Mason County, WA Environmental Rules: Flood Elevation Requirements (2026)

Mason County Code Section 14.22.040 defines flood protection elevation as one foot above the base flood elevation shown on the community's flood insurance rate maps, and Section 14.22.170, Specific standards, builds the county's freeboard requirements around that one-foot margin. Under Section 14.22.170(1)(A), new construction and substantial improvement of any residential structure must have its lowest floor, including any basement, elevated one foot or more above base flood elevation; fully enclosed space below that floor is prohibited unless designed with a minimum of two flood-vent openings totaling at least one square inch per square foot of enclosed area, with each opening's bottom no higher than one foot above grade.

Nonresidential structures face the same one-foot freeboard standard under Section 14.22.170(2), or may instead be floodproofed watertight up to one foot above base flood elevation if a registered professional engineer or architect certifies the design. Manufactured homes placed in the floodplain must likewise have their lowest floor at least one foot above base flood elevation under Section 14.22.170(5), rising to two feet above the highest adjacent grade in unnumbered A zones. Critical facilities such as hospitals, schools and emergency response installations draw a higher standard: Section 14.22.170(4) requires their lowest floor to sit three feet or more above base flood elevation whenever they must be built in the base floodplain at all.

The administrator reviews elevation certificates and floodproofing certifications to confirm compliance, and Section 14.22.070 makes noncompliant construction a misdemeanor punishable by up to a $1,000 fine, 60 days in jail, or both, for each violation.