> Pay to buy these homeowners out of their single family and convert the land to multifamily.

But that's what the market is already doing? In your example from above, you can take a $400,000 windfall and move somewhere else, or take out a mortgage against that $400,000 of new equity to pay your taxes and still come out waaaaay ahead. I don't understand your characterization of a second mortgage as "taking the house away" at all.