Principle: The residential site design should have minimal impact on the
existing site.
A sustainable design alters the existing site conditions as little as possible by preserving the elements that are present on a site as well as the natural processes and cycles that support them. This goal is hardest to achieve yet most critical on an undisturbed
natural site where any activity at all alters the site. The very placement of the house and access to it usually requires grading and removal of vegetation. Beyond this primary impact zone, every effort should be made to work around existing trees and other significant vegetation, protect unique geological formations, minimize grading, and preserve natural drainage patterns. On sites that have been drastically changed by a previous homeowner or developer, as is common in many new subdivisions, the challenge is one of saving any remnants of a natural landscape or keeping natural features and elements that were sensitively introduced to the site. The following strategies foster minimal site impact.
Preserve Existing Vegetation
All vegetation that is present on a site should be retained as much as possible, especially trees, whose size and ecological impact are most significant. The one exception to this objective is in fire-prone regions where it is desirable to remove vegetation that is close to the house (also see “Protect from Potential Wildfires” in “Natural Events and Cycles” in this chapter). Existing vegetation fulfills a number of vital environmental functions, such as stabilizing soil, retaining soil moisture, cooling summer air temperatures, reducing the impact of wind, removing carbon dioxide and dust particles from the air, and producing oxygen. Vegetation is also a habitat for many birds, animals, and insects. Removing existing vegetation diminishes these potential benefits and exposes the site to increased runoff and erosion, higher summer air temperatures, wind, and other related problems. If some of it must be removed, then vegetation that is in poor health, invasive, or a nonnative species or that is simply in the way should be removed first. Judicious pruning or the limbing of selective branches should also be considered as an alternative to eliminating vegetation.
To safeguard trees that are to remain on a site, the ground below the canopy within a tree’s drip line should not be altered or compacted in any manner. Most of a tree’s roots exist within the first several feet of soil directly below the tree canopy, though some roots can extend well beyond the tree canopy (Figures 3—1 and 13—11).
During construction, the sensitive ground below all tree canopies should be fenced off to prevent grading, movement of construction equipment, and the storage of construction materials. The proposed design should locate all structures, paved areas, and heavily used lawns outside the tree drip line as well. Structures that must be located under a tree should be elevated above the ground on posts to minimize the necessity of excavating into the ground for linear footers. Likewise, paved surfaces under trees should be porous or in the form of a deck (also see “The Wooded Site,” Chapter 13, page 456). The optimal ground surface for the area below the tree canopy is one covered with ground cover or other woody and herbaceous plants that can protect the tree roots and help to maintain soil moisture.
