Arlington

Arlington County was one of the earliest Virginia adopters of an ADU-permitting ordinance (original accessory dwelling ordinance adopted by the County Board on November 15, 2008, with the statutory term 'Accessory Dwelling' or 'Accessory Dwelling Unit' (AD/ADU)) and underwent a substantial liberalization on November 14, 2020 when the County Board voted 5-0 to expand the regime from attached-only to include detached accessory dwellings and to move most review from the discretionary 'use permit' path to an administrative by-right path. Under the post-2020 framework at ACZO Section 12.9, Arlington permits Accessory Dwellings as a by-right use in all single-family residential districts (R-5, R-6, R-8, R-10, R-10T, R-15-30T, R-20, and R2-7 two-family) subject to administrative compliance review by the Zoning Administrator. Key standards under the current Section 12.9 (as amended 2020 with minor technical amendments in 2022 and 2023): (a) size cap for an Accessory Dwelling in the principal dwelling (internal conversion) is the lesser of 35% of the gross floor area of the principal dwelling or 750 square feet; (b) size cap for a detached Accessory Dwelling is 750 square feet of gross floor area and 20 feet in height; (c) size cap for a conversion of an existing legally-constructed accessory structure (e.g., a pre-existing detached garage) is the lesser of 650 square feet or the existing footprint, with certain minor expansions allowed to accommodate code-required features; (d) one off-street parking space is required unless the parcel is within one-quarter mile of a Metrorail or high-frequency bus route (Columbia Pike / Metroway, for example), in which case the parking requirement is waived; (e) owner-occupancy of either the principal dwelling or the Accessory Dwelling is required, enforced by recorded covenant and annual self-certification; (f) no more than one Accessory Dwelling per parcel; (g) short-term rental (under 30 days) of an Accessory Dwelling is prohibited under the county's short-term rental ordinance. Arlington's regime is meaningfully more permissive than Fairfax County's post-zMOD regime because (i) Arlington eliminated the discretionary use-permit path for standard cases, moving detached ADs to administrative by-right review, whereas Fairfax still requires Board of Zoning Appeals Special Permit for detached units, (ii) Arlington's setback and height standards for detached ADs are more accommodating on the typically-smaller Arlington lots, and (iii) Arlington's transit-proximity parking waiver is explicit in the ordinance and administratively self-determining rather than requiring a variance. Arlington's regime remains more restrictive than California / Oregon / Washington ministerial regimes because the owner-occupancy mandate persists, short-term rental is prohibited, size caps are tighter than California's 1,200 sqft floor, and there is no state-law preemption in Virginia (a Dillon Rule state where all zoning authority is delegated). As of early 2026, Arlington's cumulative Accessory Dwelling permit volume since the 2020 reform is the highest of any Virginia locality — several hundred permits issued compared with a small handful annually before 2020 — reflecting both the permissive regime and Arlington's dense walkable urban fabric.

• Arlington County Zoning Ordinance (ACZO), Section 12.9 (Accessory Dwellings)
• Arlington County Board, November 14, 2020 Accessory Dwelling amendment (Recommendation 2020-C-08)
• Arlington County Board, original Accessory Dwelling ordinance adopted November 15, 2008
• Arlington County Department of Community Planning, Housing and Development (CPHD) — Zoning Accessory Dwellings guidance page
• Va. Code § 15.2-2280 (general zoning authority) and § 15.2-2286 (procedural zoning powers)
• Arlington County Missing Middle Housing Study and Expanded Housing Options (EHO) amendment, County Board action March 22, 2023

State-floor overlay: Virginia is a Dillon Rule state: Arlington County's land-use authority is a delegated power from the General Assembly. The principal enabling statutes are Va. Code § 15.2-2280 (general zoning power to classify districts and regulate size, use, and placement of structures) and Va. Code § 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. Arlington's comparatively permissive Section 12.9 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. Arlington's governance structure itself is also creature of state statute: the county operates under the 'county manager' form of government authorized by Va. Code § 15.2-800 et seq., with the County Board as the elected legislative and policy body and a professionally-appointed County Manager executing day-to-day operations. The at-large board (rather than district-based) is likewise a charter choice enabled by Va. Code.

