Semi-detached house is a house that shares a common wall with its neighbouring property on one side. The two are normally mirror images of the same design. When modern housing estates started to be built in the interwar period, they became very popular, and were marketed as being superior to terraced houses, and generally had space between them and the neighbour one side for a drive (on cheaper properties these were shared to reduce plot size). Thus, they were cheaper to construct and sat on a smaller plot than a detached house of the same size. The shared wall between houses is called a party wall. Legal agreements control any building work which effects a party wall between properties, as the wall is shared.