Abstract: A low cost building is made of modular parts in the absence of special tools. Light-in-weight wall blocks having a facade of conventional appearance include a thick, insulating mixture of a cementitious material, polystyrene, and organic fiber aggregates capable of retaining water. The blocks have interlocking top and bottom surfaces so that they are easily stacked to form walls. A floor is built by spanning foundation beams with floor trusses that have a key along their respective tops and by sliding modular floor panels between contiguous floor trusses, each floor panel having opposite edges that slidingly engage the keys of the floor trusses. A roof is made in a similar manner by sliding modular roof panels between contiguous roof trusses. Tensioning rods are extended from a top surface of the walls to the building foundation to compress the walls.