Tax l

My client owns a joint share in the freehold of a building which consists of five flats. One of the flats is occupied by my client under a 999-year lease.
The freeholders are to grant a new long lease over another flat in the building. My client understands that this will be a capital gains disposal event, but has queried whether a measure of principal private residence relief would be due to him, given that the disposal is out of the freehold interest he owns that includes his main residence.

Can you please confirm how to approach my client’s capital gains position, whether any main residence relief would be due to him in these circumstances and how this would be calculated?