Washington DC allows one accessory apartment per lot as a matter of right in the R-1A, R-1B, R-2 and R-3 residential house zones, inside the house or in a detached accessory building (11-DCMR Subtitle U §253, Subtitle D §201.1). The owner must live in either unit, the apartment may house at most 3 people, and an in-house unit is capped at 35% of the house's gross floor area. Since ZC Order 25-12 took effect on July 10, 2026, a detached carriage house may have a 650 sq ft footprint in R-1/R-2 (550 sq ft in R-3 and RF), 2 stories and 22 ft, set 7.5 ft from an alley centerline, with no parking required. Rowhouse lots zoned RF-1 do not get an 'accessory apartment' at all; they get a second principal unit (basement flat or carriage house) by right with no owner-occupancy rule. Permits come from the Department of Buildings (30-business-day review cycles) and renting requires a DLCP Two-Family Rental license.
Zoning & Size Requirements [1][2][3][4][5]
Yes — Washington allows ADUs. The size limit is a formula rather than one figure — see the size rule in the table below — under current District of Columbia and Washington zoning rules.
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| Allowed Zones | By right in the R-1A, R-1B, R-2 and R-3 residential house zones, inside the house or in an accessory building (U §253.2); special exception in the Georgetown R-1B/GT and R-3/GT zones (U §253.4). Not a permitted use in RF, RA or MU zones (E §201.6; ZC 25-12, eff. 7/10/2026): an RF-1 rowhouse lot instead gets a second principal unit (basement flat or carriage house) by right under U §301.1 with no owner-occupancy rule. |
| Max Size (Detached) | Accessory-building footprint: the greater of 650 sq ft or 30% of the required rear yard in R-1/R-2, 550 sq ft in R-3 and the RF zones (D §5003, E §5003; raised from 450 sq ft on 7/10/2026). It counts toward the 40% lot occupancy (60% for an R-3 row house, D §210.1), may hold one dwelling unit and no roof deck (U §253.8(e)). |
| Max Size (Attached) | An accessory apartment inside the house may occupy no more than 35% of the house's gross floor area, and the house must have at least 2,000 sq ft of GFA in R-1 or 1,200 sq ft in R-2/R-3, excluding garage space (U §253.7(a)-(b)). A new entrance may not face the street except a below-main-level door in R-3 (U §253.7(c)-(d)). |
| Max Size (JADU) | No JADU category in DC: a basement or English-basement apartment is simply an accessory apartment in R zones (35% GFA cap) or a second principal flat in RF zones (no size cap beyond lot occupancy and building code). |
| Max Height | Detached accessory building: 2 stories and 22 ft in the R and RF zones (D §5002.1, E §5002.1; raised from 20 ft by ZC 20-19, March 2021). In the Georgetown GT zones the 22 ft/2-story envelope applies only within 5 ft of a vehicular alley (D §1105.3-.4). |
| Max Units | One accessory apartment per lot of record in the R zones (D §201.1, U §253.1), and an accessory building may hold only one (D §5000.2(b)). RF zones allow 2 principal units per lot in RF-1, 3 in RF-4 and 4 in RF-5 (U §301.1(b)), one of which may be in an accessory building (U §301.1(c)). |
Setback Requirements [1][2][3][4][5]
Washington requires a rear setback of about 7 ft for a detached ADU, and parking requirements vary by unit size or location. Owner-occupancy is required.
| Rear Setback | Accessory building: at least 7.5 ft from the centerline of any alley, 3 ft from a rear lot line that does not abut an alley or street, and no more than 30% of the required rear yard may be occupied by accessory buildings (D §5004.1, as amended 7/10/2026). The house itself needs a 25 ft rear yard in R-1 and 20 ft in R-2/R-3 (D §207.1). |
| Side Setback | R-1 and R-2: accessory building at least 3 ft from a side lot line unless that line abuts an alley or street (D §5005.3, new 7/10/2026); no side setback rule for accessory buildings in R-3. An accessory building placed beside the house must keep the full required side yard (8 ft for detached houses, D §208.2) and sit 10 ft from the house (D §5005.1). |
| Front Setback | An accessory building may not be built in front of the principal building (D §5000.2(e)); a new house sits within the range of existing front setbacks on its block face (D §206.2). The Georgetown GT zones require the accessory building to face an alley and sit within 5 ft of the rear lot line (D §1105.2). |
| Parking Required | None for the accessory apartment: Subtitle C §701.5 requires 1 space per principal dwelling only, and an apartment in an accessory building may still share the building with a private garage, artist studio or storage (U §253.8(d)). |
| Owner Occupancy | Required and non-waivable: the owner must live in either the house or the accessory apartment for as long as the apartment exists (U §253.5, §253.10(a)); the BZA may waive at most two other §253 conditions. Occupancy of the apartment is capped at 3 people (6 for house plus apartment in the GT zones) (U §253.6). RF-zone second principal units carry no owner-occupancy rule. |
| Min Lot Size | No minimum lot size or width for an accessory apartment; the test is house size (2,000 sq ft GFA in R-1, 1,200 sq ft in R-2/R-3 for an in-house unit, U §253.7(a)). An apartment in an accessory building needs permanent access: an 8 ft-wide open passage to the street, a 24 ft improved public alley, or a 15 ft alley within 300 ft of a public street (U §253.8(c)); otherwise a BZA special exception. |
Permit Process & Fees [6][7][8][9]
An ADU permit in Washington goes through a standard permit review. Typical timeline: DOB's published service level is 30 business days per review cycle for filed projects (new construction, additions, change of use); DOB publishes no end-to-end figure. DC builders quote 4-9 months for a basement unit and longer for a new carriage house. Permit fees: New construction: $0.03 per cu ft of building plus a 10% enhanced fee and a $0.002/sq ft Green Building Fee. Alterations such as a basement conversion: $30 + 2% of construction value plus 10%, and a 0.13% green fee; a 50% filing deposit is paid up front (DOB Building Permit Fee Schedule).
