The common purpose of these two sets of regulations is to offer maximum design flexibility while setting reasonable but firm standards to protect access of light and air to public streets and adjacent buildings.

This purpose is implemented by the two alternate sets of regulations, which are based on two distinct approaches. Both sets are concerned with daylight at street level.

The daylight compensation regulations require any building generally to be built within a sky exposure curve which relates required setbacks to building heights. Within limits, the buildings may encroach outside the curve but only if extra setbacks or recesses provided elsewhere on the same street frontage of the zoning lot compensate for the encroachment. In the regulations, the sky exposure curve for three street width categories is represented in each case by a table of required setbacks related to building heights.

The daylight evaluation regulations measure and evaluate portions of sky blocked by a building as viewed from specified vantage points in the street. The building is plotted on daylight evaluation charts representing the zoning lot's available daylight from specified vantage points, measured in daylight squares as defined in paragraph (b) of Section 81-272. Daylight blockage by the building is then measured to determine the building's score on each frontage and the average score for the building as a whole. The regulations specify the minimum passing scores for compliance.