Residential zone is the term for a zone defined by the zoning plan of the respective area in which certain buildings may be erected. Residential zones are usually classified according to the restrictions on the type, height and floor space of the buildings that may be constructed in them. 

In Switzerland, there are three pure residential zones: W1 (single-storey residential), W2 (two-storey residential) and W3 (three-storey residential). In these zones, buildings may be erected up to a certain height and with a certain number of storeys. 

The permitted development ratios or the number of storeys indicates how much building area may be built per square metre of land area.