Our very large rain garden will mitigate a significant amount of stormwater [1] - we will also sell stormwater mitigation rights to future developments surrounding the plot, because we will have surplus stormwater mitigation. Since our development is in the floodplain, and is at the bottom of a large hill with a neighborhood developed before advanced stormwater management, we will hope to mitigate a lot of the local stormwater issues.
The rain garden is going to occupy some amount of the green space. The green space shown is 70,000 square feet, but the rain garden does not have to occupy all the space. Rain gardens cost anywhere from $5-13psf [2 (inflation adjusted)]. We will probably construct 5,000-10,000 square feet of rain garden. We will also be planting trees and other native plants on the property for aesthetic reasons, although we will use species that do not block out the sun for our floor-mounted solar panels.