Abstract: Modular dwellings comprise attached wall members that enclose an interior area, and a central truss system extending across the dwelling. A lower roof member is supported by a truss system lower chord and wall member, and an upper roof member is supported by a truss system upper chord and a wall member. The ends of the truss system are supported by a vertical post. The upper part of a wall section and of the upper chord are in co-planar alignment and the lower chord and the upper part of another wall section are in co-planar alignment. Wall panels include an internal frame and a tongue on one side and a groove on the other side which mate with the tongues and grooves of other like panels to compress resiliently deformable seals. A frame portion positioned at the internal side of the extension supports the extension for proper alignment during construction.