Chantilly

Fairfax County has one of the most formally-developed Accessory Dwelling Unit regimes in Virginia, using the statutory term 'Accessory Living Unit' (ALU) rather than ADU. The regime was substantially modernized under zMOD (the comprehensive Zoning Ordinance Modernization project) effective July 1, 2021, and further refined by the Board of Supervisors through amendments in 2023 and March 2024. Under the post-zMOD ordinance at Article 8 Section 8102, Fairfax offers TWO ALU approval pathways: (a) an Administrative Accessory Living Unit permit issued by the Zoning Administrator for ALUs that are interior to or entirely contained within the existing footprint of the primary dwelling and that meet all standards of Section 8102 by right — this is a ministerial approval with no public hearing; and (b) a Special Permit Accessory Living Unit reviewed and decided by the Board of Zoning Appeals (BZA) after a public hearing, required when the ALU involves detached construction (a separate accessory structure) or exterior alterations to the primary dwelling that exceed Administrative ALU thresholds. Key standards under Section 8102 (as amended March 2024): ALU floor area is capped at the lesser of 1,200 square feet or 40% of the gross floor area of the principal dwelling; the ALU may contain no more than two bedrooms; one additional off-street parking space is required; owner-occupancy of the primary dwelling OR the ALU is required (the owner must live in one of the two units); interior Administrative ALUs have no age or disability occupancy restriction (the historic requirement that occupants be 55+ or disabled was eliminated in the zMOD reform); detached Special Permit ALUs retained age / family-member occupancy restrictions in some formulations but the March 2024 amendment reduced those restrictions significantly. ALUs are permitted as an accessory use in all residential zoning districts where single-family detached dwellings are permitted (R-1, R-2, R-3, R-4, R-5, R-8, R-12, R-16, R-20, R-30, R-P, and PRC districts among others). Fairfax's regime is meaningfully more permissive than most Virginia counties (Chesterfield, Arlington's historic accessory-apartment program, Loudoun's comparatively restrictive rules) and is often cited as the model for Northern Virginia ADU policy. It is nevertheless more restrictive than California / Oregon / Washington ministerial regimes because (i) detached ALUs still require a BZA special permit with public hearing, (ii) size cap of 40% of the primary dwelling is tighter than California's 800 sqft no-further-review floor, (iii) owner-occupancy is required, and (iv) no state-law preemption of local conditions exists in Virginia (which is a Dillon Rule state where all zoning authority is delegated).

• Fairfax County Zoning Ordinance, Article 8 (Accessory Uses and Structures), Section 8102 (Accessory Living Units)
• Fairfax County Board of Supervisors, zMOD adoption, effective July 1, 2021
• Fairfax County Board of Supervisors, March 19, 2024 ALU amendment
• Fairfax County Department of Planning and Development — Accessory Living Unit (ALU) guidance page
• Va. Code Section 15.2-2280 (general zoning authority) and Section 15.2-2286 (procedural zoning powers)

State-floor overlay: Virginia is a Dillon Rule state: Fairfax County's land-use authority is a delegated power from the General Assembly. The principal enabling statutes are Va. Code Section 15.2-2280 (general zoning power to classify districts and regulate size, use, and placement of structures) and Va. Code Section 15.2-2286 (procedural zoning powers including special exceptions, special permits, and variances). Virginia has NOT enacted a preemptive statewide ADU ministerial-approval framework comparable to California's AB 68/881/3182, Oregon's HB 2001, or Washington's HB 1337; each Virginia locality regulates ADUs (or prohibits them) under its own zoning ordinance. Fairfax's comparatively permissive Section 8102 regime is therefore a local policy choice, not a response to state preemption. ADU-preemption bills have been introduced in the General Assembly in the 2022, 2023, 2024, and 2025 sessions (covering various combinations of by-right allowance, parking ceilings, and owner-occupancy prohibitions) but none have been enacted. The Virginia Housing Commission ADU Workgroup has studied potential state preemption but has not recommended specific legislation as of early 2026.

