Everything homeowners ask about building an ADU in Washington — answered with 2026 data.
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, DC. 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)). Attached ADUs can be up to 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. Washington has an ADU-friendliness score of 7/10. Check our full Washington ADU rules page for complete zoning details.
ADU construction in Washington typically costs $150,000-$550,000. The cost per square foot averages $200-$500. Garage conversions are more affordable at $110,000-$250,000, while prefab ADUs cost $150,000-$300,000. Permit fees add 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). to your budget. See our detailed Washington ADU cost guide.
Detached ADUs in Washington can be up to 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. Attached ADUs are limited to 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. Junior ADUs (JADUs) have a cap of 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). Maximum height is 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). You are allowed up to One accessory apartment per lot of record in the R zones on a qualifying lot.
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). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
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)). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
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. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Washington is typically 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. 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 availability: None: DC has no pre-approved ADU plan program.
Average ADU rental income in Washington: 1-bedroom units rent for $1,500-$3,300/month, 2-bedroom units for $1,650-$3,650/month. Short-term rentals: 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. Expected 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.
The minimum lot size for an ADU in Washington is 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. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Washington, a garage conversion typically costs $110,000-$250,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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