Rhodes Architecture + Light has designed and been involved in constructing many Attached Accessory and Detached Dwelling Units (AADU’s and DADU’s) in Seattle and neighboring cities. We’ve walked your Seattle area neighbors through the process of adding additional dwellings to their residences, and perhaps after learning about the multiple advantages, you’re considering this as well.
There are significant benefits of adding a dwelling (attached, detached… or both!) that deliver an impressive return on investment–both financially and in quality of life. We have years of experience bringing these unique projects to life and have a coordinated team of close consultants including building and engineering partners whose expertise ensures that our architectural vision is executed according to plan.
To provide additional expert insight on the structural aspects of ADUs, we’ve consulted with Ryan Reichman from Swenson Say Faget (SSF), a firm known for their innovative approach to engineering challenges. Their experience with diverse projects, including ADUs, makes them an invaluable resource for understanding the technical considerations involved in these structures. In this article, we’ve included a conversation with Ryan to give you a comprehensive understanding of ADUs from both an architectural and structural standpoint. But first, what is an accessory dwelling unit?
AADU and DADU Definitions
What is an AADU, you ask? A DADU? What defines these accessory dwellings? What makes an AADU different than a DADU?
AADU’s and DADU’s are different types of Accessory Dwelling Units (ADUs). An ADU is a small, independent residence on the same lot as another residence. The difference between them is an AADU is Attached to the main residence and a DADU is Detached.
An AADU has some sort of connection to the main home (whether it’s a sectioned off space within the main home like a converted basement or garage, or a separate structure with as little as a 5’-1” wide breezeway attaching the two).
A Detached Accessory Dwelling unit has its own regulations for separation from the primary residence – in most cases in Seattle DADU’s should be at least 10 feet away. In our area, regulations around ADU’s are changing rapidly, generally making them more attainable for the average homeowner to build. This is a great thing if you’ve ever wanted to add and ADU to your property, but it also makes it tough to keep up on the facts and nuanced criteria for building one, which is where an Architect steps in!
We can explain the most up to date codes and if your property is zoned Neighborhood Residential (NR) then depending on your lot size and shape (along with other factors), you could build both an AADU and a DADU up to 1,000 SF each. There’s a lot we can design within 1,000 SF – imagine a two-bedroom, two bath home with a roomy kitchen with sizable island open to a light-filled living room, laundry and maybe even an office space.
Return on Investment
Adding another residence (or two), is not only nice to have, but can provide significant financial benefits. Having another dwelling on your property creates an increase in property values, can be a source of rental income, or allow family or guests to stay with you rather than in a hotel. Using an existing property with extra space is a good way to take advantage of existing green space to create courtyards and nurture friends, family and community right in your backyard.
This careful addition of housing availability in single family neighborhoods has a broad positive impact on the city by increasing housing options without changing their context or culture. By using existing square-footage and existing land for additional homes it increases the overall sustainability of a property. Using available, existing, infrastructure (a basement or garage for example) not only makes someone resourceful, but reduces their overall environmental impact by reducing the need for new construction materials. Using your own land and existing utilities to add living space is much more efficient than developing new land.
Sound Intriguing? If you own your home, there are several ways to approach ADU design. We have built them into the lower stories of existing homes, included them in the design of new homes, created them as flexible bonus spaces for speculative residences and designed them as separate dwellings on a forgotten corner of existing land. We understand the complexities of codes and regulations, including the opportunities and constraints.
Rental Income
The American dream of a home with a yard is less accessible than in years past. Both renters and buyers are looking for alternative ways to live. Most of the housing market in Seatle is urban style apartments or townhomes that lack green space, often divorce their tenants from the ground altogether, and require shared entries, drives and halls. In these developments we have noticed a prioritization of interior square footage over quality, efficient designs, that leave room for outdoor space. Interest is growing in ADUs as a housing option with their own entries, green space, parking, kitchens, living areas and bedrooms and privacy.
ADU’s are a desirable option for many people who are shopping for housing, but they are also great investments for homeowners. Having rental income can greatly offset the burden of a mortgage or create monthly income. One exercise we provide as Architects is called a Proforma – basically an estimate of costs compared to returns on investment, which helps homeowners be financially informed as they design their ADU.
