Site Planning and Design: Regulation, Grading, and Drainage
This domain tests site analysis and the regulatory envelope that constrains it: what can be built, where, how water moves, and how people and vehicles arrive.
Site analysis and zoning
- Inventory: topography and contours, soils and borings, floodplain, vegetation, climate and solar path, views, noise, utilities, easements, and adjacent uses.
- Floor area ratio = gross floor area / lot area. Lot coverage = building footprint / lot area. These are different limits and both can bind.
- Setbacks, buffers, and height limits may be measured from the right-of-way rather than the property line. Read the definition before calculating.
- Buildable area = lot area minus setbacks, buffers, and easements. Net site area is not the same as gross.
Grading and drainage
- Contours show elevation; slope percentage = rise / run × 100. Paving typically needs about 2 percent minimum to drain, and accessible routes are limited to a 1:20 running slope and 1:48 cross slope.
- Stormwater: rational method Q = CiA, with C as runoff coefficient, i as rainfall intensity, and A in acres. Low-impact design uses bioswales, permeable paving, and detention to hold the post-development increase.
Worked example
A 60,000 square foot lot with a floor area ratio of 2.5 allows 150,000 gross square feet. With 25 percent lot coverage, the footprint is 15,000 square feet, which implies ten stories, provided the height limit permits it. Drainage: after development, C = 0.85, i = 2.5 inches per hour, A = 1.4 acres, so Q = 0.85 × 2.5 × 1.4 = 2.98 cubic feet per second. Before development, C = 0.2, giving Q = 0.7 cfs. Detention must manage roughly the 2.3 cfs difference for the design storm.
Traps that cost marks
- Using FAR and lot coverage interchangeably.
- Measuring setbacks from the property line when the code measures from the right-of-way.
- Grading an accessible route steeper than 1:20 and calling it a ramp without designing it as one.
- Forgetting that easements and utility corridors remove buildable area even when zoning would allow construction.