The New E.D. -- Environmental Density

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Developers often have an E.D. problem and are not even aware of it. No, not the type of E.D. temporarily cured with Viagra. Environmental Density — E.D. — is the measurement of the impact of man made construction on a site. In simple terms, E.D. is the average per acre volume of impervious surface due to land development construction. It has two very important impacts, one environmental, and one financial. One acre of land is 43,560 square feet. The lower the E.D. — square foot of impervious surface area divided by 43,560 — the lower the surface area of manmade structures that divert rain run-off, and the less environmental damage.

Some municipalities have impervious surface limitations in their regulations. These limitations can be counter to human benefits. For example, a developer faced with the limits of allowed impervious surface area would rather not propose a walking system; the regulations could mean a choice between walkways and homes. E.D., on the other hand, is not an imposed limit, but a way to measure the efficiency of the neighborhood design.

Don’t bother searching the internet for opposing articles on E.D., because we invented the term's use in relationship to modern land development right here at

From a financial perspective, the lower the volume of manmade stuff, the lower the development cost. The savings translate into more money that can be spent on higher quality development and/or a drop in the cost of housing and commercial construction. In other words higher quality development at more affordable prices. This affects everyone, worldwide.

It doesn't matter if the site is a New Urban “Smart Growth” design, a subdivision in “Garage Grove Acres”, or a Prefurbia neighborhood. E.D. is the number that can easily indicate the direct environmental impact of land development. The E.D. is essentially the Efficiency of Development.

Assuming that New Geography readers are not all engineers, I’ll use some simple examples of E.D.:

If the design is wasteful (eliminating waste in design is NOT a subject taught in land planning schools – but it should be), then costs and environmental impacts increase. Nobody but the paving and earthwork contractors being paid to build excessive infrastructure gain from wasteful development. The developer's profit decreases and the city's maintenance cost escalates from having to maintain excessive infrastructure… forever. We all pay for this!

In an urban high density development which has a very large ratio of hard surface area to organic ground (sometimes the E.D. reaches 100%), there are often opportunities to lower the inorganic percentage. Green roofs (landscaped rooftops) have an impact on E.D. because, in theory, the rainfall is held in soils that water landscaping. However, this assumes existing building structures can handle the additional weight and can be modified to properly maintain an organic area. Organic space on ground level benefits 100% of the population, as opposed to a green roof many stories above the pedestrian ways. So for the purposes of this article we will define all rooftops (urban or suburban) as negative impact square footage. Walks, streets, and driveways are all hard surface areas that divert rain. Organic areas absorb rain. Run-off from hard surface area negatively affects the environment.

Velocity is another problem. Run-off travels on hard surfaces at a much higher rate than it does on landscaped ground. The worst rates are found where there are long runs of straight street with rain traveling along gutters; curved design slows it a bit . Velocity builds momentum as more rain collects in gutters, inlets and sewer pipes. Eventually this wall of water reaches the end of developed land and spills into a natural system, carrying pollutants into major bodies of water. That oil slick on your driveway can be carried to environmentally sensitive areas hundreds or thousands of miles away in a heavy downpour.

Lower the E.D. ratio and some magnificent things happen.

A gain in organic space can reduce the disruption of the earth — the moving of dirt — which can significantly lower development costs, as well as provide surface run-off conduits which cost much less than sewer pipe. The designer must learn how to identify waste, and then take the steps to reduce it. This adds an additional element in the initial planning stages, but an extra day or two in design could reduce development costs by hundreds of thousands of dollars.
Rooftop surface can be reduced by building up, not out. The trend to build single level housing for the empty-nester market results in sprawling homes with terrible E.D. ratios, and it adds to the costs of the structures; roofs and foundations certainly are not cheap. Sprawling homes require longer streets to be reached, another increase in costs and environmental damage. A residential elevator is about $14,000 (installed) for a two story home, and $22,000 for three floors . By using them, builders can construct compact structures and plummet the E.D. ratio.

Paved areas can be reduced by changing regulations to allow vast, commercial parking areas, shared. by users that have different peak times. Some cities use progressive thinking, and allow this simple technique to lower the E.D. ratio of a region. Paved areas built to municipal standards are incredibly expensive, making the E.D. ratio even more critical.

When we developed Performance Planning Systems we wanted to create the tools to easily determine E.D. while still in the initial design phases, as well as to provide the education to recognize waste and teach how it can be reduced. E.D. relies on this new technology; tracing accurate space for the calculations would have been too tedious and time consuming in the past. Environmental Density, unlike impervious surface limits, does not impede efforts to create great neighborhoods. It’s not a restriction on what can be built, but a measure of a design's efficiency that can benefit builders, developers, and environmentalists.

Rick Harrison is President of Rick Harrison Site Design Studio and Neighborhood Innovations, LLC. He is author of Prefurbia: Reinventing The Suburbs From Disdainable To Sustainable and creator of Performance Planning System. His websites are and

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We have the solutions

We have the solutions already, all we need is the determination to actually apply them. The construction companies should be in a close relationship with junk removal companies and the recycling lines too, reusing the construction material is another factor that should pe considered in this equation. Our local junk Removal La Verne is actively involved in recycling project, it's a formula that works just great although I think we still have a lot to wait until the green roof are in place.

Environmental Density

I amy be misreading the text but it does not seem efficient to have only "impervious" and organic or natural surfaces as determinants of ED.

Most of our codes etc in NZ allow for semi-permeable surfaces as well, and in fact some, like Gobi Blocks, are counted as fully permeable because their uneven texture (grass and concrete every few inches) slows water runoff more than grass alone.
We also use mussel shells to surface walking tracks where mussel farms are nearby.

The general term used here for addressing ED is "low impact design" where a range of techniques for all the systems (e.g. swales rather than pipes) are used to address the general problem of ED.

Owen McShane, Kaiwaka, New Zealand.
Director, Centre for Resource Management Studies.

About Pervious solutions...

Your comment well taken, but the ED ratio is more concerned with the overall efficiency of the layout than the type of surfaces...

For example, a designer that does not realise they have created excessive amounts of street over a more efficient design option could not easily run the ED ratio in a timely manner before the technology existed to perform these calculations automatically. As such they came up with a design - proposed it and it got approved and built, whereas another option could (in theory) reduce the infrastucture say, 30%.

Your assumption is that the paving could be pervious, which is true, but that would still be an inefficient deisgn with pervious paving. What we are saying, what if we could reduce the paving 30% by having the ability to run options until the ED ratio is as low as it could go. Pervious paving is also not cheap - more efficient designs that pave less will always have less enviromental and economic costs.

I hope that clarifies ED.



Pervious solutions

It certainly does.
Now that I understand the focus on surface area I see how it fits into your general approach.

I am doing my best to advocate the prefurbia case down here.
We need a model development.

Owen McShane, Kaiwaka, New Zealand.
Director, Centre for Resource Management Studies.