Yep, it's the zoning laws. The requirement for the size of lots purchased to either build, or install modular homes has to change. The problem being local landowners who have McMansions on large tracts of land, or home owners associations, lobby to prevent starter homes on small lots that are adjacent, because they are only concerned about their property value for resale.
Of course, that premium value becomes less important when it comes time to pay taxes.
You might take this the wrong way, but I agree with George Carlin on the issue. We should take all of the golf courses, and turn them into public housing, and small lots for starter homes. Enough land there to create those 3.8 million homes easily.
There always seems to be those that think public housing should be an inner city project. I call bullshit. Public housing, and starter home lots should be put in the suburbs, and rural areas. You know, where the white flight class moves to get away from it.
That would create a miraculous solution to the problem in about 24 hours.