Most people who buy a home by taking out a mortgage don't think of themselves as buying on margin. Yet, this is exactly what they are doing, and more so when they take out home equity loans, second mortgages, etc.
The chart below shows how fast mortgage amounts ballooned in recent years and, worse yet, that such debt is now over 52% of the gross value of all dwellings in the US - up from a mere 18.5% in 1950 and 31.5% in 1980 (click to enlarge). According to the Fed, at the end of 2007 US housing was worth $20.1 trillion and mortgage debt came to $10.5 trillion. Housing "margin" is getting bigger and bigger.
Assuming that house prices drop 20% - a reasonable assumption given the recent performance of the Case-Shiller index (see below) - the debt-to-value ratio will jump to 65%, all other things equal.
If housing was treated like a leveraged stock portfolio, margin clerks would be getting ready for "calls". And you know what? Mortgage securitization and derivativization has done exactly that. In a few short years investment bankers and financial engineers managed to turn the most fundamentally conservative asset class into yet another "trading sardine", subject to bubbles and busts...