Everyone knows the cities where housing is most expensive--New York, San Francisco, and Washington. But The Washington Post's Emily Badger breaks down some of the reasons why these areas are so expensive.

Land is obviously part of the problem. San Francisco and Boston, hemmed in by water, have only so much of it left to build on. But while some of the obstacles are natural — the Bay Area's got those hills, too — others are man made.

While NIMBYs and zoning are some of these man made factors. they are not the only issues.

This last factor — building permit delays — turns out to be a significant one, according to an analysis by Trulia chief economist Ralph McLaughlin. New York, Boston and San Francisco all take much longer on average than Atlanta or Charlotte to grant building permits to developers.

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