Centreville

Loudoun County, Virginia regulates accessory residential construction under the Loudoun County Zoning Ordinance (LCZO) — a comprehensive ordinance adopted by the Board of Supervisors that governs land use in all unincorporated areas of the county and in the seven incorporated towns whose zoning the county does NOT separately administer (Hamilton, Hillsboro, Lovettsville, Middleburg, Purcellville, Round Hill each zone independently; Leesburg, the county seat, also zones independently). As a Dillon Rule state, Virginia gives counties only the powers the General Assembly has expressly delegated; Virginia Code Section 15.2-2304 permits localities to adopt affordable-dwelling-unit (ADU) programs under specific conditions, and Section 15.2-2286 authorizes accessory-use regulation broadly. Loudoun County uses TWO distinct regulatory tracks that are often confused: (a) an "Affordable Dwelling Unit (ADU)" program under LCZO Article 7 (inclusionary-zoning requirement on certain residential rezonings of 50+ units in the Suburban Policy Area) — this is an AFFORDABILITY program, not a backyard-accessory-cottage program; and (b) an "Accessory Residential Structure" / "Tenant House" / "Accessory Dwelling" allowance for genuine backyard / in-law units scattered through LCZO Article 5 (Use Regulations) for the rural and agricultural zoning districts and Article 3 (District Regulations) for suburban and urban districts. For the backyard-ADU use case this research file concerns, the controlling provisions are the accessory-dwelling / tenant-house allowances in the applicable base zoning district — which vary substantially between the Rural Policy Area (AR-1, AR-2, JLMA districts: generous accessory-unit and tenant-house rights on larger agricultural parcels) and the Suburban Policy Area (R-1, R-2, R-3, R-4, R-8, R-16, PD-H, PD-CC, etc.: more restrictive accessory-dwelling rules, typically one accessory dwelling per lot subject to size and occupancy limits). The 2023 comprehensive Zoning Ordinance Rewrite (ZOR) — Loudoun's first full rewrite since 1993 — reorganized these provisions and updated standards to conform with 2020s state legislation; the current LCZO reflects the ZOR effective date of January 2023 plus subsequent amendments through 2025. Unlike California (Gov. Code 65852.2), Virginia has no statewide statutory ADU preemption; each locality sets its own rules within the Dillon Rule grant. This makes the county ordinance the dominant controlling authority for unincorporated parcels, rather than a thin local layer atop a preemptive state floor.

• Loudoun County Zoning Ordinance (LCZO) — Revised Zoning Ordinance effective January 2023 (Zoning Ordinance Rewrite / ZOR)
• LCZO Article 7 — Affordable Dwelling Unit (ADU) Program (inclusionary zoning)
• Virginia Code Section 15.2-2304 (Affordable Dwelling Units) and Section 15.2-2305 (Accessory apartments)
• 2023 Zoning Ordinance Rewrite (ZOR) adoption and background materials
• Loudoun County Codified Ordinances (Codified Municipal Code) — ecode360

State-floor overlay: Virginia is a Dillon Rule state with no statewide ministerial-ADU preemption. Virginia Code Section 15.2-2305 (as amended) expressly authorizes localities to permit accessory apartments within single-family detached dwellings, but leaves to the locality the choice of procedure (by-right / administrative permit / special-exception), size caps, occupancy rules, and design standards. Section 15.2-2304 authorizes inclusionary Affordable Dwelling Unit programs. Section 15.2-2286 provides the underlying accessory-use police power. Because there is no statewide floor comparable to California Gov. Code 65852.2, Loudoun's ordinance is the dominant controlling authority on unincorporated parcels; an applicant's backyard ADU rights depend almost entirely on the base zoning district of the parcel. The Virginia Residential Code (VRC, based on the IRC 2018 with state amendments, adopted by the state Board of Housing and Community Development under the Virginia Uniform Statewide Building Code) governs building-construction standards statewide and applies to any ADU regardless of zoning permissibility; localities cannot override the VRC by ordinance.

