September 2, 2014
By Paul Emrath, Ph.D.
Report available to the public as a courtesy of HousingEconomics.com
Data collected during a 2014 survey allow NAHB to compute, for the first time, summary statistics for the typical residential subdivision being built in the United States. The summary statistics for development projects with at least 4 housing units (a conventional threshold for defining subdivisions) are as follows:
- Median size: 24 acres.
- Median number of housing units: 60.
- Share that include retail space: 12%.
- Share that include other (non-retail) commercial space: 11%.
For many purposes, however, results broken down for particular types of subdivision will be more useful, as aggregate statistics lumping together multifamily and single-family development, for example, may be difficult to interpret. The balance of this article, therefore, in addition to briefly summarizing the survey methodology, will discuss characteristics typical for six major types of subdivision.
Collecting and Analyzing the Data
In January of 2014, NAHB sent a survey electronically to 2,042 NAHB members whose primary activity was land development. A total of 266 developers responded (a 13 percent response rate). The survey asked developers to provide basic information on up to three projects currently have under way or completed recently. Eighty-eight developers provided information on 1 project, 56 on 2 projects, and 111 on 3 projects; so the sample contains information on a total of 533 projects. In the U.S., most jurisdictions require that projects have a minimum number of housing units to classify it as a subdivision. Although practices vary, a common threshold is 4 housing units. The vast majority (96 percent) of projects captured in the NAHB survey contain at least 4 housing units. This article presents statistics only for these 4+ unit developments and refers to them as subdivisions. For readers interested in 1-3 unit projects, results are tabulated and shown separately in the “detailed tables” (also available under Additional Resources).
Acres per Subdivision
Geographic size, like most subdivision characteristics, varies widely. At the top end, some projects span many square miles, and averages tend to be driven strongly by these relatively few extreme cases. For example, the average size of subdivision in survey sample is 241 acres, compared to the median of only 24. Because the median better reflects projects being developed by a large share of NAHB members, this report generally uses medians to describe “typical” subdivisions (although for completeness, the detailed tables contain averages as well as medians).
Among other things, the NAHB survey provided developers with a brief summary of the official definition of a metropolitan area (a densely settled urban area and surrounding counties tied to the urban area by frequent commuting) and asked if each development was inside or outside a metro area. The median size of subdivisions is 20 acres inside metropolitan areas and 40 acres outside of metros (Figure 1).
The survey also collected information on the type of housing built in the subdivision: single-family detached, townhomes, multifamily or some mix of these types. The “mixed” subdivisions contain every possible combination, including subdivisions with all three housing types, and tend to be relatively large. The median size of mixed subdivisions is 85 acres, compared to 22 acres for single-family-only, 10 acres for townhome-only, and 12 acres for multifamily-only developments.
Homes per Acre
Size of a subdivision can also be measured by the number of housing units it contains. In this respect, subdivisions inside and outside of metropolitan areas are quite similar. The median number of housing units in subdivisions built inside metro areas is 59. The median in subdivisions built outside of metro areas is 63.
When comparing subdivisions by type of housing built in them, mixed developments, which tend to be the largest in terms of total acreage, also contain the largest number of housing units. The median in mixed subdivisions is 291 housing units, compared to 48 housing units in single-family-only, 56 in townhome-only, and 86 in multifamily-only subdivisions (Figure 2).
Prior to the 2014 survey, NAHB lacked estimates of the number of housing units per subdivision. Such estimates can be useful in a number of contexts. For example, NAHB is currently projecting that multifamily starts and new single-family sales together will sum to a little over 850,000 housing units in 2014. Assuming this represents housing in subdivisions of average size,[*] the 850,000 should be built in a total of about 2,500 subdivisions. Although this is a rough estimate, it is the first of its kind NAHB has been able to generate.
Given data on number of housing units and total acreage, it’s also possible to calculate the gross density of a subdivision. We’ve already seen that subdivisions inside metropolitan areas cover fewer acres but contain roughly the same number of housing units, so it follows that the development tends to be denser inside metros. The median for subdivisions developed inside metro areas is 2.9 total units per acre, compared to 1.8 units per acre for non-metro subdivisions. The median for subdivisions with only multifamily units is 11.2 units per acre, compared to 5.9 for townhome-only projects, 3.1 for mixed, and 2.1 for single-family-only subdivisions (Figure 3).
