As housing fads change, some styles will go out of favor. Great rooms replace lots of small rooms.

Technology changes, too, will make some houses obsolete; older roof technology, plumbing, HVAC, inefficient systems, too little insulation and no space for more.

Houses that are obsolete for their location will sell as teardowns. In that situation, the land value is measured by the selling price plus the cost of removing the old house from the site. Assessors can work with those figures very well. Connect the dots from one to the next to the next, and the land value map shapes up.