| Approval Process | The Department of Buildings (DOB) issues the building permit through the Citizen Access Portal and ProjectDox; a new dwelling unit is a 'filed project' reviewed by zoning, structural, MEP, fire, green/energy and, in historic districts, OP Historic Preservation. Exterior work needs a DC Surveyor plat; the finished unit needs a Certificate of Occupancy; renting needs a DLCP Two-Family Rental Basic Business License. |
| Review Timeline | DOB's published service level is 30 business days per review cycle for filed projects (new construction, additions, change of use); DOB publishes no end-to-end figure. DC builders quote 4-9 months for a basement unit and longer for a new carriage house. |
| Permit Fees | New construction: $0.03 per cu ft of building plus a 10% enhanced fee and a $0.002/sq ft Green Building Fee. Alterations such as a basement conversion: $30 + 2% of construction value plus 10%, and a 0.13% green fee; a 50% filing deposit is paid up front (DOB Building Permit Fee Schedule). |
| Impact Fees | None (DC levies no development impact fee); only DOB's 10% enhanced fee and Green Building Fee apply, plus DC Water and DDOT charges if you add a service line or work in public space |
| Plan Check | 30 business days per DOB review cycle (DOB Service Level Agreements, Feb 2024); the next-day Homeowner's Center and Digital Walk-Thru tracks exclude new dwelling units |
| Pre-Approved Plans | None: DC has no pre-approved ADU plan program |
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Construction Costs [10][11][12][13][14]
Building an ADU in Washington typically costs $150,000-$550,000 ($200-$500 per sq ft); a garage conversion runs $110,000-$250,000.
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| Average Build Cost | $150,000-$550,000 |
| Cost per Sq Ft | $200-$500 |
| Garage Conversion | $110,000-$250,000 |
| Prefab ADU (Installed) | $150,000-$300,000 |
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Rental Income Potential [15][16][17][18][19][20]
A 2-bedroom ADU in Washington rents for around $1,650-$3,650/mo, an estimated 6-17 year simple payback: a $150,000 basement unit at $2,200/mo (HUD FY2027 1BR FMR $2,204) recoups in about 6 years, a $500,000 carriage house at $2,500/mo in about 17.
