Retaining Walls: Redwood City vs San Mateo
How do retaining walls rules compare between Redwood City, CA and San Mateo, CA?
Redwood City and San Mateo have similar restriction levels.
Redwood City, CA
San Mateo County
A building permit is required for a fence built on top of a retaining wall in unincorporated San Mateo County. Under the California Building Code, retaining walls over 4 feet high (measured from the bottom of the footing) — or any wall supporting a surcharge — also require a permit.
View full Redwood City rules →San Mateo, CA
San Mateo County
Retaining walls in San Mateo under 4 feet in height (measured from bottom of footing) are generally exempt from building permits under the California Building Code. Walls over 4 feet require a building permit and may need engineered plans. San Mateo's hillside terrain makes retaining walls common.
View full San Mateo rules →Key Facts Comparison
| Fact | Redwood City | San Mateo |
|---|---|---|
| Fence on retaining wall | Building permit required | - |
| Wall permit threshold (CBC 105.2) | Over 4 ft from bottom of footing | - |
| Surcharge walls | Permit required at any height | - |
| Abrupt grade shift | Max 12 ft fence/wall (Sec. 8.332.020) | - |
| Fence measured from | Lowest adjacent natural grade | - |
| Code authority | California Building Code + Ch. 8.332 | - |
| Permit Exempt | - | Under 4 ft, no surcharge |
| Permit Required | - | Over 4 ft or with surcharge |
| Engineering | - | Licensed engineer for 4+ ft |
| Drainage | - | Critical — especially hillside |
| Building Division | - | 650-522-7200 |
Highlighted rows indicate differences between cities.
Redwood City FAQ
Do I need a permit for a retaining wall in unincorporated San Mateo County?
Under California Building Code Section 105.2, a permit is required for retaining walls over 4 feet measured from the bottom of the footing, or any wall supporting a surcharge regardless of height. A fence built on top of any retaining wall also requires a building permit.
Can I add a fence on top of my retaining wall?
Yes, but it requires a building permit, and the combined fence height is measured from the lowest adjacent natural grade, so it may exceed the 4-foot or 6-foot limits and need a Fence Height Exception. Section 8.332.020 caps walls at abrupt grade shifts at 12 feet.
San Mateo FAQ
Do I need a permit for a retaining wall in San Mateo?
Walls under 4 feet not retaining a slope or surcharge are generally exempt. Walls over 4 feet or with surcharges require a building permit and engineering.
Why is drainage important for retaining walls?
San Mateo's hillside terrain creates significant water pressure behind walls. Proper drainage with weep holes or French drains prevents hydrostatic failure.
Can a retaining wall also serve as a fence?
Yes, but the combined structure must meet both retaining wall and fence regulations. The total height may trigger different permit requirements.
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