Arlington County administers several county- and state-level overlay regimes that materially affect Accessory Dwelling siting: (1) the Chesapeake Bay Preservation Act (CBPA) Resource Protection Area (RPA) and Resource Management Area (RMA) buffers, mandated by Va. Code § 62.1-44.15:67 et seq. and administered locally through the Arlington County Chesapeake Bay Preservation Ordinance (Chapter 61 of the Arlington County Code) and the ACZO — 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 (and because Arlington is an 'Intensely Developed Area' under CBPA regulations, RMA effectively covers the entire county); (2) the Arlington County Floodplain Management Ordinance (Chapter 48 of the County Code, incorporated into ACZO Section 10.1) covering FEMA Special Flood Hazard Areas along the tidal Potomac River (Rosslyn waterfront, Roaches Run, Gravelly Point / Daingerfield Island area), Four Mile Run (the principal freshwater flood corridor transecting South Arlington from Bailey's Crossroads through Shirlington and Crystal City to the Potomac), Long Branch, Lubber Run, Donaldson Run, Windy Run, and Pimmit Run; (3) Local Historic Districts (LHDs) under ACZO Section 11.3 and Chapter 31.1 of the County Code — the seven designated Local Historic Districts are Maywood, Ashton Heights, Lyon Village, Buckingham, Fort Myer Heights, Colonial Village, and Westover, each with its own design guidelines and HALRB review scope; (4) the Reagan Washington National Airport (DCA) approach and departure corridors — Arlington's southern and eastern boundary is defined in part by DCA (the airport is on reclaimed land immediately across the Potomac / Boundary Channel from the Pentagon, with the 14th Street Bridge approach directly over Arlington), producing FAA Part 77 imaginary surfaces and DCA DNL noise contours that cover much of South Arlington (Aurora Highlands, Long Branch Creek, Crystal City residential pockets, Alcova Heights); (5) Joint Base Myer-Henderson Hall (Fort Myer) does have an Air Installation Compatible Use Zone component at Davison Army Airfield 14 miles south at Fort Belvoir (not Arlington), but Fort Myer itself is a garrison installation with limited aviation footprint — a small helicopter pad at Fort Myer is used for ceremonial and administrative flights to the Pentagon helipad and does not have a formal AICUZ. 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 — Arlington has no WUI overlay comparable to California's CAL FIRE VHFHSZ system, and the dense urban fabric with virtually no wildland-urban interface makes fire-zone overlay moot. Seismic retrofit zones do not apply in Virginia.

