"The binding restriction for single family residential lots is not the lot size (in contrast to lots with commercial and multifamily buildings) but the number of dwellings allowed on these lots. While many single-family homes in the District are constructed on large lots, they are often in zones where only detached housing is allowed (R-1-A, and R-1-B). We found 21,000 homes just in these zones, which have no hope for densification even when there could be plenty land. For these, the only path to adding density is by way of an Accessory Dwelling Unit, such as a basement rental or small standalone apartment (sometimes known as a granny flat)."