| Avg Rent (1 Bedroom) | $1,500-$3,300/mo |
| Avg Rent (2 Bedroom) | $1,650-$3,650/mo |
| Short-Term Rental | Yes with limits: a Short-Term Rental endorsement on a Basic Business License, the property must be the host's primary residence, and an in-law suite qualifies (D.C. |
| Est. Annual ROI | 6-17 year simple payback: a $150,000 basement unit at $2,200/mo (HUD FY2027 1BR FMR $2,204) recoups in about 6 years, a $500,000 carriage house at $2,500/mo in about 17 |
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Recent Law Changes
What's Changed Recently
- July 10, 2026 (ZC Order 25-12, 73 DCR 28): accessory-building footprint raised from 450 sq ft to 650 sq ft in R-1/R-2 and 550 sq ft in R-3/RF; new 3 ft side (R-1/R-2) and 3 ft rear lot-line setbacks; up to 30% of the required rear yard may be occupied; RF/RA/MU zones now say expressly that 'accessory apartments' are not permitted there (second principal units are); RF accessory-building units no longer need a pre-2013 building
- August 25, 2023 (ZC Order 18-16/19-27, 70 DCR 11297): zone names consolidated; the Georgetown R-19/R-20 zones became R-1B/GT and R-3/GT, and the minimum-house-size table for an in-house apartment now reads R-1 2,000 sq ft, R-2/R-3 1,200 sq ft
- March 19, 2021 (ZC Order 20-19, 68 DCR 2908): accessory-building height raised from 20 ft to 22 ft and the alley-centerline setback cut from 12 ft to 7.5 ft
- Entry re-verified 2026-09-20 against the 10/1/2025 unofficial compilation of 11 DCMR and the July 10, 2026 final rulemaking; the earlier 12 ft alley setback, 20 ft height, 450 sq ft footprint and R-19/R-20 references were pre-2021 text
Tips for Building an ADU in Washington
- Check your zone on the DC Zoning Map first: R-1 to R-3 lots get one accessory apartment by right; RF-1 rowhouse lots (most of Capitol Hill, Shaw, Petworth, Columbia Heights) get a second principal unit by right instead, with no owner-occupancy rule but no 'accessory apartment' label
- Worked example, R-2 lot 40 ft wide: required rear yard 20 ft x 40 ft = 800 sq ft, 30% = 240 sq ft, so the 650 sq ft floor governs; two stories at 22 ft gives up to roughly 1,300 sq ft of building for one dwelling unit, with no roof deck and at most 3 occupants (U §253.6, §253.8(e))
- A carriage-house apartment must have permanent access: an 8 ft open passage from the alley building to the street, a 24 ft improved public alley, or a 15 ft alley within 300 ft of a street; a narrower alley means a BZA special exception under U §253.8(f)
- Historic districts (Capitol Hill, Georgetown, Cleveland Park, Mount Pleasant and others): OP Historic Preservation or the Old Georgetown Board reviews a new carriage house or a new street-facing basement door as part of the DOB permit, and DOB's Feb 2024 SLA sheet says historic approval must be in hand before the permit application
- Before renting: get the Certificate of Occupancy, then a DLCP Two-Family Rental Basic Business License ($219 for 2 years or $438 for 4, with a housing-code inspection, U §253.13), then register with DHCD's Rental Accommodations Division; a unit with a post-1980 C of O owned by natural persons with 4 or fewer DC rental units is exempt from rent stabilization once the claim of exemption is filed (D.C. Code §42-3502.05(a)(2)-(3))
- Short-term rental: the STR law lets a host who lives on the property list an in-law suite with a Short-Term Rental BBL endorsement and $500,000 liability cover; renting it out while you are away counts as a vacation rental limited to 90 nights a year (D.C. Code §30-201.02, §30-201.06)
- Cost basis: a DC design-build firm's January 2026 page puts a custom detached carriage house at $450,000-$600,000 all-in (foundations, utility trenching, design); DMV guides dated June 2026 put new ADU construction at $200-$275 per sq ft economy to $375-$500+ high-end and basement finishing at $30-$75 per sq ft before kitchen, egress and separate systems; Arlington 2025 permit data puts garage conversions at $110,000-$160,000 (industry sources, not rules)
- DOB's next-business-day Homeowner's Center and Digital Walk-Thru tracks do not cover a new dwelling unit (change of occupancy, new bathrooms, new utility connections), so plan on the 30-business-day filed-project cycle and file complete plans to avoid a second cycle
- Cost basis (2026-09-20 re-verification): short term allowed: Yes with limits: a Short-Term Rental endorsement on a Basic Business License, the property must be the host's primary residence, and an in-law suite qualifies (D.C. Code §30-201.02); a host-absent 'vacation rental' is capped at 90 nights a year (§30-201.06(e))
ADU Builders in Washington
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Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Zoning Regulations of 2016 (11 DCMR), unofficial compiled PDF updated 10/1/2025 (Subtitle D §§201, 207-210, 1105, 5000-5005; Subtitle E §§201, 5000-5004; Subtitle U §§201, 250, 253, 301; Subtitle C §701.5) Government Data — Accessed 2026-09-20
