Choose an AADU when you want a separate living unit within your home. Choose a DADU when you want a detached home on the same property and have space for one. Consider a garage conversion when an existing garage could serve your needs—but remember, a garage conversion is a way to reuse a building, not a separate ADU type. If the finished space is a permitted dwelling, an attached garage conversion may be an AADU and a detached garage conversion may be a DADU.
The right option depends on who will use the space, how much independence they need, what your property can support, and how much change you’re ready to take on.
| Option | What it is | Often suits homeowners who… | Main trade-off |
|---|---|---|---|
| AADU | A permitted living unit within the main home | Want to use existing interior space and preserve most of the yard | The unit shares the home’s structure, so layout, access, light, and privacy matter |
| DADU | A permitted living unit in a separate building on the same property | Prioritize separation and independent access | Uses yard space and requires planning for site access, construction, and utilities |
| Garage conversion | Remodeling an existing garage for a new use, from a flex room to a dwelling unit | Have an underused garage that may be practical to adapt | The structure may need substantial work; a room conversion is not automatically an ADU |
What’s the difference between an AADU, DADU, and garage conversion?
Seattle defines an accessory dwelling unit (ADU) as a separate living space within a home or elsewhere on the same property. The City calls a permitted unit within the main home an attached accessory dwelling unit (AADU) and a permitted unit outside the main home a detached accessory dwelling unit (DADU). ADUs must be established through the permit process to be legal.Seattle’s ADU guidance
A garage conversion describes the construction approach, not the legal type of unit. A garage can become an office, workshop, studio, or other flexible space. If the goal is a separate dwelling, the project needs to meet the requirements for an ADU and receive the appropriate permits.
When an AADU may be the better fit
An AADU creates a separate living space within the existing home. A basement or lower level may offer a starting point, especially when you want room for a family member, caregiver, guest, or tenant without adding a new building to the yard.
Using existing interior space can preserve more outdoor area, but it doesn’t guarantee a simple project. The layout, ceiling height, access, natural light, ventilation, moisture conditions, utilities, and privacy all affect whether the space can work well as an independent home. Stairs and sound between units may also matter to the future occupant.
Seattle requires a construction addition or alteration permit to add an ADU within an existing home. The unit must meet applicable city requirements, which may include building, mechanical, electrical, energy, and land-use standards.Seattle’s ADU guidance
Explore AmyWorks’ ADU services or basement remodeling if you’re considering an interior project.
When a DADU may be the better fit
A DADU is a separate home on the same property as the main house. Its physical separation can make it a good option when privacy and independent access are priorities—for example, for a relative who wants their own space or for a household considering a future rental.
A detached unit affects more than the building footprint. Consider how it will fit on the lot, how people and construction crews will reach it, how it relates to neighboring properties, and what utility connections the project may need. Seattle’s rules and permit requirements depend on the property and the proposed work, so assess site conditions before settling on a design.
Seattle’s ADUniverse offers a property-feasibility tool and a gallery of pre-approved DADU plans. The City says pre-approved plans can make permitting more predictable and that, in most cases, permits may be issued in 2–6 weeks. That refers to the permit process—not the entire design and construction timeline—and site-specific requirements still apply.Seattle SDCI’s ADU guidance
See AmyWorks’ DADU services to learn more about detached-unit projects.
When a garage conversion may be the better fit
A garage conversion starts with a structure you already have. Depending on your needs, the finished space could be an office, gym, studio, workshop, guest area, or part of a plan for a separate dwelling.
The intended use shapes the scope. An office or hobby room is different from an independent unit with its own living facilities. Before planning a conversion, look at the garage’s structure and slab, moisture conditions, insulation, lighting, windows, ventilation, heating, electrical system, and plumbing needs. If the garage is detached, connecting utilities may add work and complexity. You’ll also be giving up some or all of the space’s current use for parking or storage.
The City’s permit path depends on the project and site. Some garage work may qualify for a subject-to-field-inspection permit; other projects require an addition or alteration permit. Creating an ADU requires a construction permit, and separate electrical or plumbing permits may also be needed.Seattle’s garage permit guidance Seattle’s ADU guidance
Learn more about AmyWorks’ garage remodeling services.
Choose the intended use before the construction type
Start by picturing the person who will use the space and what their daily life will require.
- If you want a separate living unit within the main home and can make the layout work, explore an AADU.
- If independent access and more physical separation matter most, explore a DADU.
- If you mainly need a flexible room for work, exercise, hobbies, or guests, a garage conversion may be enough.
- If an existing garage might become a dwelling later, discuss that future possibility before choosing a limited remodel scope.
A phased plan can help keep options open, but it should be grounded in the property’s conditions and the intended future use. Seattle provides tools to research a property and its permit history; a site-specific conversation can help identify questions to resolve before design work advances.
Frequently asked questions
Is a garage conversion automatically an ADU?
No. A garage conversion can create a flexible room without creating a dwelling unit. It is an ADU only when the project establishes a separate living space that meets the applicable requirements and is permitted. An attached garage conversion may be an AADU; a detached garage conversion may be a DADU.
Which option usually costs less?
There is no reliable answer without knowing the property and scope. Reusing an existing basement or garage may avoid building a new detached structure, but repairs, upgrades, utility work, or code requirements can affect the total. A DADU involves a separate building and site work. Compare options after assessing the existing conditions and the level of independence you want.
Do I need a permit to convert a garage in Seattle?
The permit requirements depend on the work and the property. Seattle requires a construction permit to create an ADU, and electrical or plumbing work may need separate permits. Check the City’s garage permit guidance and ADU guidance for the relevant project type.
Can an existing detached garage become a DADU?
It may be possible, but the existing building and property need to support the proposed use. The garage may need structural, utility, or other upgrades, and the project must follow the applicable permit and code requirements. Assess the site and structure before assuming that a conversion will be straightforward.
Plan around the project, not just the finished space
Construction affects daily routines as well as the final layout. Ask how crews will access the work area, whether belongings need to be moved, and how the project could affect parking, storage, yard use, or household privacy while work is underway. These practical considerations can influence whether adapting an existing space or building separately is the better fit.
AmyWorks helps homeowners compare ADU building, DADU construction, and garage remodeling based on the property and the way the space will be used. If you’re weighing the options, contact AmyWorks to discuss your goals and possible next steps.