Flexible Design for Modern Living
In an era of evolving family dynamics, ADUs represent more than just additional square footage - they’re a reimagining of community, connection, and intergenerational support. These versatile living spaces create unique connections where family and visitors can maintain independence while remaining connected.
Imagine a property where generations coexist with intentional proximity - where aging parents can live comfortably with independent space - yet only steps from their adult child's home, where young professionals can launch their careers with affordable housing and immediate family support, and where kids can experience growing up within a nurturing familial environment - a village.
Another situation we’ve encountered is building an ADU to be rented out now, while living in the larger residence to raise a family, with future plans to move into the ADU as empty-nesters. This forward-thinking strategy not only addresses immediate housing needs but also provides a built-in plan for financial security through future rental income and lifestyle adaptation in later years.
There are so many ways to imagine an ADU, and we’ve seen a lot over 30 years of designing them. They can be used as a home office and transitioned to a dwelling upon retirement or for aging parents, for hobbies, teen hang-outs, home offices, businesses and rental income.
We have transformed lots into dynamic living environments, integrating shared and private green spaces and courtyards between 2-3 dwellings. By reimagining even smaller inner-city lots of 5,000-7,500 square feet, we create interconnected landscapes that balance individual privacy with communal interaction, maximizing both functional and aesthetic potential of limited urban land.
Sustainable by Design
The size and efficiency of AADU and DADU dwellings means that they cost less and use less energy to build and operate. Less land is excavated, less concrete, steel and wood are
utilized, the buildings require less energy to heat and cool, clean and maintain–all leading to a quantifiably sustainable building. Since the overall volume of these residences is smaller, the energy used on lighting, heating, ventilation and air conditioning is much smaller too! This makes the use of small photovoltaic systems able to provide all the electricity required by the Dwelling Unit.
Building Community
When thoughtfully designed, clustered buildings can yield intriguing and welcoming courtyards and gardens facilitated by multiple smaller residences on the same land. These private yards allow all residences to open to green space as well as communal areas which encourage gathering.
As of 2024, Americans are spending 62% of their time at home (up 50% from 2019), according to the U.S. Bureau of Labor Statistics, so spending that time adjacent to green space, in gardens, and having community on our land has a measurably positive effect on our mental health, overall health and wellness.
Rhodes Architecture + Light believes in the benefits of smaller homes, ADUs, and residences oriented in a way that creates interesting green spaces between. These are in line with our commitment to fostering wellness through designing interior spaces with ample daylighting; dwelling spaces that feel open to the land, trees, and gardens. Having views to yards and community spaces improve our interior environments and, on Seattle’s sunny days, lure people into shared outdoor space that benefit families and pets.
Size and Efficiency
It is worth recalling that the average American home was just 909 square feet in 1949, then 2,080 square feet in 1990, growing to 2,480 square feet in 2024. In the recent past, people lived in the square footage allowed, in Seattle, for ADU’s.
Building smaller dwellings within the city reduces congestion on highways, allowing people to live closer to amenities and work, and reducing overall energy-use. The development of outlying land for new construction worsens multiple issues including longer commute times, loss of forests and productive or environmentally beneficial land, and often larger homes that require more energy to operate and construct. ADUs use land more fully while requiring less maintenance and resulting in smaller footprints and eventually less material to be demolished and disposed of (or recycled).
Size also affects efficiency in the sense that it encourages a close-knit neighborhood community within a single lot where resources (gardens, utilities, outdoor spaces) are shared. We’ve written extensively about the advantages of smaller homes in this three-part series.
On Being Small in Seattle
The City of Seattle, and many surrounding jurisdictions, encourage the construction of accessory dwellings, promoting these small homes in residential and multifamily zones.
In July of 2019, Seattle’s mayor, Jenny Durkan, signed legislation to remove barriers to the creation of these additional homes in Seattle’s single-family zones and it is currently permissible to build up to three single-family dwellings on most residential lots (a single-family residence + two attached dwellings or an attached and a detached dwelling) under specific guidelines. Seattle also aids people in owning and renting or even selling these additional dwellings, noting encouragement for these added homes:
“ADUs increase the supply and variety of housing options in our single-family neighborhoods.
Backyard cottages share many characteristics with small, detached homes: a single unit with no shared walls in quiet residential neighborhoods. Given their smaller size and lack of additional land cost, ADUs can offer a more affordable housing option in neighborhoods where homes are out of reach to most people. This is a small but important step we can take to address Seattle’s acute housing crisis.