• Chesapeake Bay Preservation Act Resource Protection Area (RPA) — 100-foot buffer — and Resource Management Area (RMA) — Accessory Dwelling designs that cantilever over, or place impervious surface within, the 100-foot RPA buffer require an RPA Exception. Arlington's RPA Exception process involves a Water Quality Impact Assessment (WQIA), CPHD Chesapeake Bay Program Environmental Review, and in more impactful cases a Planning Commission review. Adding 45-90 days to the overall AD timeline for an RPA Exception is typical. Parcels along Four Mile Run (much of Shirlington, Nauck, Green Valley, Aurora Highlands, Crystal City residential), Lubber Run (Lubber Run Park area, Columbia Heights / Penrose neighborhoods along the creek), Donaldson Run (Rock Spring, Yorktown, Dover Crystal neighborhoods), Windy Run (Windy Run Park area in far North Arlington), and Pimmit Run (northwestern boundary with Falls Church and McLean) should confirm RPA status via the Arlington County Map Gallery RPA viewer before design. Because Arlington is an Intensely Developed Area, RMA provisions apply to every parcel countywide — new impervious surface anywhere in the county requires stormwater best-management-practice review under Chapter 60 of the County Code (Chesapeake Bay-related stormwater regulations).
• Arlington County Floodplain Management Ordinance — FEMA NFIP participant — Accessory Dwellings in an SFHA must have lowest floor elevated to or above Base Flood Elevation plus Arlington'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. Arlington's substantial-improvement trigger uses a single-permit cycle rather than a 10-year cumulative lookback (a meaningful distinction from Fairfax's 10-year cumulative approach), so individual AD permits are evaluated for the 50% threshold in isolation. Owners along Four Mile Run (Nauck / Green Valley, Shirlington, Long Branch Creek, Crystal City residential), Long Branch, Lubber Run, Donaldson Run, Windy Run, and Pimmit Run should verify current FIRM status (2013 effective panels plus LOMRs through 2024-2025) via the county's Flooding page and the FEMA Map Service Center before AD design. Flood insurance is federally required for SFHA parcels with federally-backed mortgages.
• Airport noise and safety zones — Reagan Washington National Airport (DCA) primary; Dulles (IAD) and Joint Base Andrews secondary — Accessory Dwelling siting inside DCA's Part 77 approach surfaces or within the DCA DNL 65+ noise contour is subject to height limits (typically 25-35 feet depending on distance from DCA — detached AD maximum height of 20 feet under Section 12.9 is well within Part 77 limits on almost all Arlington parcels but should be verified for South Arlington parcels within the Runway 01 approach surface), and noise attenuation requirements for residential construction in DNL 65+ areas (additional insulation, acoustical glazing, STC-rated walls per the Uniform Statewide Building Code). Unlike Fairfax's Dulles-area noise contours, DCA's noise impact on Arlington is dominated by single-event overflight noise rather than DNL-averaged impact, so AD owners in South Arlington should expect frequent overhead noise even when nominally outside the DNL 65 contour. Owners in Aurora Highlands, Long Branch Creek, Crystal City residential, Alcova Heights, Pentagon City residential, and lower Arlington Ridge should verify Part 77 surface status via the FAA's 7460-1 obstruction evaluation tool before detached AD design; projects clearly within 20 feet of grade rarely trigger notification but projects at the 20-foot maximum on parcels near the airport sometimes do.
• Local Historic Districts (LHDs) under ACZO Section 11.3 and Chapter 31.1 of the County Code — HALRB review — An Accessory Dwelling on a parcel within a locally-designated LHD requires a Certificate of Appropriateness from the HALRB before the Zoning Administrator can issue the AD approval under Section 12.9. HALRB review typically adds 60-120 days to the overall AD timeline. The HALRB is particularly protective of street-facing elevations, roof forms, porch and entry details, and material authenticity; interior Accessory Dwellings not visible from the street are typically approved readily, while detached Accessory Dwellings visible from a public right-of-way often require significant design iteration. Maywood is the largest single-family LHD and sees the most AD review traffic; the garden-apartment LHDs (Buckingham, Colonial Village, Westover) rarely see single-family AD applications because the underlying parcels are multi-family apartment ownership. Parcels that are individually National Register-listed but not in an LHD are not subject to local HALRB review for AD additions. HALRB design guidelines for each LHD are published on the Historic Preservation Program page; owners contemplating an AD in an LHD should review the specific design guidelines and conduct a pre-application consultation with HALRB staff before engaging an architect.

Because Arlington County contains NO incorporated towns or sub-jurisdictions, all Accessory Dwelling permitting across the entire county is handled by a single set of county offices — there is no 'unincorporated vs. incorporated' distinction as in Fairfax (where Herndon / Vienna / Clifton operate independently) or as in most other Virginia counties. Every residential parcel within the Arlington County boundary submits through the same county process. Accessory Dwelling permitting is administered primarily by the Arlington County Department of Community Planning, Housing and Development (CPHD), with the CPHD Zoning Division handling the Accessory Dwelling zoning review and the CPHD Inspection Services Division handling building permits, trade permits, and inspections. Under the post-2020 Section 12.9 framework, almost all Accessory Dwelling applications follow a single administrative by-right path: (a) applicant files an Accessory Dwelling application through the Permit Arlington online portal with site plan, floor plans, elevations (for detached ADs), parking-space demonstration or transit-proximity justification for waiver, and signed owner-occupancy affidavit; (b) the Zoning Administrator reviews for Section 12.9 compliance, typically within 30-45 days for administratively-complete applications; (c) on approval, applicant proceeds to building permit through the same Permit Arlington portal with construction documents sealed by a Virginia-licensed architect or engineer for any structural work, reviewed by the CPHD Inspection Services Division under the 2021 Virginia Uniform Statewide Building Code (adopted locally with the Virginia Uniform Statewide Building Code mandated state floor); (d) trade permits for electrical, plumbing, and mechanical work; (e) construction with inspections (footing, slab, framing, rough-in electrical / plumbing / mechanical, insulation, final); (f) Certificate of Occupancy issued by CPHD Inspection Services on final inspection pass. Parcels requiring Chesapeake Bay Preservation Act RPA review, FEMA floodplain review, or Historic District review follow parallel sub-processes that add specific procedural steps. The small fraction of Accessory Dwelling projects that cannot meet Section 12.9 standards by right (e.g., larger than 750 sqft, taller than 20 feet, non-conforming setbacks) require a variance from the Board of Zoning Appeals (BZA) via a noticed public hearing under Va. Code § 15.2-2309 — a path used rarely but available.