Loudoun County administers several overlay regimes that materially affect ADU siting in the unincorporated county: (1) FEMA Special Flood Hazard Areas along the Potomac River frontage, Goose Creek, Catoctin Creek, Little River, Broad Run, Sugarland Run, and related tributaries, with a county floodplain-management ordinance imposing local freeboard above BFE; (2) the Rural Policy Area / Transition Policy Area / Suburban Policy Area framework of the Loudoun 2019 General Plan (adopted 2019) which determines the density-and-use posture of a parcel and indirectly its accessory-dwelling allowance — this is not a narrow overlay but a broad policy-area system that functions overlay-like for ADU purposes; (3) the Historic & Cultural Conservation Districts (H-1 overlay and Historic Site & District overlays), including the Waterford National Historic Landmark District, the Middleburg Historic District extensions into unincorporated Loudoun, and the Goose Creek Historic District, where ADU exterior design and siting are subject to Historic Preservation Review Committee (HPRC) approval; (4) the Dulles International Airport noise and safety overlays — Loudoun County contains the majority of Washington Dulles International Airport (IAD) and a substantial portion of eastern Loudoun lies within the Dulles Airport Impact Overlay District (AIOD), with sub-zones for noise contours (Ldn 65 and above) and safety zones where residential construction including ADUs is subject to noise-attenuation and avigation-easement requirements, and in the tightest safety zones effectively prohibited; (5) scenic / open-space overlays along the Mountainside Development Overlay District (MDOD) in the Catoctin and Bull Run mountain slopes, and the Scenic Creek Valley Buffer (SCVB) for parcels adjacent to designated creek valleys; and (6) the Data Center Overlay / eastern-Loudoun industrial-use footprint — not directly an ADU overlay, but the substantial growth of data-center and adjacent commercial zoning in eastern Loudoun has compressed the available residential-accessory-dwelling opportunity in that area. Loudoun does NOT have a Very High Fire Hazard Severity Zone regime comparable to California — Virginia does not designate VHFHSZ on the CAL FIRE model — but rural parcels in wooded Catoctin / Blue Ridge foothill areas are subject to the general Virginia Department of Forestry fire-prevention standards.

• FEMA Special Flood Hazard Areas — Loudoun County Floodplain Management Ordinance — The county administers the FEMA NFIP floodplain regulations for unincorporated parcels. Principal SFHA extents follow the Potomac River (northern boundary with Maryland), Goose Creek (central east-west drainage through Leesburg, Ashburn, and east into Fairfax), Catoctin Creek (central north-south drainage through Waterford / Lovettsville / Purcellville vicinity), Little River (southeast), Broad Run (Brambleton / Dulles area), and Sugarland Run (Sterling / eastern Loudoun). ADUs in an SFHA require lowest-floor elevation to or above Base Flood Elevation plus county freeboard (typically 1-2 ft above BFE), flood vents on enclosures below BFE, anchoring against floatation, and a post-construction Elevation Certificate. Community Rating System participation gives Loudoun NFIP policyholders modest flood-insurance discounts. FEMA FIRM panels for Loudoun County were last comprehensively updated in the 2010s with partial revisions since; owners should verify current effective panels before design.
• Dulles International Airport Impact Overlay District (AIOD) — noise and safety zones — Washington Dulles International Airport (IAD) lies substantially within Loudoun County — the airport property itself straddles Loudoun and Fairfax counties but the majority of the airport impact footprint on residential areas falls in Loudoun. The AIOD imposes graduated restrictions: in the highest-noise Ldn 75+ contour, residential construction including ADUs is generally prohibited or subject to very tight noise-attenuation construction (STC-rated windows, acoustic-sealed doors, forced-air HVAC with acoustic treatment); in Ldn 65-75, new residential is subject to avigation-easement recording against the parcel title and enhanced noise-attenuation; in Ldn 60-65, disclosure to occupants is required but construction is otherwise conventional. Safety zones (Part 77 airspace, runway protection zones, inner safety zones, outer safety zones) impose separate restrictions — some on height rather than use. ADU applicants near IAD should pull the current AIOD map and confirm overlay status before design; the airport impact area extends several miles beyond the physical airport fence in each direction.
• Historic & Cultural Conservation Districts — Historic Site & District Overlay (H-1 / H-2) and HPRC review — Loudoun has several historic overlay districts administered by the Loudoun Historic Preservation Review Committee (HPRC): the Waterford National Historic Landmark District (federally-designated, one of only a handful of intact 18th-century Quaker villages in the country), the Goose Creek Historic District, the Middleburg-extension historic areas, the Aldie Historic District, the Lincoln Historic District, and others. ADUs in these overlays require a Certificate of Appropriateness from HPRC before a building permit can issue; exterior design (materials, roof form, window pattern, siting, setbacks) must be compatible with the district's historic character. This adds typically 60-120 days to the ADU review timeline and materially constrains design choices (pre-approved modern-prefab ADU kits generally will not pass HPRC review in the tighter districts).
• Mountainside Development Overlay District (MDOD) — Catoctin and Bull Run mountain slopes — The MDOD applies to steep-slope mountainous parcels in the Catoctin Mountains (running north-south through central Loudoun) and the Bull Run Mountains (southern Loudoun border). The overlay imposes limits on grading, tree clearance, ridgeline-silhouette construction, and visually-exposed exterior materials. An ADU on an MDOD parcel is subject to these design standards on top of the base-district accessory-dwelling rules; tile / metal roofing, natural-material siding, and earth-tone color palettes are typical requirements. The steep-slope limitations also affect siting: ADUs generally must be placed on ground with less than 25% slope, with substantial additional review for 15-25% slopes.
• Rural Policy Area / Transition Policy Area / Suburban Policy Area — Loudoun 2019 General Plan framework — The 2019 General Plan divides unincorporated Loudoun into three broad policy areas: (a) Rural Policy Area, covering western Loudoun (Waterford, Lincoln, Philomont, Unison, Aldie, Middleburg-adjacent, Hillsboro-adjacent) and characterized by AR-1 / AR-2 agricultural zoning with strong tenant-house and farm-worker-housing allowances on large parcels; (b) Transition Policy Area, covering a buffer zone between rural and suburban (Leesburg-south, western Ashburn, Beaverdam) with mixed low-density residential zoning; (c) Suburban Policy Area, covering eastern Loudoun (Ashburn, Sterling, Brambleton, Dulles-area, Sugarland, Cascades) with R-1 through R-16 and planned-district zoning and more restrictive accessory-dwelling rules. An applicant's ADU rights are primarily determined by which policy area the parcel falls in AND the specific base zoning district within that area. The Policy Area framework also determines which county investments (road, water, sewer, school) are available — Rural Policy Area parcels are mostly on well/septic with no public sewer planned.
• Chesapeake Bay Preservation Act — Virginia statewide overlay administered locally — All of Loudoun County is within the Chesapeake Bay Watershed (via the Potomac River drainage), but Loudoun is NOT a Tidewater locality required to designate Resource Protection Areas and Resource Management Areas under the stricter Chesapeake Bay Preservation Act provisions (which apply to the 84 Tidewater localities in eastern Virginia). Loudoun instead implements general stormwater and erosion-and-sediment-control requirements under the Virginia Stormwater Management Program and Virginia Erosion and Sediment Control Program administered by the Department of General Services and the Department of Environmental Quality. An ADU project disturbing 2,500 sqft or more triggers stormwater-management review; under 2,500 sqft typically only the local E&SC plan is required. This is a Virginia-statewide regime rather than a uniquely-county one, noted here only because applicants often conflate it with the Tidewater Chesapeake Bay Preservation Act which does NOT apply in Loudoun.

