Better Site Design: Open Space

Open space design, also known as conservation development or cluster development, is a better site design technique that concentrates dwelling units in a compact area in one portion of the development site in exchange for providing open space and natural areas elsewhere on the site. The minimum lot sizes, setbacks and frontage distances for the residential zone are relaxed in order to create the open space at the site. Open space designs have many benefits in comparison to the conventional subdivisions that they replace: they can reduce impervious cover, stormwater pollutants, construction costs, grading, and the loss of natural areas. However, many communities lack zoning ordinances to permit open space development, and even those that have enacted ordinances may need to revise them to achieve greater water quality and environmental benefits.

It should also be noted that the benefits of open space design can be amplified when it is combined with other better site design techniques such as narrow streets, open channels and alternative turnarounds (see Narrow Streets and Alternative Turnarounds Fact Sheets).

This site (Figure 1) represents a typical single-family residential subdivision developed in the early 1990's in the Chesapeake Bay watershed. Lots are a uniform size and shape, cul-de-sacs dominate the landscape, existing natural resources (forests, streams, wetlands) are only moderately protected, street widths are excessively wide, driveway widths and lengths create unnecessary impervious cover, and lawn turf is the dominant feature in the developed site. Total impervious cover: 12.0 acres (27% of total site).

This site design (Figure 2) incorporates many of the principles of open space design. Specifically, lots are narrower and varied in shape, existing natural resources (forests, streams, wetlands) have been preserved wherever possible, a significant portion of the site is retained as natural open space, street widths are narrow, houses are closer to the road thus creating shorter driveway lengths, a minimum 100 ft. buffer is provided along all intermittent and perennial streams, stormwater is managed in a "treatment train" with bioretention facilities coupled with a wet extended detention pond, and lawn turf is minimized. Total impervious cover: 9.1 acres (20% of total site).

The codes and ordinances that govern residential development in many communities do not allow developers to build anything other than conventional subdivisions. Consequently, it may be necessary to enact a new ordinance or revise current development regulations to enable developers to pursue this design option. See the Open Space Ordinances in the Ordinance Category.

Open space design is widely applicable to most forms of residential development. The greatest stormwater and pollutant reduction benefits typically occur when open space design is applied to residential zones that have larger lots (less than two dwelling unit per acre). In these types of large lot zones, as great deal of natural or community open space can be created by shrinking lot sizes. However, open space design may not always be a viable option for high density residential zones, redevelopment or infill development, where lots are small to begin with and clustering will yield little open space. In rural areas, open space design may need to be adapted, especially in communities where shared septic fields are not currently allowed by public health authorities.

Open space design can be employed in nearly all geographic regions of the country with the result of different types of open space being conserved (forest, prairie, farmland, chaparral, or desert).

Several site planning techniques have been proposed for designing effective open space developments (Arendt, 1997, and DE DNREC, 1997). Often, a necessary first step is adoption of a local ordinance that allows open space design within conventional residential zones. Such ordinances specify more flexible and smaller lot sizes, setbacks, and frontage distances for the residential zone, as well as minimum requirements for open space and natural area conservation. Other key elements of effective open space ordinances include requirements for the consolidation and use of open space, as well as enforceable provisions for managing the open space on a common basis.

There are a number of real and perceived barriers to the wider acceptance of open space designs by developers, local governments and the general public. For example, despite strong evidence to the contrary, some developers still feel that open space designs are less marketable than conventional residential subdivisions. In other cases, developers contend that the review process for open space design is more lengthy, costly, and potentially controversial than that required for conventional subdivisions, and thus, not worth the trouble.

Local governments may be concerned that homeowner associations lack the financial resources, liability insurance or technical competence to maintain open space adequately. Finally, the general public is often suspicious of cluster or open space development proposals, feeling that they are a Trojan horse for more intense development, traffic and other local concerns.

In reality, many of these misconceptions can be directly addressed through a clear open space ordinance and by providing training and incentives to the development and engineering community.

Once established, common open space and natural conservation areas must be managed by a responsible party able to maintain the areas in a natural state in perpetuity. Typically, the open space is protected by legally enforceable deed restrictions, conservation easements and maintenance agreements. In most communities, the authority for managing open space falls to a homeowner or community association or a land trust.

Annual maintenance tasks for open space managed as natural areas are almost non-existent, and the annual maintenance cost for managing an acre of natural area is less than $75 (CWP, 1998). It may be useful to develop a habitat plan for natural areas that may require periodic management actions.

Recent redesign research indicates that open space design can provide impressive pollutant reduction benefits compared to the conventional subdivisions they replace. For example, the Center for Watershed Protection (1998) reported that nutrient export declined by 45% to 60% when two conventional subdivisions were redesigned as open space subdivisions. Other researchers have reported similar levels of pollutant reductions when conventional subdivisions were replaced by open space subdivisions (Maurer, 1996; DE DNREC, 1997; Dreher, 1994; and SCCCL, 1995). In all cases, the reduction in pollutants was due primarily to the sharp drop in runoff caused by the lower impervious cover associated with open space subdivisions. And indeed, in the redesign studies cited above, impervious cover declined by an average of 34% when open space designs were utilized.

