What's the difference between an ADU and a DADU in simple terms?

An ADU (Accessory Dwelling Unit) is any secondary living space on a residential lot , attached or detached. A DADU (Detached Accessory Dwelling Unit) is the specific type that's a completely separate structure in your backyard, with no shared walls or roof with the main house.

Can I build both an ADU and a DADU on the same lot in Washington?

Yes, in most King County cities. Washington State legislation passed in 2023-2024 establishes two ADUs per lot as a baseline right on single-family lots in Growth Management Act jurisdictions. Local setbacks and lot coverage rules still apply. We confirm feasibility for your specific parcel during a site visit.

How much does a DADU cost in King County in 2026?

A new detached ADU in King County typically runs $175,000 to $550,000 depending on size, lot conditions, and finish level. A 600-800 square foot DADU , the most common size , lands between $230,000 and $380,000 including design, permits, construction, and utility connections.

How long does it take to get an ADU permitted in King County?

Permit timelines vary by city. In Kirkland and Bellevue, expect 3-6 months. In Seattle, 4-9 months depending on project type and current backlog. In SeaTac, typically 2-5 months. We manage the permit process end-to-end and know the current review timelines for each jurisdiction we work in.

Do I need to live on the property to build or rent a DADU in Washington?

No. Washington State legislation has eliminated owner-occupancy requirements for ADUs in most jurisdictions. You can build a DADU as a rental investment without living on-site. Confirm your specific parcel's rules during our feasibility consultation.

What's the maximum size for a DADU in King County?

It varies by city. In Seattle, detached ADUs can be up to 1,000 square feet (or 60% of the primary home's floor area). In Kirkland and Bellevue, the limit is typically 1,000-1,200 square feet depending on zone. SeaTac follows similar limits. We verify the specific rules for your parcel during feasibility.

Which is better for rental income: ADU or DADU?

DADUs typically generate more rental income because of the privacy and independence they offer tenants. A 700 sqft DADU in Kirkland or SeaTac might rent for $2,200-$2,800/month. A comparable attached ADU in the same neighborhood might fetch $1,600-$2,200/month. The gap varies by location, unit quality, and finish level.

Does Love Construction build both ADUs and DADUs?

Yes. We build attached ADUs (basement conversions, garage conversions, additions) and fully detached DADUs throughout King County. We're a design-build firm , design, permits, and construction are under one contract with one point of contact. See our location-specific guides for [ADU builders in Kirkland, WA](/blog/adu-builders-kirkland-wa) and [ADU builders in SeaTac, WA](/blog/adu-builders-seatac-wa). ---

Do You Need Permits for an ADU in Seattle? What the City Requires

Yes, and usually more than one. What Seattle's SDCI requires to permit an attached or detached ADU, the steps in order, and what the pre-approved plan route changes.

Yes. Seattle requires a permit for every accessory dwelling unit, attached or detached, and the city is blunt about the alternative: SDCI's guidance on accessory dwelling units states that these units "are not legal unless they have been established through a permit process." There is no square footage below which a second kitchen and a lockable entrance stop counting as a dwelling unit. Both types are established through a construction addition/alteration permit, and for a detached unit that permit sits alongside several others.

That last point is where most homeowners are surprised. "Do I need a permit" has a short answer, but "which permits" does not.

One project, several separate permits

The construction permit covers the structure. It does not cover everything the structure needs.

SDCI Tip 116B, Establishing a Detached Accessory Dwelling Unit (DADU), updated August 18, 2023, names the others directly. You need a separate electrical permit from SDCI for any electrical work, and unless the property owner is performing all of that work themselves, the electrical contractor is the one who applies for it. Any plumbing work requires a plumbing permit from the Seattle and King County Health Department, which is a different agency from the one issuing your building permit. If you are connecting to an existing side sewer, you will need a side sewer permit as well.

There is also a charge that is not a permit and catches people after the fact. Both Tip 116A and Tip 116B state that you will pay a King County capacity charge when you construct an accessory dwelling unit. The bill arrives from King County about three months or more after the sewer connection is reported. King County then bills every three months for 15 years, or you may pay the remaining balance in one lump sum at a discount any time during that period. It is a real obligation attached to the unit, and it is not part of the permit fee.

How the application actually moves

Seattle publishes the sequence as four steps, and knowing the order matters mostly because of what sits in the middle of it.

Step one is a site plan and a Building and Land Use Pre-Application submitted through the Seattle Services Portal. The city confirms the legal description and address of the property and assigns a record number. Some projects are told at this point that they require a Pre-Application Site Visit.

Step two is scheduling an intake appointment. Seattle's own tip sheets say plainly that "the next available appointment may be 2-4 months out." That sentence does more to explain ADU timelines than any cost estimate does, and it is worth reading before you plan around a move-in date.

Step three is preparing the drawings, and the scope is wider than people expect. Seattle asks for a detailed site plan and floor plans for both the main house and the new unit, energy calculations, exterior elevation drawings if you are building a new structure or changing the building envelope, and full structural plans including framing plans, foundation plans and sections. The plans have to state the number of ADUs proposed, whether the unit is part of a new or existing accessory structure, the size of the unit, the total lot coverage for the site, and the number of parking spaces provided for the ADUs if any.

