Short answer: In CBC, “average grade plane” is the “grade plane.” You calculate it as the reference plane that represents the average of the finished ground levels adjoining the building at all exterior walls. Where the finished ground slopes away from an exterior wall, establish the reference plane for that wall using the lowest points in the area between the building and the lot line—or, if the lot line is more than 6 feet (1829 mm) away, between the building and a point 6 feet (1829 mm) from the building—then average around the building to set the grade plane (§ 202).
How to apply it on a sloped lot
- Identify finished ground levels that adjoin each exterior wall. Use the post-construction “finished” grades, not preexisting terrain (§ 202).
- For any wall where the adjacent ground drops away from the building, determine the lowest points within the strip between that wall and the lot line (or within 6 feet (1829 mm) of the wall where the lot line is farther). Use those lowest points when establishing the reference plane that is then averaged with the others (§ 202).
- Average the resulting elevations around all exterior walls to set the building’s grade plane. CBC uses this plane to determine items such as stories “above grade plane” (§ 202). San Francisco codes that reference “stories above grade plane” rely on this CBC definition.
Don’t confuse this with Planning Code height measurement
San Francisco zoning height is measured under Planning Code § 260, which starts from curb level or averaged cross-sections of ground and uses different rules for up- or down‑sloping streets. That system is separate from the CBC grade-plane calculation used for building code purposes (e.g., stories above grade plane) and can yield different “heights” for the same project.
What the code text does not specify
CBC § 202 tells you to average the finished ground adjoining all exterior walls and to use the lowest points within the lot-line/6‑foot band where the ground slopes away, but it does not prescribe exactly how many measurement points to take along a wall or a specific averaging protocol beyond that. If your site has complex terracing or retaining walls close to the building, coordinate the point selection and documentation method with the San Francisco Department of Building Inspection, since § 202 itself does not detail those procedural steps.