Fairfax County administers several county- and state-level overlay regimes that materially affect ALU siting: (1) the Chesapeake Bay Preservation Act (CBPA) Resource Protection Area (RPA) and Resource Management Area (RMA) buffers, mandated by Va. Code Section 62.1-44.15:67 et seq. and administered locally through the Chesapeake Bay Preservation Ordinance (Chapter 118 of the Fairfax County Code) and the zoning ordinance — RPA buffers protect all tidal shores, tidal wetlands, connected non-tidal wetlands, and perennial non-tidal streams at 100 feet, while RMA buffers extend the water-quality zone to a wider countywide footprint; (2) the Fairfax County Flood Plain Management Ordinance (Chapter 118-6 of the County Code and Article 4 Section 4102 of the zoning ordinance) covering FEMA Special Flood Hazard Areas along the Potomac River, Occoquan River, Bull Run, Pohick Creek, Accotink Creek, Little Hunting Creek, Cameron Run, Difficult Run, Sugarland Run, and their tributaries; (3) Historic Overlay Districts (HODs) governed by Article 7 Section 7102 of the zoning ordinance — the principal HODs are the Fairfax Courthouse HOD (around the historic 1800 Fairfax County Courthouse in the City of Fairfax area with county-jurisdictional portions immediately adjacent), the Clifton HOD (Town of Clifton's locally-designated historic district, which extends into unincorporated county parcels in the Clifton CDP), the Ash Grove HOD, the Colchester HOD, the Lake Barcroft HOD, the Lorton Historic District, and several Civil War battlefield overlays including the Historic Bull Run Battlefield area and Mount Vernon Historic District; (4) Airport Noise Contour Overlay Districts addressing airspace around Washington Dulles International Airport (IAD, operated by Metropolitan Washington Airports Authority), Reagan Washington National Airport (DCA) approach corridors, and Davison Army Airfield (KDAA) at Fort Belvoir — the Dulles airspace cuts across northwestern Fairfax including Herndon, Chantilly, and parts of Great Falls, while DCA approach corridors affect eastern Fairfax along the Potomac; (5) the Runway Protection Zone and Noise Impact Area around Davison AAF at Fort Belvoir in southeastern Fairfax; (6) Highway Corridor Overlay Districts (HCODs) along I-66, I-95, Route 7, Route 29, Route 28, and the Fairfax County Parkway, with architectural and setback controls that interact with exterior ALU design. Coastal Commission jurisdiction does NOT apply (Virginia has no California-style Coastal Commission; CBPA is the functional analog). Very High Fire Hazard Severity Zones are NOT a Virginia regulatory category — Fairfax has no WUI overlay comparable to California's CAL FIRE VHFHSZ system.

