Housing fashions flare and fade nearly as fast as clothing fads. The backyard fire pit? It will go the way of the ruffled tuxedo shirt. The soaring two-story foyers? They might as well be women's shoulder pads.

Obsolescence is more serious. People will reject the obsolete. Their needs have changed, and technology has rendered the old stuff useless.

Obsolescence is not just a function of age. An old house can be rehabbed so it nicely meets the needs of a buyer willing to spend a couple million dollars. But unless owners periodically invest in repairs and upgrades, their homes will fall so far below the standards of current buyers that they become obsolete. Property value then lies almost entirely with the land.

What's obsolete? It could be an extremely small stand-alone house, say 900 square feet or less. Buyers who want only one or two bedrooms, which are all you can jam into such a small house, today look for condos. Other relics: