Redwood City, CA Environmental Rules: Flood Elevation Requirements (2026)

Chapter 41 of the Redwood City Code applies to every area of special flood hazard inside city limits. Those areas are the ones shown on the FEMA flood insurance study dated November 17, 1981, the flood insurance rate maps dated May 17, 1982, and all later amendments. The maps are on file with the City Engineer at 1017 Middlefield Road.

For a house, duplex or other residential building, new construction and substantial improvement must put the lowest floor, including any basement, at least one foot above the base flood elevation (the 100-year flood level) or the design flood elevation, whichever is higher. Section 41.6 adds that every subgrade enclosed area in a residential structure is treated as a basement, so below-grade garages and storage rooms are prohibited. A nonresidential building can either meet the same elevation or be floodproofed with watertight walls, structural components that resist hydrostatic and hydrodynamic loads and buoyancy, and a certificate from a registered professional engineer or architect delivered to the Floodplain Administrator.

An enclosed area below the lowest floor that is used only for parking, building access or storage must let floodwater flow in and out. The non-engineered method requires at least two openings with a total net area of one square inch for every square foot of enclosed area, with the bottom of every opening no higher than one foot above grade. Otherwise an engineer or architect must certify the design. Two-car detached garages and small low-cost sheds can sit below the base flood elevation if they are used only for parking or limited storage, are built of flood-resistant materials, are anchored, keep mechanical and utility equipment at or above the base flood elevation, and have flood openings.

A project counts as a substantial improvement when its cost equals or exceeds 50 percent of the structure's market value before work starts. Substantial damage uses the same 50 percent test. Repairs to correct identified health, sanitary or safety code violations, and alterations to a historic structure that keep its designation, are excluded. Building permit work must begin within 180 days of issuance to keep its start-of-construction date.