The only sustainable way to increase demand for vacant houses is to spur the formation of new households. Admitting more skilled immigrants, who tend to earn enough to buy homes, would accomplish that while paying other dividends to the U.S. economy.
A decline in the national housing price level would need to be substantial to trigger a significant rise in foreclosures, because the vast majority of homeowners have built up substantial equity in their homes despite large mortgage-market financed withdrawals of home equity in recent years.
Even though some down payments are borrowed, it would take a large, and historically most unusual, fall in home prices to wipe out a significant part of home equity. Many of those who purchased their residence more than a year ago have equity buffers in their homes adequate to withstand any price decline other than a very deep one.
