Principal Private Residence rules.
If you acquire a second residence i.e. you have two properties and live part time in each one, it is worthwhile making a formal election for one of them to be your Principal Private Residence (PPR).
Make this election within two years of the date when you started to occupy both residences, simply by writing a letter to HMRC.
If you don’t make the election within two years, then HMRC can decide (based on how long you reside in each residence or where your family lives etc.) which one is your main residence is - and you are then stuck with their decision – and it may not be to your advantage. By electing for one of them to be your PPR, you will have the right to change your PPR status from one residence to the other which can prove extremely tax efficient.
For example: A couple live in Wiltshire and buy a flat in London for their student son. After their son graduates, and leaves the flat, the couple (now retired) use the flat during the week to visit galleries/friends etc. Years later, they sell the flat making a profit of £170,000 and will have to pay Capital Gains Tax of about £40,000.
If they had simply written to HMRC to make a PPR election within two years of using the flat, they could have made the London flat the PPR and consequently, most or even all of the profit would be exempt from Capital Gains Tax.
In fact, even if they had made a PPR election on behalf of their Wiltshire home, they could later have switched the PPR to the London flat, which, if nothing else, always makes the last 18 months of the gain exempt (9 months from April 2020).
So, the advice is: make that election - even if it you think you will never use it!