Capital gains - second property that is part of our primary residence

We own a semi-detached house in London and an adjacent ground-floor flat, with a patio between the two properties and the gardens merged into one for more than 20 years. The flat has always been used as part of our primary residence (never rented) for our children, work and other family things. This relationship between the two properties is evident on Google maps, photos, bills (e.g. us always paying council tax etc). We are now splitting the properties to downsize. My view is that we should be exempt from capital gains on the flat. Thoughts on this? And on liaising in advance with HMRC to come to an agreement?

Hi ,
 

This cannot be confirmed on this forum as further information is required on the 2 properties to determine if it is in fact 1 or 2. HMRC cannot give clearance before a gain is declared  . Thank you .