If you are looking for attic space, consider using attic trussed rafters if the building configuration is favourable. You can get up to 60% living space of the footprint and because a truss is an engineered product, you would also get the structural calculations and design drawing package – all compliant with EC. The local authority may allow a replacement roof without planning, even though the roof pitch would be steeper than usual, so its worth paying a visit to enquire.