• Chesapeake Bay Preservation Act Resource Protection Area (RPA) — 100-foot buffer — and Resource Management Area (RMA) — ALU designs that cantilever over, or place impervious surface within, the 100-foot RPA buffer require an RPA Exception. Fairfax's RPA Exception process involves a Water Quality Impact Assessment (WQIA), DPD Environmental Review, and in more impactful cases a Planning Commission review. Adding 60-120 days to the overall ALU timeline for an RPA Exception is typical. Owners with parcels along Difficult Run (much of Great Falls, Oakton, and Vienna-area CDPs), Accotink Creek (much of Fairfax Station, Burke, and Annandale area), Pohick Creek (western Lorton, Springfield), and the Potomac shore (Mount Vernon, Belle Haven, River Bend) should confirm RPA status via the Fairfax County RPA Map Viewer before design.
• Fairfax County Flood Plain Management Ordinance — FEMA NFIP participant — ALUs in an SFHA must have lowest floor elevated to or above Base Flood Elevation plus Fairfax's freeboard (the county adopted a 2-foot freeboard for residential construction in 2014, exceeding the FEMA 1-foot minimum), flood vents on any enclosed area below BFE, structural anchoring, and a post-construction Elevation Certificate. The substantial-improvement trigger (Section 4102 >50% of structure value over any 10-year lookback) is cumulative — multiple smaller renovations can aggregate to trigger full floodplain compliance for the whole structure. Owners along the Potomac, Difficult Run, Accotink Creek, Pohick Creek, and the Occoquan should verify current FIRM status (2023 effective panels plus LOMRs) via the county's Floodplain Viewer before ALU design. Flood insurance is federally required for SFHA parcels with federally-backed mortgages.
• Airport noise and safety zones — Washington Dulles International (IAD), Reagan Washington National (DCA), and Davison Army Airfield (KDAA) at Fort Belvoir — ALU siting inside the 65+ DNL noise contour or within a Part 77 approach surface is subject to height limits (typically 35 feet for detached ALUs under the base zoning, lower within Part 77 approach surfaces), noise attenuation requirements for residential construction in DNL 65+ areas (additional insulation, acoustical glazing, STC-rated walls per the Uniform Statewide Building Code), and avigation easement recording for new residential construction. Owners in Chantilly near Dulles, in western Herndon, in Great Falls within Part 77 surfaces, along the Potomac in eastern Fairfax, or within the Davison AAF AICUZ footprint in Lorton should verify Airport Overlay and Part 77 status before a two-story detached ALU design. The MWAA has aggressive flight-track monitoring for Dulles overflights and the DCA noise complaint history is well-documented in MWAA annual reports.
• Historic Overlay Districts (HODs) under Zoning Ordinance Article 7 Section 7102 — An ALU on a parcel within a locally-designated HOD requires a Certificate of Appropriateness from the ARB before the Zoning Administrator can issue the ALU approval. ARB review typically adds 60-120 days to the overall ALU timeline. The ARB is particularly protective of street-facing elevations and roofline characteristics; interior Administrative ALUs (not visible from the street) are typically approved readily, while detached ALUs visible from a public right-of-way can require significant design iteration. Clifton's HOD is the most restrictive in practice due to the small parcel sizes and the district's intact 19th-century character. Parcels that are individually National Register-listed but not in an HOD are not subject to local ARB review for ALU additions.

All ALU permitting in unincorporated Fairfax County is handled by the Department of Planning and Development (DPD, zoning / ALU application review) in coordination with Land Development Services (LDS, building permits, site plans, inspections). The three incorporated towns (Herndon, Vienna, Clifton) and the two adjacent independent cities (City of Fairfax, City of Falls Church) operate their own permitting offices and are NOT served by the county process; residents of those jurisdictions must apply through their municipal building / zoning offices. For all other Fairfax County parcels, ALU approval follows one of two tracks based on the ALU type: (a) Administrative ALU (interior, contained within existing primary-dwelling footprint, meeting all Section 8102 standards by right) — submitted to the DPD Zoning Administrator, reviewed administratively without public hearing, typically approved in 30 to 60 days, followed by a separate LDS building permit for any interior alterations and a DPD zoning sign-off; (b) Special Permit ALU (detached accessory structure or exterior alterations beyond Administrative thresholds) — submitted to the DPD Zoning Administrator, noticed to adjacent property owners per Va. Code Section 15.2-2204 and the Fairfax County procedures, heard at a public hearing by the Board of Zoning Appeals (BZA), typically decided within 4-6 months of complete application. Both pathways culminate in LDS-issued building / trade permits (electrical, plumbing, mechanical) under the Virginia Uniform Statewide Building Code, LDS inspections (footing, framing, rough-ins, final), and a Certificate of Occupancy. Parcels served by public water and sewer (most of the county east of the Fairfax County Parkway) use Fairfax Water and the Fairfax County Department of Public Works and Environmental Services (DPWES) Wastewater Management; parcels on private well and septic (concentrated in western Fairfax, including much of the Great Falls, Clifton, and Fairfax Station areas) require Fairfax County Health Department review of septic capacity.