If you have no more empty spots in your City, you can always demolish a building. You can use this ability in order to replace the demolished building with a type of building that may be missing from your City.

Foundries, Lumberjacks' Houses, and Stonecutters' Houses require a large tile in the City. Farmers' Houses and Mansions require a small tile.

Select one of the buildings, and you'll see the information icon here:



Press this icon and select "demolish": 



You can then build a new building on the vacant site.