The mansard roofs in Paris -- taxes were based on number of floors up to the roof line. Mansard roofs disguise the top floor as part of the roof. Narrow build

The mansard roofs in Paris -- taxes were based on number of floors up to the roof line. Mansard roofs disguise the top floor as part of the roof. Narrow buildings -- these buildings were taxed based on the number of linear feet facing the canal. In the picture, the door in the middle is the door for a house that is only as wide as that door, "the skinniest house in the world". Covered / Bricked over windows -- buildings were taxed based on the number of windows. Building owners could save money simply by bricking over some windows.