a plurality of frame members including a plurality of first frame members and a plurality of second frame members, wherein each said frame member is comprised of an adjustable frame element including a pair of opposing studs and integrated two-part top plate connecting said pair of opposing studs at an upper integration point;

a plurality of first noggings connecting a respective first stud of said pair of opposing studs of each said first frame member with a respective first stud of said pair of opposing studs of each said second frame member, and a plurality of second noggings connecting a respective second stud of said pair of opposing studs of each said first frame member with a respective second stud of said pair of opposing studs of each said second frame member, thereby defining at least one free-standing tunnel module;

a plurality of ceiling joists connecting said two-part top plate of each of said first frame members to a respective said two-part top plate of each of said second frame members, thereby securing said at least one free-standing tunnel module;

at least one vertical wall panel secured to an exterior wall area of said at least one free-standing tunnel module, thereby vertically enclosing said at least one free-standing tunnel module;

at least one horizontal ceiling panel secured to an exterior ceiling area of said at least one free-standing tunnel module; and

at least one horizontal floor panel positioned over an existing floor surface, wherein said at least one horizontal ceiling panel and said at least one horizontal floor panel horizontally enclose said at least one free-standing tunnel module.