ADU and DADU, Greater Seattle Area
ADU vs DADU in Seattle, WA: What Is the Difference?
An ADU, or accessory dwelling unit, is a separate home on the same lot as a house. A DADU is the detached kind: its own building in the yard, often called a backyard cottage. An attached ADU sits inside or against the house, such as a basement conversion. Seattle allows two ADUs per lot.
Below are the Seattle code rules for each type, the state rules for nearby cities, and what each one costs to build. To have us check what your lot can take, Book a Consultation.
What is the difference between an ADU and a DADU?
The difference is attachment. ADU is the umbrella term, and a DADU is one kind of ADU. The Seattle Department of Construction and Inspections (SDCI) defines both on its Accessory Dwelling Unit page (read October 2026): "A legally permitted unit in the home is called an attached accessory dwelling unit (AADU). A legally permitted unit on the property (but not within the home) is called a detached accessory dwelling unit (DADU)."
The Seattle Municipal Code draws the same line. SMC 23.84A.008, as amended by Ordinance 127376 (PDF page 299, read October 2026), defines an accessory dwelling unit as one that "Is located within the same structure as a principal dwelling unit or within an accessory structure on the same lot as a principal dwelling unit". Units count as attached "if they share a common or party wall or have walls containing floor area that are located within 2 feet of each other."
Both kinds are complete homes. Both sit on the same lot as the house, and both need a permit.
| Question | Attached ADU (AADU) | Detached ADU (DADU) |
|---|---|---|
| Where it sits | Inside or against the house | Its own building on the lot |
| Common forms | Basement, lower floor, new wing | Backyard cottage, converted garage |
| Own entrance | A locking door separate from the main entry | A lockable entrance door |
| Separation from the house | Fire and sound separation, or interconnected smoke alarms in an existing house | A separate building, with its own heating controls |
| Size cap in Seattle | 1,000 sq ft with up to 2 bedrooms, 1,200 sq ft with 3 or more | Same cap |
| Our Basic starting price | $150,000+ conversion, $250,000+ new attached unit | $325,000+ (300 to 600 sq ft) |
Sources: SDCI Tips 116A and 116B (updated August 18, 2023); SMC 23.42.022.G.1, read October 2026; prices from Cost of an ADU or DADU in the Greater Seattle Area, updated October 2026.
What does the Seattle code say about ADUs and DADUs?
Seattle's ADU rules sit in one section, SMC 23.42.022, rewritten by Ordinance 127376 (PDF pages 25 to 30, read October 2026). Four subsections set the basics:
- 23.42.022.C: "No lot may have more than two accessory dwelling units."
- 23.42.022.D: "Accessory dwelling units may be attached, detached, or stacked."
- 23.42.022.E: "Unless otherwise provided in the standards of the underlying zone, accessory dwelling units shall be subject to the same standards as principal dwelling units."
- 23.42.022.F: "Accessory dwelling units must be located on the same lot as the principal dwelling unit."
So a Seattle lot can hold the house plus two ADUs, in any mix of attached and detached. SDCI puts it this way: "While some property may be large enough to have four or more units, only two of them may be ADUs." A DADU follows the zone's height, setback and lot coverage rules for a house unless the zone says otherwise.
How big can an ADU or DADU be in Seattle?
The cap is the same for both types. Under SMC 23.42.022.G.1, an ADU with up to two bedrooms "may not exceed 1,000 square feet", and one with three or more bedrooms "may not exceed 1,200 square feet" (read October 2026). Under 23.42.022.G.1.c, an ADU may reach 1,500 square feet on an LR zone lot in a frequent transit service area, if the lot also meets a purchase history condition.
Three areas do not count toward the cap (23.42.022.G.2): up to 250 square feet of an attached garage, all underground stories, and up to 35 square feet of long-term bicycle parking. That makes a basement unit easier to fit under the limit.
One rule favors attached units in older houses. Under 23.42.022.H.4, an attached ADU may exceed 1,000 square feet if the part of the house it occupies "existed as of July 23, 2023."
Our ADU pricing guide lists typical sizes of 300 to 800 sq ft for a conversion, 400 to 1,000 sq ft for an attached ADU and 300 to 1,000 sq ft for a DADU. Our 600 to 1,000 sq ft DADU tier runs to 1,200 sq ft in Bellevue.
Can a garage or basement become an ADU or a DADU?
Yes. A basement becomes an attached ADU. A detached garage or shed becomes a DADU. Seattle eases the rules for older structures: under SMC 23.42.022.H.3.b (read October 2026), "Conversion of an existing accessory structure to a detached accessory dwelling unit is permitted notwithstanding applicable lot coverage or yard or setback provisions in this Section 23.42.022 or the applicable zone." An existing accessory structure is one that stood before July 23, 2023 (23.42.022.H.2).
