The major cost in time is also weather related. Materials are damaged if they sit in the rain, things have to be done in a specific order, in many places the livable build season is short (in the rocky mountain states, eg.) and the opportunity fir error is huge. If things arrive as a mostly prefab kit, then things are faster to put up on site, which avoids tons of complexity. It especially avoids a lot of the challenges with multiple subcontractors and scheduling.<p>Making demolition cheaper and easier would be a major improvement. Of course, demolition is one area where care is very important, for safety of the workers and the environment. It is amazing the types of hazmat that are in old buildings and houses. (Asbestos, lead, mercury, etc.) Containing it all is very hard.