ADUs also support the housing needs of a wide range of households, like older adults looking to downsize, seniors with fixed incomes, lower- and middle-income households, homeowners with disabilities or other needs, and multigenerational families. Households can benefit from the flexibility, stability, or supplemental income that an ADU provides to meet their evolving household needs” – Source: City of Seattle Website
So, what is allowed in an AADU and a DADU?
Accessory dwellings are prescribed and constrained by the Land Use Code. Again, using City of Seattle as the benchmark, two additional 1,000 square foot dwellings can be added to most primary residences on the same land, under separate permits. Seattle requires the second accessory dwelling (the third residence) to meet either green building or affordable housing standards.
The overall land coverage of structures, the total floor area (called “floor area ratio”, a percentage of land/lot size) and residential setbacks (standard required front, rear, side yards) govern what can be built and where. A site study and some calculations are necessary to determine feasibility, which your Architect can provide.
Neither AADUs nor DADUs require parking to be built. Access to your land, parking and garage space may be a significant determinant too. Alleys in Seattle yield bonus space to your rear yard and the ability to build and access an AADU or DADU as well. Both need to have kitchens, living, sleeping space, bathroom facilities and a separate locking entrance door and meet residential building codes. There are also codes relating to fire separation between units, which can have a hand in determining the best layout of a lot.
Design Experience is Important
Accessory attached and detached residences in Seattle are allowed under separate permits, but have the same requirements generally as primary residences, and need to meet all land use codes and building codes.
The permit submission requires a site study, site plan, floor plans, elevations, sections, construction/material details, structural engineering and must meet the Seattle/Washington State Energy Code.
The Architects at Rhodes Architecture + Light are seasoned professionals and well-versed in site planning, solving the inevitable issues, producing drawings and permitting your AADU or DADU.
Considerations in Design and Planning
ADUs can be positioned on the property to maximize privacy and minimize impact on the main dwelling. Consider orienting the ADU to maximize natural light and views, consider shadowing of other structures and outdoor spaces, while maintaining privacy from neighbors and the main house.
Consider orienting the ADU to form a courtyard between it and the main home. Imagine what you will do with your shared green space! Think about the orientation of that greenspace.
Ensure that ADUs adhere to local zoning (land use) regulations regarding setbacks, separation, height, location.
Careful space layout within an ADU is crucial when designing to fit an existing footprint or creating an efficient new layout. We design the interior to maximize every square foot. This may include using multi-functional furniture and built-in storage solutions depending on the space available.
Use vertical space effectively, especially in areas with height restrictions. Consider loft beds, high ceilings with storage, or built-in shelving units to increase efficiency.
Be Mindful of Constraints
The overall size of the land you own is important. A DADU requires a 3,200 square foot lot size in Seattle. The DADU can be built to 1,000 gross square feet and this does not include garage areas, storage areas accessed from the exterior, porches and covered decks less than 25 square feet in area and 35 square feet of covered bicycle parking. The AADU can also be built up to 1,000 gross square feet (and there are exceptions to this rule). A residential lot with one AADU or one DADU can house up to eight people unless the residents of both units are related to each other.
All jurisdictions have standard front, side and rear yards/setbacks which must be calculated, and which will constrain the amount of residential floor area that can be built on a given lot. In Seattle the total residential lot coverage (the footprints of all structures) is limited to 35% of the total lot area (Which may increase soon under the “One Seattle Plan”). A DADU can, however, use a portion of your rear yard and having an alley behind your land potentially allows a rear alley accessed DADU. Access and parking/garages are a consideration; AADU’s and DADU’s do not require new parking to be added in Seattle but their construction cannot remove existing parking.
Your existing residence and its placement on the lot, the land’s placement on the block and considerations like the location of existing utilities will influence the design and location of AADU’s and DADU’s. Working with an experienced Architect allows you to plan and calculate the most beneficial layout of accessory units and any addition to your residence.
In Seattle, environmental issues are important too and may include slopes, soils, water (wetlands and streams). In Seattle, the tree codes are strict and require an arborist to consult on the retaining of and protection of certain trees. This includes detailed design and layout considerations, so the size and location of existing trees are known and considered. A plan by an experienced Architect at Rhodes Architecture + Light may open possibilities and define constraints that the average homeowner is not aware of. See our blog for detailed information about studying your land in preparation for development.
