Fairfax County, VA Environmental Rules: Flood Elevation Requirements (2026)
Key Facts
- Freeboard required
- 18 inches above 100-yr flood
- Applies to
- New homes, additions, manufactured homes
- Applies beyond floodplain
- Abutting/containing lots too
- Federal program
- National Flood Insurance Program
Summary
New and substantially improved homes in or near a Fairfax County floodplain must sit at least 18 inches above the calculated 100-year flood elevation, a freeboard standard set in the Zoning Ordinance.
The lowest part (i.e., the bottom of the floor joists or top of a concrete slab on grade) of the lowest floor,
including any basement must be at least 18 inches above the water-surface elevation of the 100-year flood level calculated in accordance with the Public Facilities Manual. This requirement for an 18-inch vertical separation applies to development within a floodplain, any lot where a floodplain is located, or on any lot that abuts a lot where a floodplain is located, for the following:
(1) Any new or substantially improved dwelling or manufactured home.
(2) Any proposed addition to an existing dwelling, unless it is permitted under subsection 3.A(9)(b) above.
For development on land outside the floodplain, this subsection B may be modified by the Director in accordance with the standards set forth in the Public Facilities Manual.
C. No structure or substantial improvement to any existing structure may be allowed unless adequate
floodproofing is provided in accordance with this section or under the USBC.
D. Stable vegetation must be protected and maintained in the floodplain to the extent possible.
E. Storage of herbicides, pesticides, or toxic or hazardous substances as set forth in Title 40, Code of Federal
Regulations, Parts 116.4 and 261.30 et seq., is prohibited in a floodplain.
Official source re-checked September 7, 2026: no newer edition of the code had been published.
Full Breakdown
Subsection 5105.6.B sets an 18-inch freeboard above the calculated 100-year flood level for the lowest floor, including any basement, of new or substantially improved dwellings and manufactured homes, and for additions to existing dwellings not otherwise exempted. The rule applies not just inside the floodplain but on any lot containing floodplain or abutting a lot with floodplain. The County separately tracks federal base flood elevation data under subsection 5105.2.C for National Flood Insurance Program compliance and must notify FEMA of elevation changes within six months of learning of them.
Violations & Fines
Failing to elevate the lowest floor 18 inches above the 100-year flood level violates subsection 5105.6.B and can block building permit approval or final inspection sign-off for the structure.
Frequently Asked Questions
How high above flood level must a Fairfax County home be built?
Does the 18-inch rule apply outside the mapped floodplain itself?
Sources & Official References
Other rules in Fairfax County
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