Step four is uploading all of it as PDFs against your record number. Seattle notes that if everything is uploaded well ahead of the appointment, the appointment may be moved up. After intake you pay a permit intake fee and a plan review fee based on the value of the work, and only then does review begin. If corrections are needed you receive correction letters in the portal and upload revised plans. On waiver requests the tips are direct: "Do not assume that your waiver request will be granted."

Two details at the end are worth knowing in advance. Seattle states there is no public comment period and no appeal opportunity to the Hearing Examiner or City Council for these permits, so a neighbour's objection is not a stage in the process the way it is for larger projects. And occupancy is gated on the final inspection: in the city's words, "once we give you the final inspection approval, a tenant may occupy the accessory unit."

The pre-approved plan route, and what it does not cover

Seattle maintains a gallery of pre-approved DADU designs through its ADUniverse site. SDCI's position on the benefit is specific: using one, "in most cases, you can get your permit in just 2-6 weeks," against up to several months for a typical plan review, at a reduced permit fee that reflects the time saved.

The part that gets misread is what was pre-approved. The city reviewed the structure, not your property. To use one of these plans you connect with the plan's designer, who prepares the full site plan your construction permit still requires. Your setbacks, your rear yard, your trees, your utility connections and your lot's constraints are all still evaluated on their own. The route removes the structural review, not the site review.

What the code checks about the lot before it checks the design

For a detached unit in Seattle's Neighborhood Residential zones, Tip 116B sets out limits that decide feasibility before a design exists:

The lot must be at least 3,200 square feet.
The DADU cannot exceed 1,000 gross square feet, excluding garage areas, storage areas accessed from the exterior, porches and covered decks under 25 square feet, and gross floor area below grade. Up to 35 square feet can be exempt if it is designed for bicycle parking.
Accessory structures, including DADUs, cannot exceed 40 percent of the required rear yard area. A DADU may cover an additional 20 percent of that required rear yard specifically in order to protect trees, provided it removes no exceptional trees and no trees over two feet in diameter measured 4.5 feet above the ground.
Entrances may face the nearest side lot line or the rear lot line if they are 10 feet from the property line, unless an alley abuts that side of the lot.
DADUs are not permitted on lots in shoreline districts.

In lowrise zones the detached unit is capped at 650 square feet, and each principal unit may have only one accessory dwelling unit. An attached unit in a single-family home is capped at 1,000 square feet excluding garage area, unless the part of the home where the unit sits already existed as of December 31, 2017.

On the question of two units, Seattle allows lots to have up to two accessory dwelling units, either two attached or one attached and one detached. The second unit has to meet the criteria in SMC 23.44.041.A.2: either it meets green building standards, or it is an affordable unit reserved for income-eligible households. In the RSL zone, each principal dwelling unit may have only one accessory dwelling unit.

Site conditions can override all of this before it becomes relevant. If any part of the property is mapped as an environmentally critical area, that shapes what is buildable and where, which is worth checking at the very start rather than after a design exists. Our early checklist for Seattle's environmentally critical areas covers what to look for. If the attached-versus-detached distinction itself is still the open question, our guide to ADU and DADU differences in Washington State works through which one a property can support.

Two requirements Seattle removed, and one it kept

Parking is not required for any accessory dwelling unit. There is a condition attached: you cannot remove an existing, required off-street parking space in order to build the unit unless you replace that space somewhere else on the lot in conformance with the code.

Owner occupancy is no longer required. Seattle removed it through Council Bill 119544, adopted in July 2019 and in effect since August 8, 2019. The owner does not have to live on the property to rent the unit out.

What Seattle kept is a cap on residents. On a lot with one accessory dwelling unit, the total number of residents across both the primary and the accessory unit cannot exceed eight people unless all residents of both units are related to each other. On a Neighborhood Residential lot with two accessory units, the total number of unrelated residents cannot exceed twelve.

If a unit already exists without a permit

This comes up on purchases and inheritances more than on new projects. Seattle's process is that on receiving a complaint about an unpermitted accessory dwelling unit, SDCI may inspect it and send the owner a Notice of Violation. The owner is then required either to legalize the unit or to remove the features that make it a separate unit. Penalties are provided under Seattle Municipal Code sections 23.90.018, 23.90.019 and 23.90.020, and tenants displaced when an illegal unit is removed may be entitled to relocation assistance paid by the owner under SMC 22.206.160C.1.j.

Selling a property with a permitted unit is simpler than people assume. If the new owner keeps the accessory dwelling unit, no permit is required from SDCI. A permit is only needed if the new owner wants to remove it.

These are Seattle's rules, and only Seattle's

Every limit above comes from Seattle's own tip sheets and municipal code. The 3,200 square foot minimum lot, the 1,000 square foot cap, the rear yard percentages and the two-unit rule are Seattle numbers. Bellevue, Kirkland, Issaquah, Newcastle and SeaTac each write and administer their own zoning code, and a detached unit that is straightforward on a Seattle lot may be sized, sited or reviewed differently a few miles away. The first question on any project is which jurisdiction the parcel actually sits in, and the answer determines which rulebook applies to everything that follows.

The practical starting point is not a design. It is confirming your zone, your lot size, your rear yard area and whether anything on the parcel restricts where a structure can go, because those four answers decide what is worth drawing. If you want that assessed on a specific property along with the permit path it implies, our ADU and DADU construction service is where to start that conversation.