Their construction costs material resources. The only exception is where you have turned on the Manipulation that allows every nation to have buildings completed each turn. Unlike units, they do not carry over any production overflow, but they do (with one exception) start with any existing overflow from a unit. (The exception is space ship parts even though they are listed at the bottom of the appropriate "buildings" menus.) In some similar games, invasion can destroy buildings. That does not happen in C-evo except for the automatic destruction of a palace, courthouse, or town hall.