Accessory dwelling units or ADUs have been trending terms in the housing industry for the past few years, as many U.S. cities have recently altered zoning laws to accommodate secondary units on single-family residential lots. The housing type, which can be attached to or detached from a main residence, not only provides additional space for a homeowner or their guests, but also increases density and offers a solution to increasing the affordable housing stock.
In an effort to create unique and accessible ADU plans in the Pacific Northwest and beyond, Seattle-based Johnston Architects has designed a number of ready-to-build, detached accessory dwelling units (DADUs). Plans for four designs, including the firm’s initial Twisp Cabin and new Casita, Origami, and Gable DADUs, are available for purchase. The firm also has developed software to allow future clients to customize the Casita plan to fit their specific needs and lot requirements.
The whole concept to create and sell these DADU plans started with Twisp Cabin back in 2005. Ray and Mary Johnston, founding partners of the firm, were looking to build a getaway cabin in Washington’s Methow Valley. The objective was to have a small footprint and architecturally explore the possibilities of a simple box.
“It was fairly organic in the way it began,” explains Jack Chaffin, partner at Johnston Architects. “Essentially, people came to the website or visited Ray and Mary’s house and said, ‘Wow, I would like to have one of these.’”
Delivering on their goal of creating a simple, yet attractive, tiny home, the original DADU has a 1,200-square-foot rectangular floor plan with a lofted interior and 15-foot-tall windows on the west side to let in the views of the sky, valley, river, and Cascade Mountains. A large roof overhang protects the house from the intensity of the sun and offers covered space for outdoor living.
On the first level, the plan is split by a central walkway. The front half toward the deck includes a full kitchen, dining, and living area, while the back half houses a pantry, a full bath, a bedroom “cabinet” with two barn doors, and stairs to the loft level. Upstairs, owners have a secondary bedroom area, an additional bath, and more living space.
“The basic premise for the Twisp Cabin was to create a simple dwelling with high-quality materials,” says Chaffin. “There’s a good deal of cedar, concrete, and steel. It’s a simple palette that endures for a long time and weathers very well on-site.”
In 2013, the firm replicated the plan for the first time on Orcas Island in the San Juan Islands of the Washington coast. Besides the sloped and rocky site, the overall Twisp concept, including the floor plan and materials, were relatively unchanged. Today, the firm is working on a cross-laminated timber version in Oregon and is offering the architectural plans to anyone interested across the U.S. for $7,000, according to the firm’s website.
During the peak of the pandemic, the Johnstons retreated to the original Twisp Cabin and ultimately decided to expand the living space on the 11-acre lot by developing the Casita DADU prototype behind the original cabin built 16 years ago.
Completed in March of this year, the Casita model encompasses roughly 790 square feet, broken down into 450 square feet of interior space, a 64-square-foot sauna, and 240 square feet of covered patio and outdoor shower space.
Like its sister structure, the dwelling includes a sloped roof with an overhang, large windows, and concrete floors, but because of its decreased size, the interior mirrors more of a studio guest cottage with a bedroom, a wood stove, a kitchenette, storage, and a full bath.
“The Casita was sort of an evolution of what we were doing with the Twisp Cabin,” says Chaffin. “We came up with the Casita as sort of this very simple, yet we hope elegant, way of creating multiple different types of DADUs to accommodate a person’s or family’s needs.”
Chaffin mentions the Casita was developed not only to address the need for DADUs in urban environments like Seattle, but also to illustrate the concept in rural areas or tourist towns. “Kind of unbelievably there’s as bad a housing shortage there as here, and COVID has just made that worse,” he says.
Offering a multitude of potential layouts, the Casita has been componentized with numerous pre-designed rooms that can be added or subtracted. While designing the DADU on the firm’s website, options include three different sized living spaces, various bedroom and bunkroom layouts, two different kitchen and bath modules, and the optional sauna. Individuals select their preferred components, and the digital model updates automatically.
As the technology expands, individuals will be able to include additional parameters, such as solar orientation or exterior material choices. Users may place the components on their site with property lines, setbacks, and environmentally critical areas illustrated. As the concept continues to evolve, the firm also hopes to add the capability to create real-time plans, sections, elevations, and a bill of materials.
Looking ahead, the firm hopes both the Twisp Cabin and Casita plans will help homeowners fulfill their DADU goals, but the partners are also open to new designs as well.
“It’s one of those things that we’ve put out in the office to our architects,” says Chaffin. “If they come up with a product, house, or DADU that they think is really cool and fits a niche that we’re not addressing right now, we definitely would add that into the mix.”