A Seattle ADU feasibility conversation is an early review of your goals, property, and likely project questions—before you invest in a detailed design. It can help you compare an attached accessory dwelling unit (AADU) with a detached one (DADU), identify site or utility issues to investigate, and choose a practical next step.
It is not a permit approval or a guarantee that a particular design can be built. The goal is to understand what needs closer review and make better-informed decisions about design, permits, schedule, and budget.
Start with how you want to use the space
Seattle ADUs can take two basic forms. An AADU is a separate living space within the main house. A DADU is a separate building on the same property. Both need to be legally established through the permit process, and both must meet applicable requirements.
Before focusing on square footage, consider who will use the space and how. Is it for a parent, an adult child, guests, a home office, or a future tenant? Will a private entrance matter? What level of privacy, storage, accessibility, and separation from the main home do you want?
The intended use helps shape the right questions. An AADU may involve adapting a basement or another area inside the home. A DADU may offer more separation, but it depends on what can fit on the lot and how a new structure can be accessed and connected to services.
What does a Seattle ADU feasibility review look at?
The property itself—not just a sketch of a proposed unit—should guide the conversation.
For a potential DADU, the review may consider the lot and property lines, existing buildings, access to the proposed building area, privacy, drainage, sewer and other utility routes, and construction access. Trees, slopes, retaining walls, easements, and environmentally sensitive or shoreline conditions may also affect what needs further investigation.
For an AADU, the existing home matters: its layout, ceiling heights, windows, access, utilities, and the conditions behind walls and ceilings. Existing permits or plans can also help clarify what is already known about the space.
A survey, site visit, or professional review may be needed to confirm details that online property records or photos cannot establish. The applicable zoning, height, location, and building standards should be checked for the specific property; generic size or setback summaries may not reflect current requirements or site conditions.
Look at utilities early
Water, sewer, electrical service, heating and cooling, ventilation, and internet all contribute to a comfortable, functional living space. Their locations and capacity can influence the design and scope, so they are worth discussing before the floor plan is finalized.
Seattle notes that an ADU project may require an electrical-service change or a new service from Seattle City Light. Sewer connections and related charges may also need to be considered. Ask which utility details are known, which need to be verified, and whether any potential upgrades could affect the project budget.
Understand permits—and check current Seattle guidance
Seattle requires permits to establish an ADU legally. The City’s ADU guidance describes the permit process and standards that may apply, including building, electrical, mechanical, energy, land-use, environmental, and shoreline requirements.
Seattle’s ADUniverse includes a process guide and a gallery of pre-approved DADU plans. Its property feasibility tool, however, is a preliminary screening resource: the City says it reflects outdated regulations while Seattle updates materials for recent state requirements. Treat its results as a starting point, not a definitive answer about what your property can support. For more detailed guidance, consult SDCI and a design or homebuilding professional.
A pre-approved DADU design may be worth considering if it fits your goals and property. It does not remove the need to evaluate the site and project requirements. A custom design may make more sense when the lot, existing home, or household needs call for a different approach.
Discuss schedule and budget without relying on a guess
There is no single timeline for every Seattle ADU. Design, permit review, utility work, material selections, site conditions, and construction all affect how long a project takes. A responsible early conversation should explain the stages and identify what needs to be confirmed—not promise an approval date outside anyone’s control.
The same is true of budget. A conversion inside an existing home and a new backyard structure involve different work. Utility connections, access, drainage, foundations, and other site conditions can affect scope. Ask which assumptions are driving an early estimate and what information would make it more reliable.
What to bring to your first conversation
You do not need completed plans to start. Helpful information includes:
- The property address and any survey, site plan, or past plans you have
- Photos of the yard or the interior area you are considering
- A short description of who will use the space and how
- Priorities such as privacy, access, storage, or keeping part of the property usable during construction
- Any known concerns about utilities, trees, slopes, drainage, or existing structures
- A rough sense of your budget and timing expectations, if you have one
What should you know by the end?
A useful feasibility conversation should leave you with a clearer sense of whether to explore an AADU, a DADU, or another remodeling option—and which questions need answers before design moves forward. The next step might be gathering property information, checking a utility connection, comparing layouts, or moving into concept design.
AmyWorks works with Seattle-area homeowners on ADU projects and home remodeling. Explore its ADU building services and DADU building services, or contact AmyWorks to discuss your property and goals.
Seattle ADU feasibility FAQs
Does Seattle’s online ADU tool confirm that I can build an ADU?
No. The City describes the tool as a preliminary assessment and currently notes that it reflects outdated regulations. Use it for early orientation, then confirm property-specific questions with SDCI and a qualified design or homebuilding professional.
Do AADUs and DADUs both need permits?
Yes. Seattle says an ADU is not legally established unless it has gone through the permit process. The type of permit and additional reviews depend on the project and property.
Is an AADU or a DADU easier or less expensive?
There is no universal answer. Converting space inside a home and building a separate structure involve different design, construction, and utility considerations. The existing home, site conditions, access, and intended use all matter.
How long does it take to build an ADU in Seattle?
The schedule depends on the project’s design, site, permit path, utility work, and construction. A feasibility conversation can identify likely stages and questions, but it cannot guarantee a permit or completion date.