Along with reduced imperviousness, open space designs provide a host of other environmental benefits lacking in most conventional designs. These developments reduce potential pressure to encroach on resource and buffer areas, as enough open space is usually reserved to accommodate resource protection areas. As less land is cleared during the construction process, the potential for soil erosion is also greatly diminished. Perhaps most importantly, open space design reserves 25% - 50% of the development site in green space that would not otherwise be protected, preserving a greater range of landscapes and habitat "islands" that can support considerable diversity in mammals, songbirds and other wildlife.

Open space developments can be significantly less expensive to build than conventional subdivisions. Most of the cost savings are due to savings in road building and stormwater management conveyance costs. In fact, the use of open space design techniques at a residential development in Davis, California provided an estimated infrastructure construction costs savings of $800 per home (Liptan and Brown, 1996). Other examples demonstrate infrastructure costs savings ranging from 11 to 66%. The following table lists some of the projected construction cost savings generated by the use of open space redesign at several residential sites.

Table 1. Projected Construction Cost Savings for Open Space Designs from Redesign Analyses
Residential Development	Construction Savings (%)	Notes
Remlik Hall ¹	52	Includes costs for engineering, road construction, and obtaining water and sewer permits
Duck Crossing ²	12	Includes roads stormwater management, and reforestation
Tharpe Knoll ³	56	Includes roads and stormwater management
Chapel Run ³	64	Includes roads, stormwater management, and reforestation
Pleasant Hill ³	43	Includes roads, stormwater management, and reforestation
Rapahannock ²	20	Includes roads, stormwater management, and reforestation
Buckingham Greene ³	63	Includes roads and stormwater management
Canton, Ohio⁴	66	Includes roads and stormwater management
Sources: ¹ Maurer, 1996; ² CWP, 1998; ³DE DNREC, 1997; ⁴ NAHB, 1986

While open space developments are frequently less expensive to build, developers find that these properties often command higher prices than homes in more conventional developments. Several regional studies estimate that residential properties in open space developments garner premiums that are 5 to 32% higher than conventional subdivisions and moreover, sell or lease at an increased rate. In Massachusetts, cluster developments were found to appreciate 12% faster than conventional subdivisions over a twenty year period (Lacey and Arendt, 1990). And in Atlanta, the presence of trees and natural areas measurably increased the residential property tax base (Anderson and Cordell, 1982).

In addition to being aesthetically pleasing, the reduced impervious cover and increased tree canopy associated with open space development reduces the size and cost of downstream stormwater treatment facilities. The resulting cost savings can be considerable, as the cost to treat the quality and quantity of stormwater from a single impervious acre can range from $2,000 to a staggering $50,000. The increased open space within a cluster development also provides a greater range of locations for more cost-effective stormwater practices. Clearly, open space developments are valuable from an economic as well as an environmental standpoint

Anderson, L.M. and H.K. Cornell. "Residential Property Values Improved by Landscaping With Trees." Southern Journal of Applied Forestry pp 162-6.

Arendt, Randall. 1994. Rural by Design. American Planning Association. Planners Book Service. Chicago, IL

Arendt, Randall. 1996. Conservation Design for Subdivisions: A Practical Guide to Creating Open Space Networks. American Planning Association Planners Book Service. Chicago, IL

Center for Watershed Protection (CWP). 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Ellicott City, MD.

Delaware Department of Natural Resources and Environmental Control (DE DNREC) and The Environmental Management Center of the Brandywine Conservancy. 1997. Conservation Design for Stormwater Management.

Dreher, D.W. and T.H. Price. 1994. Reducing the Impacts of Urban Runoff: The Advantages of Alternative Site Design Approaches. Northeastern Illinois Planning Commission. Chicago, Il.

Lacey, J. and R. Arendt 1990. An Examination of Market Appreciation for Clustered Housing with Permanently Protected Open Space. Center for Rural Massachusetts, Amherst, MA. 14 pp.

Liptan, T. and C.K. Brown. 1996. A Cost Comparison of Conventional and Water Quality-Based Stormwater Designs. City of Portland. Bureau of Environmental Services. Portland, OR. 16 pp.

Maurer, George. 1996. A Better Way to Grow: For More Livable Communities and a Healthier Cheasapeake Bay. Chesapeake Bay Foundation, Annapolis, MD.

Montgomery County (Pennsylvania) Planning Commission. Guidelines for Open Space Management in the Land Preservation District. Norristown, PA

National Association of Homebuilders (NAHB). 1986. Cost Effective Site Planning. Washington, D.C.

Schueler, Thomas R. 1995. Site Planning for Urban Stream Protection. Center for Watershed Protection. Ellicott City, MD.

South Carolina Coastal Conservation League (SCCCL). 1995. "Getting a Rein on Runoff: How Sprawl and Traditional Towns Compare." SCCCL Land Development Bulletin (Number 7). Charleston, SC.