Density is the degree to which an area is filled or occupied. In the context of housing and planning policy, it generally refers to the quantity of people or buildings in an area. It is useful to understand and measure density both as a way of gauging how land is used and to help make informed decisions about new development and what physical and social infrastructure is required to support it.

Measuring housing density

There are many ways of measuring housing density, none of which is perfect. The London Plan (see below) relies on two principal measures: the number of homes (units) per hectare (u/ha) and the number of habitable rooms per hectare (hr/ha).

The way in which a site area is measured for planning purposes is not always consistent and without a consistent approach it is hard to make meaningful comparisons. For example, the same location can have very different housing densities if the number of homes is measured according to gross site area (including land used for surrounding shops, services, roads and public realm) as opposed to the net built area (which restricts the calculation only to the land on which the residential buildings stand).

Choosing what site area to measure is particularly important when it comes to larger and mixed-use schemes. Open spaces, roads, parking and non-residential uses are generally an integral part of these developments. The London Plan convention of stripping out non-residential uses for the purposes of the density calculation, produces higher housing densities than the unadjusted alternative.

Housing density policy in London

The London Plan contains a number of policies that relate to housing density, including: where certain densities are more acceptable than others; how density should manifest itself architecturally; and how a scheme should fit in with its surroundings. At the heart of this approach sits the London Plan’s density matrix (Figure 3) which is based on homes per hectare and habitable rooms per hectare. It sets out indicative density ranges suitable for areas with different levels of public transport accessibility (PTAL) according to three neighbourhood types - suburban, urban and central. The matrix is intended as guidance and the London Plan makes clear that it should not be applied mechanistically when making planning decisions.

^{Source: London Plan 2015}

While the density of an application for residential development provides a measurement of how intensively a piece of land is being used, it says nothing about the quality of the homes being built or how the type of homes meets the housing need of an area. The density matrix is a useful cross-check for planners when looking at particular schemes but, as discussed later in Planning Policy, it currently fails to capture the complexity of London’s settings, and has no regard to other factors which are relevant to determining how many homes there should be in a development.