martin wilburn
It would entirely depend on the condition of the structure but to include it as a habitable part of the house it needs to comply with building regulations. Ordinarily yon need to create a 100mm cavity and a 100mm wall internally different ways to do this, a 75mm stud wall filled with kingspan and over boarded with a 25mm kingspan board with 12.5mm bonded plaster board over the top would give the required insulation. Cost depends entirely on size and condition