The building code still applies in full. SDCI Tip 116B (updated August 18, 2023) says: "If converting a structure to a DADU, you must fully comply with the Seattle Energy Code." Tip 116A says the same for converted space in a house. A garage built to park cars usually needs new insulation, windows and heat to get there. Our Garage Conversion Across the Greater Seattle Area page covers that work.
What does each type need to pass inspection in Seattle?
SDCI Tip 116B (updated August 18, 2023) says "DADUs include living, sleeping, kitchen and bathroom facilities and have a lockable entrance door." The rest of the list differs by type:
- Attached ADU. Tip 116A (updated August 18, 2023): "The AADU's entrance must be a locking door separate from the entry door to the main dwelling unit." In an existing house, fire and sound separation is required between the two units, and it is waived if interconnected smoke alarms run through the whole structure.
- DADU. Tip 116B: "The DADU must have heating controls separate from those in the primary dwelling unit."
- Both. Smoke detectors and carbon monoxide alarms, and an emergency escape window or door in each sleeping room.
Permits follow the same split. SDCI lists a construction addition or alteration permit for a unit inside the house or a detached unit, and a construction permit to establish use for an existing unit built without one.
Are ADU and DADU rules different in Redmond and other cities near Seattle?
Yes, in the details. Each city writes its own code, and state law sets a floor that the cities near Seattle must meet. Under RCW 36.70A.680 and RCW 36.70A.681 (read October 2026), cities and counties planning under the Growth Management Act must follow the state ADU rules. In urban growth areas, a city "must allow at least two accessory dwelling units on all lots" that allow single-family homes. The allowed configurations are one attached and one detached, two attached, or two detached. The same section says a city:
- "must permit accessory dwelling units in structures detached from the principal unit";
- "may not establish a maximum gross floor area requirement for accessory dwelling units that is less than 1,000 square feet";
- may not set a roof height limit under 24 feet, unless the house itself is limited to less;
- "must allow accessory dwelling units to be converted from existing structures, including but not limited to detached garages".
Subsection (4) excludes lots with critical areas or their buffers. Local details still differ. Our ADU pricing guide notes that Redmond requires sprinklers in detached ADUs, and attached units need rated walls. We confirm the rules for your address at the first visit.
Does an attached ADU or a DADU cost more to build?
A DADU costs more. A new detached unit needs its own foundation, roof, walls and utility runs across the yard, and a conversion reuses what the house already has. At our Basic finish level, conversions start at $150,000+, new attached units at $250,000+ and 300 to 600 sq ft DADUs at $325,000+ (our ADU pricing guide, updated October 2026). Every finish level, size and added cost is in How Much Does It Cost to Build an ADU?
Should you build an ADU or a DADU on your lot?
Start with the space you already have. A dry basement with good ceiling height, or a detached garage, is usually the shorter route: per our ADU pricing guide, a conversion takes 3 to 5 months on site, and a new DADU takes 5 to 10. Choose a DADU when the yard has room and the household wants a separate building with its own door, heat and yard side.
Our own work shows what a lower level takes. Our Finished Basement Remodel in Seattle, WA, completed in July 2024 for about $120,000, added closets and one plank floor through a level with its own outside door. It is not an ADU: a legal unit would also need its own kitchen, the fire and sound separation and a permit.
A new unit runs months on site next to your home, and How to Manage Home Renovation Stress covers how we run that time. The service is on ADU Builders Across the Greater Seattle Area. For other work in the city, see Home Remodeling Contractors in Seattle, WA.
Frequently Asked Questions
Does an ADU or a DADU need parking in Seattle?
No. SMC 23.42.022.I (read October 2026) says "No off-street motor vehicle parking is required for an accessory dwelling unit."
Is a tiny house a DADU in Seattle?
Only on a foundation. The SDCI Accessory Dwelling Unit page (read October 2026) says "Tiny houses, with foundations, are considered DADUs." A tiny house on wheels is treated like a camper trailer and cannot be lived in on a Seattle lot.
Is a pre-approved DADU plan faster to permit?
Usually. The city keeps a gallery of pre-approved DADU plans on its ADUniverse site, and SDCI says that in most cases a permit on one of those plans takes 2 to 6 weeks (read October 2026). We can build from one.
What happens to an ADU when the house is sold?
It stays with the property. SDCI Tip 116A (updated August 18, 2023) says "A permit is not required from SDCI if the new owner chooses to keep the ADU." Removing it takes a permit to remove the features that make it a separate unit.
