The main purpose of stipulating the density of built use or degree of building coverage is to stipulate the density of development and the construction height of physical structures, and the proportion of a development site which may be built on. The density of built use can be determined by including stipulations on
- the site occupancy index or plot coverage rate,
- the floor-space index or floor area,
- the cubing ratio or building volume,
- the number of full storeys
- the height of physical structures.
The site occupancy index is the ratio of the actual surface area of a plot to permissible coverage. It is expressed as a simple ratio of built surface area to site area. This value makes it possible to calculate the proportion of the surface area of a development site which may be covered. The floor-space index indicates the relationship between the total floor space of all of the storeys in a building and the size of the development site. The total floor space is calculated on the basis on the external dimensions of all full storeys; i.e., it includes external and internal walls and stairways, although exceptions are possible for certain types of physical structure and sections of buildings.
The cubing ratio indicates the maximum volume of buildings per square metre of plot area. It is calculated on the basis of the external dimensions of buildings from the floor of the lowest full storey to the ceiling of the uppermost full storey. In respect of industrial, commercial and special-use areas, planning designations may be made without the need for details on the number of storeys. Depending on the building use category, the Land Utilisation Ordinance sets limits to the site occupancy index, floorspace index, and cubing ratio, which may for specific reasons be exceeded.
When the degree of building coverage is set in a binding land-use plan, the site occupancy index or the proportion of the site to be covered by physical structures must always be stated; the number of full storeys and the height of construction need to be set if failure to do so might be detrimental to the public interest, and especially to the appearance of the locality or landscape.