I got to see the first modular house built in Santa Monica.
The modules are made from 4 inch square steel tubes. Mounted on the tubes are exterior wall materials on the outside, plumbing and electric in the walls with insulation and plywood walls inside ready for further finishing. The structure is expansive and beautiful.
Getting the inspectors to sign-off is a problem since they’ve never done it before.
Modular houses are the norm in Japan. 
Modular housing has made sense for fifty years but unions have made them impossible in most countries.
Japan has weak unions and no cheap labor so factory production is necessary; it has been highly refined.
The modular home in Santa Monica is fun (for those of us who love welding) but not cheap and definitely not the wave of the future.
Click to enlarge photos.