A wall-sign allowance formula turns something you measure on the building into the most sign area you may put on it. The input varies by code: feet of frontage, square feet of wall, or a flat number. Confirm which one before you multiply, because feet and square feet give wildly different answers.
How one real code handles it
San Luis Obispo, California uses a percentage of the wall with a hard cap, in section 15.40.470(A)(2):
“Wall signs may be a maximum of one hundred square feet or fifteen percent of the building face where the sign is attached, whichever is less.”
“The building face area is determined by multiplying the length of the linear frontage of the tenant space or building by the average height of the tenant space or building.”
Here is that formula on two made-up storefronts:
| Storefront | Building face | 15% of face | Result |
|---|---|---|---|
| 40 ft wide, 16 ft average height | 640 sq ft | 96 sq ft | 96 sq ft (under the 100 cap) |
| 50 ft wide, 16 ft average height | 800 sq ft | 120 sq ft | 100 sq ft (the cap wins) |
The same subsection adds that wall signs “shall not exceed maximum cumulative areas of Section 15.40.460,” so the per-sign answer is not the end of the math on a building with other signs. (Section history: Ord. 1667, 2019. We read it on September 28, 2026. The storefront dimensions are invented.)
What to check on your job
Confirm the eligible frontage or wall, what counts inside the sign area, how separate letters are measured, every per-sign and combined cap, and whether other signs already use part of the allowance. Do that before you subtract existing signs or add a second elevation.