Idylwood Environmental Rules: Flood Zones Rules (2026)

Fairfax County participates in the National Flood Insurance Program (NFIP) and enforces the Floodplain Regulations in Chapter 82 of the County Code. FEMA Flood Insurance Rate Maps (FIRMs) designate Special Flood Hazard Areas (SFHAs) throughout the county, with significant mapped Zone AE (1% annual chance flood with BFE) along: the Potomac River and tidal tributaries; Difficult Run; Accotink Creek; Pohick Creek; Cub Run; Bull Run; Little Rocky Run; and their tributaries. Zone A (no BFE determined) covers some smaller streams. Zone X shaded indicates 0.2% annual chance (500-year) floodplain with reduced risk. Any development in an SFHA — including new construction, substantial improvements (over 50% of market value), additions, and fill placement — requires a Floodplain Development Permit from Fairfax County Land Development Services. Structures must be elevated so that the lowest floor (including basement) is at or above BFE plus the county freeboard (typically 1-2 feet above BFE for residential). Substantial damage (repair cost over 50% of pre-damage value) triggers the same elevation requirements as new construction. Critical facilities (hospitals, emergency operations) require 500-year flood protection. Non-residential structures may be dry floodproofed as an alternative to elevation. Fill placement and development in floodways (the primary flood-carrying channel) is prohibited if it causes any rise in BFE. Flood insurance is required for all federally-backed mortgages on properties in SFHAs. The Chesapeake Bay Preservation Act RPA 100-foot buffers often overlap with floodplains and add further restrictions on land disturbance. Failure to comply violates both county ordinance and federal NFIP requirements, potentially resulting in loss of flood insurance and significant civil penalties.