It is important to bear in mind that this is gross density—i.e., density based on total acreage in the entire development. It is not net residential density, which would use only by the actual land used for residential purposes as the base—subtracting any area set aside for environmental reasons, parks, or simply open space; as well as any land used for commercial or other non-residential uses, and possibly streets (some jurisdictions have an official definition of net residential density that includes streets, but practices vary).
For anyone particularly interested in much land in allocated to housing in a strict sense, the standard reference is lot size Census Bureau’s series on the Characteristics of New Single-family Homes Sold. The series starts with a median lot size of just over 10,000 square feet starts in 1976. After that, median lot size drifts downward and has been consistently below 9,000 square feet in the twenty-first century—about 8,600 in 2013.
Given that an acre is 43,560 square feet, this works out to a little over 5 homes per acre in the typical single- family subdivision, if nothing but the land for the lots is included. This is substantially above the median gross density for single-family subdivisions calculated from NAHB’s survey, implying that the amount of land allocated to open space, streets, etc. in the typical subdivision tends to be substantial.
Land Not Used for Housing
Although the NAHB survey didn’t collect data on total acreage set aside for non-residential purposes, it did ask about some specific uses, such as land set aside for retail or other commercial structures.
Whether inside or outside a metropolitan area, roughly 12 percent of subdivisions include some retail space. By type of housing, 41 percent of mixed housing subdivisions include retail space, compared to 23 percent of multifamily-only, 8 percent of townhome-only, and a scant 4 percent of single-family-only developments. Similarly, 12 percent of subdivisions outside and 10 percent of subdivisions inside metro areas include commercial space other than retail. Thirty-six percent of mixed housing developments include non-retail commercial space, compared to 20 percent of multifamily-only, and 4 percent of both single-family-only and townhome-only subdivisions (Figure 4).
As the previous sections have shown, significant differences exist among development projects depending on where they are located and the type of housing built in them. For convenience, this section collects results for each of the six types of subdivisions discussed above.
Subdivisions Inside Metropolitan Areas:
- Median size: 20 acres.
- Median number of housing units: 59.
- Median gross density: 2.9 units per acre.[†]
- 12% include retail space.
- 10% include other (non-retail) commercial space.
Subdivisions Outside Metropolitan Areas:
- Median size: 40 acres.
- Median number of housing units: 63.
- Median gross density: 1.8 units per acre.[†]
- 12% include retail space.
- 12% also include other (non-retail) commercial space.
- Median size: 22 acres.
- Median number of housing units: 48.
- Median gross density: 2.1 units per acre.[†]
- 4% include retail space.
- 4% include other (non-retail) commercial space.
- Median size: 10 acres.
- Median number of housing units: 56.
- Median gross density: 5.9 units per acre.[†]
- 8% include retail space.
- 4% also include other (non-retail) commercial space.
- Median size: 12 acres.
- Median number of housing units: 86.
- Median gross density: 11.2 units per acre.[†]
- 23% include retail space.
- 20% also include other (non-retail) commercial space.
Subdivisions with a Mix of Housing Types:
- Median size: 85 acres.
- Median number of housing units (including all types): 291.
- Median gross density: 3.1 units per acre.[†]
- 41% include retail space.
- 36% also include other (non-retail) commercial space.
Cross tabulations and considerably more detail for each type of subdivision are available in the detailed tables. NAHB is currently planning to repeat its development project survey in mid-2015, in order to see if any of the characteristics of typical subdivisions are changing. Possible changes would be especially worth investigating if the recovery in home building activity (which is still running well below its long-run average) gathers momentum.
[*]The average number of housing units in a subdivision (342) rather than the median is used in this case, because the medians don’t mathematically sum to the total number of units in the country.
[†]Gross density is based on all land in the development, including land set aside for open space, non-residential uses, etc. Also, many of the standard operations of arithmetic don’t apply to medians; so median gross density can’t be obtained by dividing the median number of housing units by median acreage.
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