The impervious surface math, and why it shapes the design
Winnetka allows a maximum of 50 percent of lot area to be covered by all impermeable surfaces. The building component is capped separately at 25 percent of lot area — 27 percent for pre-FAR buildings in R-5 and R-4 — and the balance is available for driveways, patios and walks so long as the combined total stays under 50 percent. In the R-5, R-4 and R-3 districts, a maximum of 30 percent of the required front yard may be impermeable.
In practice that is a landscape-friendly ratio, but it is also a budget you have to spend deliberately. If the driveway and the front walk eat the front yard allowance, the back terrace shrinks. We run those numbers at concept stage, not at permit stage, and we look hard at permeable paver assemblies where they buy back coverage without giving up the look.