Construction Costs
The construction costs for AADUs and DADUs can vary significantly based on a wide range of factors. As of 2025, the average cost to build an AADU or DADU on a property in our area is around $500-$800 per square foot, with the potential for higher costs based on size, finishes, location, property and utility considerations. DADUs tend to cost more than AADU’s due to the need for separate structure (exterior walls), utilities and foundations. Urban areas with higher land values and labor costs tend to have higher overall construction expenses. While larger ADUs/DADUs generally cost more in total, there is some economy of scale in ADU construction; as the size increases, the cost per square foot tends to decrease.
Maintenance and Management of AADUs and DADUs
Maintenance and management of Attached Accessory Dwelling Units (AADUs) and Detached Accessory Dwelling Units (DADUs) represent critical considerations of responsible property ownership. These additional living spaces require consistent, thoughtful care to ensure their longevity, safety, and continued value, just like any home.
Essential Maintenance Approach
Successful ADU maintenance begins with a comprehensive, proactive strategy. Homeowners should conduct monthly inspections of critical systems, focusing on smoke detectors, air quality, and basic structural integrity. Annual professional assessments of plumbing and electrical systems help prevent potential issues before they escalate. Developing a dedicated maintenance budget that accounts for routine repairs, potential upgrades, and unexpected issues helps homeowners maintain the unit’s value and functionality. By approaching ADU maintenance as a holistic, strategic process, property owners can transform these spaces into reliable, efficient, and valuable assets. Seasonal maintenance plays a crucial role, with careful attention to exterior elements like gutters, roofing, and weatherproofing. Our Architects are trained in designing for the wet conditions we experience here in the PNW and incorporate a broad selection of materials to proactively address moisture related challenges. We are also versed in designing for low-maintenance structures, carefully considering the construction cost relative to the overall maintenance costs of roofing, siding, foundations, and finishes.
Sustainable Maintenance Practices
Modern ADU management emphasizes sustainability and efficiency. Implementing smart home technologies can significantly reduce maintenance costs and improve energy performance. Owners should consider eco-friendly materials and landscaping solutions that minimize long-term maintenance requirements while supporting environmental sustainability.
Management Considerations
Managing an ADU as a rental involves more than physical maintenance. For rental properties, owners should consider establishing an L.L.C., managing a robust tenant screening processes, establishing clear communication channels, and creating systematic approaches to rent collection and maintenance requests. Staying current with local building codes is important, requiring periodic safety inspections and potential updates to ensure continued compliance. It is also important to know and consider your local jurisdiction’s rules around long-term and vacation rentals.
Structural Considerations: Interview with Ryan Reichman
Now that we’ve covered the ins-and-outs of ADU design from an architectural perspective, let’s hear from Ryan Reichman with Swenson Say Faget (SSF) about some of the structural considerations.
RA+L: What are some things you would like the homeowner to know before embarking on an ADU project?
Ryan: I think the key to any kind of DADU or ADU is that just because it’s small from a square footage perspective, it’s not any less complicated than a full house. In fact, in many regards, it’s more complicated because you’re trying to really understand the use of the space.
There isn’t a lot of square footage available, so you’re compacting and distilling down all the ideas of what makes a functional dwelling into a small footprint. It takes a lot of thought and care to do that. It should be treated with the same care and understanding as any kind or size of residential structure.
If I could give advice to homeowners before embarking on an ADU project, I would say hiring a team of experts is critical, starting with your Architect. And finding the right Architect who is a good fit for the project. And of course, my response would always be to hire Rhodes Architecture + Light. I have collaborated with them for 25 years and cannot express how much I enjoy working with the firm.
Some of the challenges we run into are part and parcel with the idea of distilling everything down into a small space. For example, with the lateral system for the house, we must ask how we are going to resist wind and seismic forces, because there’s not necessarily the same length of exterior wall that’s available for use in buttressing. At the same time, there are not a lot of interior walls available as we want the interior to be open, allowing for more flexibility.
The designs I’ve noticed from Architects are essentially building a box by which the interior can then be divided in any number of different ways. Having that flexibility available for the Architect’s client requires a careful understanding of how we would approach the structural system.
