
I found this which might give you a few ideas:
https://www.krwaccountants.co.uk/krw...velopment.html
I’d suggest that you try and find an accountant who is familiar with similar activities and have an initial chat to scope the issues ahead of commercial discussions.