Hi,
Capital gains liability can only be calculated after you have disposed of the property.
Private Residence Relief will apply for the period the property was your main residence, so no Capital Gains Tax will occur for that period. Capital Gains Tax may be payable for the period the property is rented out. You can see guidance on Private Residence Relief here:
HS283 Private Residence Relief (2024)
If you have more than one property, you need to advise us in writing, which is your main residence and from when, otherwise HMRC may have to make that decision for you, based on the facts of the case.
Thank you.