Fairfax County ADU rules — what you can build and when.
Interior ADUs are allowed today. Detached backyard cottages become by-right in 2027.
Verify locally — rules are changing ahead of July 1, 2027. This page reflects our working understanding at publication. Always confirm with the Fairfax County zoning department before you design or file. We handle that verification as part of every Readiness Assessment.
Fairfax in one screen.
- Interior ALUs allowed today by administrative permit
- Detached ADU today: special permit + 2-acre lot + owner-occupancy
- Interior cap ~800 sf / 40%; detached cap ~1,200 sf
- SB 531 makes detached ADUs by-right in 2027 and removes owner-occupancy
Reading between the lines.
Fairfax County today runs a two-track system. Interior conversions — a finished basement, over-garage unit, or attic apartment — are allowed by administrative permit for most homeowners. Detached backyard cottages, by contrast, are gated behind a special-permit process and a two-acre minimum lot, which excludes the vast majority of Fairfax neighborhoods.
That changes on July 1, 2027. SB 531 preempts Fairfax's owner-occupancy requirement, drops the two-acre lot minimum for detached units, and caps the local permit fee at $500. For homeowners on standard suburban lots — the ones currently locked out — Fairfax is one of the counties where SB 531 makes the biggest practical difference.
Homeowners planning a detached ADU here should use the 2026 runway to complete a Readiness Assessment, land a design, and hold engineered plans that can be filed the week SB 531 takes effect. Fairfax's permit intake will be one of the busiest in the state on day one.
Fairfax answers.
Can I build a detached ADU in Fairfax County today?
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Only with a special permit and a lot of at least two acres, with owner-occupancy required. Most Fairfax lots do not qualify under current rules.
How big can an interior ALU be in Fairfax County?
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Interior Accessory Living Units are generally capped at about 800 square feet or 40 percent of the main dwelling, whichever is smaller.
How does SB 531 change Fairfax County ADU rules?
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On July 1, 2027, SB 531 makes detached ADUs by-right on single-family lots statewide, preempts Fairfax's owner-occupancy requirement, and caps the local permit fee at $500.
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