Density

Article

In planning and placemaking, density is an important tool for measuring the intensity or quantity of units or items occupying a given square area, such as homes/dwelling units per acre, parking spaces per thousand square feet of retail, or households per square mile.  Understanding the various kinds of densities that can be used to describe a given city, district, or land parcel helps in planning, designing and combining the appropriate types and sizes of roads, utilities, sidewalks, buildings, and landscaping to properly serve the uses and create more livable places.

Because cities and settlements are mainly comprised of housing, population and housing density are particularly relevant measures.  The physical character of a residential or mixed-use neighborhood is largely determined by its housing density; crowding would be the result of excessive population density for a given housing density. 

Source: L.A. Housing Department

Specific building types are associated with ranges of housing densities. The illustration on the right depicts a very simple spread of residential densities with low density single-family homes ranging from 4 to 10 dwelling units per acre, medium density townhomes (with horizontally attached units) at 20 to 40 dwelling units per acre, and high density apartments (with horizontally and vertically attached units) at 50 to 100 units per acre.  Real-world examples of high, medium, and low-density housing are illustrated below:

High density neighborhood

Medium density neighborhood

Low density neighborhood

There are also building type sub-classifications which break down residential densities into finer gradations, as shown in this image from the Affordable Housing Design Advisor. 

Each building type has different physical characteristics and density ranges.  These ranges may vary according to local building codes and other conditions. 

The residential density matrix below describes characteristics for various housing types:

Source: WRT/Solomon E.T.C.

It is difficult to correlate density solely to building type or height because factors such as FAR and site coverage also influence density calculations and perceptions. 

For example, a high rise apartment tower with a large open-space area and a dense, mid-rise development with a smaller but better monitored open space area may have an equivalent overall site density, despite being very different in character in livability.

For other land uses (e.g. workplaces or retail areas), measures such as F.A.R., employees per square foot, and parking spaces per thousand square feet of leasable space provide additional indications of density.  It can be difficult to directly compare different types of use districts solely using density measures.   While height, F.A.R., and density are good starting points, the frequency of entries, building articulation, setbacks, and streetscape design largely influence street activity and livability, and should also be included in comparisons.

Density and District Form

Credit: FTB

In most urban areas, density follows a gradient of district types spanning from rural (lower density) to urban (higher density), called the urban transect.  A variety of factors contribute to the density and complexity seen in different urban forms (see PDF for more information). 

Finally, a sharply defined downtown core district, as shown in the image at right, is one way density can be used to create contrast, interest, and place distinctiveness.

Floor Area Ratio

A Floor Area Ratio (F.A.R.) is the mathematical proportion formed by a building’s built floor area (for all floors) divided by the area of its land parcel.  For example, a one-story building with a floor plan length and width of 100 feet by 100 feet on the same size land parcel would have an F.A.R. of 1.0. 

Source: NYC Zoning Handbook

A four (4) story building with a floor plan length and width of 50 feet by 50 feet on the same 100 foot by 100 foot parcel would also have an F.A.R. of 1.0, as shown in the image at right.

F.A.R. is a limited “yardstick” that can convey a general intensity of constructed building space on a given piece of land, but it will not indicate the character of that built volume in terms of the building’s height, length, or width, or the amount of remaining open space (unbuilt land) on the parcel.  As the figures below illustrate, developments with the same F.A.R. can have very different physical characteristics - that may or may not be compatible with the urban scale and density desired in various types of neighborhoods and street environments. 

Resources

PDFs:

• American Dwelling Types