Hi,
When you acquire a second property or more, you need to notify HMRC in writing, which property is to be your main residence, within 2 years. If you do not do this, HMRC will determine which property is your main residence, based solely on the facts of the case.
Private Residence Relief, allows an additional 9 months, when calculating the number of months that the property was your main residence. You can see guidance here:
HS283 Private Residence Relief (2023)
Thank you.