Density is a planning metric that describes the spatial and physical dimensions of crowding in human settlements. It is typically described in terms of the number of dwellings that occupy a unit area of land (dwellings per acre, or du/A in the United States). Dwellings per acre may refer to an individual building on a single lot, or to a collection of buildings in a larger area.
 
Gross density refers to the number of dwellings per gross area of land being measured, including both city blocks and right-of-ways. Net density refers to the number of dwellings per the exclusive area of blocks and lots alone. When density is used to describe Neighborhoods, Districts or Corridors, towns, cities or metropolitan regions, then the qualifiers ‘net or gross’ refer to the number of dwellings per aggregate gross or net land area to be considered.

Density is an ambiguous and confusing planning measure. In modern American cities, crowding is not only a function of the number of dwellings per unit land area, but also of dwelling average size, parking ratios, and ratios of private and shared open space per dwelling. Typical assumptions in conventional codes assume 1000-square-foot dwellings, two cars per dwelling and 500 square feet of open space per dwelling. These numbers vary by transect zone in Form-Based Codes.

Specific density numbers, a reflection of dwelling detachment, attachment and stacking, describe particular architectural patterns of building fabric and urban space. One to ten du/A is considered low density, 11 to 60 du/A middle density, and 61 du/A and up high density. These numbers also correspond roughly to popular understandings of low-rise, mid-rise and high-rise residential types.

Density has a very poor track record as a predictor of architectural and urban form. Codes whose entitlement mechanism is an FAR envelope typically regulate densities for new buildings numerically without reference to architectural typology. For example, an area within an existing Neighborhood may be designated to receive an average density of 40 dwellings per acre, independent of the area of the project site. This entitlement is literally interpreted by developers as the maximum allowable massing for their project.

Architects take these allowable envelopes, construe them to be their building’s raw form, skin them in arbitrary styles and turn them into an instant self-referential project. Such buildings are intentionally incompatible with their surroundings, sometimes too large for their lots, sometimes violently repetitive and out of scale and in all cases sharing no common formal ingredients with the building fabric and space patterns within and among the urban blocks where they are sited. This design strategy has ravaged traditional Urbanism since the 1930s and the unpredictability it promotes has turned density into the most potent political grudge of Neighborhood groups protesting new development.

The antidote to such chaos by design is interpreting allowable density not as a set of repeating buildings of an average density, but as a set of unique buildings in a variety of densities. When blended, these types fit under whatever prescribed average density they may be subject to. Blended density practices promote a harmonious, richly scaled, intentional building fabric and well-formed space figures both within new projects and among new projects and surrounding buildings. They result in a visual reading of building continuity at the scale of the block, and a richness of place-making, from the public realm of streets and parks to the private realm of the space framed within individual projects.