Datum Point

In the City of Los Angeles, the datum point is the lowest point of elevation, located within the property, of the finished surface of the ground, paving, or sidewalk within five feet around the perimeter of the building. It is the reference elevation from which the height of the building is measured.

On plans, the datum point must be clearly identified on the plot plan. On all sections and elevations, the datum point should be called out and the building height shown as a vertical distance from it. Plan checkers will look for this notation early in the review and will return the project for correction if it is missing or ambiguous.

The datum point determines whether a building meets the maximum height limit for its zone. Get the datum point wrong – pick a higher elevation than is allowed, fail to show it, or measure from the wrong reference – and the height calculation is wrong, which means the project may be over the limit without the designer realizing it.

A common confusion: retaining walls cannot be used to raise the effective elevation of finished grade for purposes of measuring the height of a building or structure. The plan checker will look for retaining walls, dirt fills, and other constructs that artificially elevate the ground around the building. If the perimeter has been built up, the datum point is still measured from the lowest point of natural or pre-construction grade.

On hillside lots, the datum point matters more than almost anywhere else. The natural fall of the ground around the perimeter creates dramatic differences in height measurement depending on which point is used. A practitioner who has read the correction sheet will identify the datum point at concept and design the building around it.

Sources: LAMC Section 12.21.1 (Building Height); City of Los Angeles Department of Building & Safety, Plan Check Correction Sheet PC/STR/Corr.Lst.106A.