Usually with the roof framing or the floor framing, we’re talking about small spans. Those are not as much of a challenge, which is good and helps to keep the costs of the ADUs down because we’re not doing any great acrobat spans with the framing. However, we have done some very creative things for the lateral design to get that to work.
RA+L: What are some considerations if a person is trying to build an ADU without an Architect, and what areas of the project require the need for a structural engineer?
Ryan: I think if homeowners attempt to do an ADU project without a team of architects and engineers, there are a number of challenges they will run into fairly quickly.
One is, especially with detached dwelling units, that there are some specific code issues related to how it’s placed on the site. Those jurisdictional requirements can cause some headaches for homeowners who don’t understand the limitations.
Then when it comes to the structure itself, when we work with homeowners who buy a set of plans off the shelf, they are often surprised when they find they actually need a local structural engineer to provide calculations for the framing and a lateral system.
Often by the time we’re discussing that with the homeowner, they’ve already gone through a very frustrating, sometimes year-long process trying to get their project to go. They thought they were saving money, but it winds up being a loss in terms of dollar signs and results in stress and headaches.
With some residential projects, there are prescriptive methods within the building code that you can try to apply, in which case the need for a structural engineer is not as prevalent. However, most additional dwelling units, because of the size, don’t necessarily fit those prescriptive requirements very well.
For example, if you want windows or doors, the amount of exterior wall that is taken out with those openings, again, goes back to those key challenges or components that we’re often faced with, which is that lateral system component.
RA+L: If a person had an option to do either an attached or a detached accessory dwelling unit, would you have any recommendations as to which way they should lean?
Ryan: I think it really depends on what the use of that space is going to be. Is it going to be a rental? In that case, having some separation might be a positive. If it’s going to be used as a mother-in-law unit for family, having something that’s attached to the main house might be a nice feature.
Next is how the design plays out. I think the detached structure gives a lot more architectural freedom because you’re starting out with a new idea. It may be in response to the architecture of the main house, but it doesn’t have to be. Whereas if you’re looking at an attached dwelling unit, there are more constraints as to how you are interfacing with the existing home.
Additionally, when attaching or creating an addition to an existing house, from a structural perspective, there is a need to review and understand what the existing house structure is, how that attachment is being made, and if the new addition is imposing loads or forces on the existing structure.
We also work with a lot of homeowners who are repurposing the basement to create an additional dwelling unit. In those circumstances, there are ceiling heights that need to be worked with, as well as plumbing and other mechanical systems that must be modified to allow the basement to be used as an occupied space.
When putting an additional dwelling unit over an existing garage, the key is really understanding what the existing garage structure consists of. For example, in older construction, a garage may just be a slab-on-grade with no actual foundation element. That can lead to some additional work on the structure.
The more knowledge that the structural engineer has up front will help with the documentation of the new structure to match the true existing conditions, which is really important.
With a renovation, there are always going to be things uncovered during construction that weren’t anticipated. Usually, there are dollar signs associated with those items.
The more knowledge that we have of the existing conditions at the beginning of the project means that we can use the documents up front to weed out any costs that might be associated with the renovation beforehand instead of during construction. From a homeowner’s perspective, this is crucial to the success of the project.
RA+L: What does SSF bring to the project?
We pride ourselves on not going with canned solutions but really thinking through the project's goals.
I personally love listening to architects present the overall goals, objectives, and themes of the project and then figure out how we can work as part of that team and part of the process to achieve those goals. How do we play a role and see that vision take place?
Something that’s not necessarily common with engineers–instead of throwing out roadblocks or saying that something is too difficult to do, I would much rather work with the team and figure out what we can we do and how we can make it work.
If I know what the goals of the project are, and if there are areas to be creative and cost-effective at the same time, then that allows us to be a true partner in the overall success of a project.
In Conclusion
Accessory Dwelling Units (ADUs) represent more than just an architectural trend—they are a transformative approach to housing that addresses affordability, sustainability, and community connection.
For 30 years, Rhodes Architecture + Light has been helping homeowners reimagine their properties, creating flexible living spaces that adapt to changing family dynamics and urban needs. By thoughtfully designing AADUs and DADUs, we can unlock the potential of existing urban lots, provide innovative housing solutions, and contribute to a more sustainable, interconnected community.
Whether you’re seeking additional income, space for family, or a more efficient living arrangement, ADUs offer a compelling reason to think outside of the box.
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