“Airspace diagonal” means a hypothetical two-dimensional inclined plane which intersects the horizontal plane of a lot at its front property line and which is projected at a given angle to the horizontal surface over the lot area in accordance with the following:

The elevation of the horizontal plane (b) shall be the same as “grade” defined in Section 17.04.310 if grade is at or above the top of curb elevation. If the building grade is below top of curb elevation, then the horizontal line shall be at the elevation of the top of curb. (Ord. 2694 § 2, 1982)