The two storey is generally more expensive because of the inceased area of external walling and chimney height and the larger windows, the pitched roof is generally a void space. The dormer utilises the attic space by using a high pitch on the roof. the roof structure and coverings are more expensive but this is more than off set by the savings in the external walling and windows. The void spaces in the dormer are normally used for the services, the inscreased depth of roof members enable a higher level of insulation to be installed ie. rockwool and thus can provide a better insulated house. The dormer roof can be constructed by a special prefabricated truss or can be cut on site to suit the design.