CGT & Barn Conversion

A client of mine owns a former farmhouse and associated outbuildings - in total the land is approx 1.5 acres, but it may be possible to claim that this is appropriate for the house.  The property was purchased in approx 2002. Planning permission has been granted on the barn and my client is thinking of selling existing property (claiming PPR) and moving in to the converted barn.

When the Barn is eventually sold (could be a number of years in the future) will PPR relief apply on the whole gain or only from the period when property became main residence (prior to this the garden was part of the previous residence)? Would the answer be different if old house retained until barn ultimately sold?