Open space ratio

Open space ratio (OSR) is the older device that requires open space on a zoning lot in proportion to the floor area built on it — the bigger the building, the more open ground it owes. It is the partner of the height factor in non-contextual residence districts: together they rewarded height, since raising the building freed ground area that then satisfied the growing open-space obligation.

The lawns of superblock housing are OSR arithmetic rendered as landscape, and they are often not spare land: the open space is committed to the existing building's floor area, constraining infill. OSR contrasts with lot coverage, which caps the footprint as a share of the lot regardless of how much floor area stands on it.