Hi,
If the property is jointly owned, you can split the calculation into two calculations, 50/50 each. You can see a Capital Gains Tax calculator to help you with your calculation here:
Tax when you sell property
You can see guidance on calculating Private Residence Relief here:
HS283 Private Residence Relief (2024)
There is no definitive list of what is permitted to be a Capital Allowance to set against a capital gain and what is routine maintenance. You can see guidance regarding enhancement expenditure here:
CG15180 - Expenditure: enhancement expenditure
If a gain does arise, then you have 60 days from the completion date to report and pay the Capital Gains Tax. At the end of the tax calculator, there is the option to register for a capital gains account, to report and pay the Capital Gains Tax.
Thank you.