The Loudoun County Department of Building & Development (B&D) and the Department of Planning & Zoning (DPZ) jointly administer accessory-dwelling / tenant-house permits for parcels in unincorporated Loudoun County. Unincorporated Loudoun is the overwhelming majority of the county's 520 square miles of land area — the seven incorporated towns (Hamilton, Hillsboro, Leesburg, Lovettsville, Middleburg, Purcellville, Round Hill) together cover a small fraction of the total area, and the two towns of Leesburg and Purcellville administer their own zoning independent of the LCZO. For an unincorporated parcel, the ADU permit path is: (1) DPZ Zoning Permit review confirms the use is permitted in the base zoning district (this is the gating step — a rural AR-1 / AR-2 parcel's tenant-house rights differ materially from a suburban R-1 / R-2 / R-3 parcel's accessory-dwelling rights, and the Transition Policy Area adds further nuance); (2) B&D Building Permit review confirms VRC compliance (structural, electrical, plumbing, mechanical, energy, residential code); (3) Loudoun County Health Department review if the parcel is served by on-site well and septic (a very large fraction of Rural Policy Area parcels; Loudoun's Department of Health issues well and septic permits under Virginia Department of Health regulations); (4) fire-marshal review if the structure requires emergency-access confirmation (most rural parcels on long private driveways do); (5) Loudoun Water utility connection review if served by public water/sewer in the eastern suburban portion of the county; (6) entrance / driveway permit from Loudoun County or VDOT if the driveway connects to a state road (most rural parcels); (7) building permit issuance, construction, inspections, certificate of occupancy. The typical by-right tenant-house or accessory-dwelling application in the Rural Policy Area takes 90-150 calendar days from application to permit issuance assuming no health-department delay on well/septic; suburban lots with clean utility service often move faster.