- Zoning Commission Notice of Final Rulemaking, Z.C. Order No. 25-12 (Omnibus Text Amendments), 73 DCR 28, effective July 10, 2026 (Subtitle D §§5003-5005, 1105; E §5003; U §§301, 310) Government Data — Accessed 2026-09-20
- DC Office of Planning setdown report, ZC Case 25-12 Omnibus Text Amendment (June 30, 2025) — items 16-18 and 24 on accessory buildings and accessory apartments Planning Department — Accessed 2026-09-20
- DC Office of Zoning Register Roundup, July 10, 2026 (ZC Orders 25-08 and 25-12 published) Planning Department — Accessed 2026-09-20
- Zoning Regulations of 2016 (Unofficial Version) index page, DC Office of Zoning Planning Department — Accessed 2026-09-20
- DOB Building Permit Fee Schedule (Department of Buildings) Government Data — Accessed 2026-09-20
- DOB Plan Review and Permit Timelines — Service Level Agreements (Feb 15, 2024) Government Data — Accessed 2026-09-20
- DOB Overview of Permitting Process Government Data — Accessed 2026-09-20
- DOB Permitting FAQs (filing fee 50%, homeowner as contractor) Government Data — Accessed 2026-09-20
- Four Brothers Design + Build — Washington, DC Accessory Dwelling Units (detached ADU $450k-$600k; updated Jan 16, 2026) industry — Accessed 2026-09-20
- Renewal Homes DMV — Accessory Dwelling Units 2026 Guide (per-sq-ft tiers, June 18, 2026) industry — Accessed 2026-09-20
- IJS Limited — Basement Renovation Costs & ROI in Washington, DC (Sept 7, 2025) industry — Accessed 2026-09-20
- Four Seasons Home Improvement — Arlington ADU report 2026 (2025 permit data: garage $110k-$160k, basement $120k-$175k) industry — Accessed 2026-09-20
- DC Structures — Virginia prefab ADUs (kit/prefab pricing, Dec 17, 2025) industry — Accessed 2026-09-20
- HUD FY2027 Small Area Fair Market Rents, Washington-Arlington-Alexandria HMFA, District of Columbia ZIP codes (effective Oct 1, 2026) Government Data — Accessed 2026-09-20
- HUD FY2027 Fair Market Rents, Washington-Arlington-Alexandria, DC-VA-MD HMFA Government Data — Accessed 2026-09-20
- DLCP Housing Business licenses — One-Family, Two-Family and Apartment rental categories and fees Government Data — Accessed 2026-09-20
- D.C. Code §30-201.02 Restrictions on short-term rentals Government Data — Accessed 2026-09-20
- D.C. Code §30-201.06 (90-night vacation rental limit) Government Data — Accessed 2026-09-20
- D.C. Code §42-3502.05 Rent stabilization exemptions Government Data — Accessed 2026-09-20
Frequently Asked Questions
Can I build an ADU in Washington?
Yes. ADUs are allowed in By right in the R-1A, R-1B, R-2 and R-3 residential house zones, inside the house or in an accessory building (U §253.2); special exception in the Georgetown R-1B/GT and R-3/GT zones (U §253.4). Not a permitted use in RF, RA or MU zones (E §201.6; ZC 25-12, eff. 7/10/2026): an RF-1 rowhouse lot instead gets a second principal unit (basement flat or carriage house) by right under U §301.1 with no owner-occupancy rule. in Washington. The maximum size for a detached ADU is Accessory-building footprint: the greater of 650 sq ft or 30% of the required rear yard in R-1/R-2, 550 sq ft in R-3 and the RF zones (D §5003, E §5003; raised from 450 sq ft on 7/10/2026). It counts toward the 40% lot occupancy (60% for an R-3 row house, D §210.1), may hold one dwelling unit and no roof deck (U §253.8(e)).. Owner-occupancy is required: Required and non-waivable: the owner must live in either the house or the accessory apartment for as long as the apartment exists (U §253.5, §253.10(a)); the BZA may waive at most two other §253 conditions. Occupancy of the apartment is capped at 3 people (6 for house plus apartment in the GT zones) (U §253.6). RF-zone second principal units carry no owner-occupancy rule.
How much does an ADU cost in Washington?
ADU construction in Washington typically costs $150,000-$550,000. Garage conversions run $110,000-$250,000, and prefab ADUs cost $150,000-$300,000.
How much can I rent an ADU for in Washington?
Average ADU rents in Washington are $1,500-$3,300/mo for a 1-bedroom and $1,650-$3,650/mo for a 2-bedroom unit.
Do I need parking for an ADU in Washington?
None for the accessory apartment: Subtitle C §701.5 requires 1 space per principal dwelling only, and an apartment in an accessory building may still share the building with a private garage, artist studio or storage (U §253.8(d)).. Check with Washington planning department for the latest District of Columbia ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Washington?
The permit review timeline in Washington is DOB's published service level is 30 business days per review cycle for filed projects (new construction, additions, change of use); DOB publishes no end-to-end figure. DC builders quote 4-9 months for a basement unit and longer for a new carriage house.. Permit fees: New construction: $0.03 per cu ft of building plus a 10% enhanced fee and a $0.002/sq ft Green Building Fee. Alterations such as a basement conversion: $30 + 2% of construction value plus 10%, and a 0.13% green fee; a 50% filing deposit is paid up front (DOB Building Permit Fee Schedule).
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