LindfieldRes

Hi - any help hugely appreciated. We are building a two storey side extension and have been granted planning permission. We also have an agreed party wall act with the neighbours, who are really nice. They have a single storey garage that is built up the boundary line. We want to build up to the boundary and butt up against this wall, but the builder raised two issues - if there was no gap or cavity left, you could get ventilation issues. Also the soffit on the end extension would cross the boundary. He suggested we need to leave a minimum of 50mm gap, to allow for ventilation, and the minimum size of the soffits above. Does anyone know what's best in this situation. No gap, but then how would the soffits work above as they would cross the boundary line. 50mm or 100mm cavity? Thanks again