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A Rigorous Test of Design Discipline

Codes and zoning requirements make executing a refined DADU project a game of inches

Dadu Sept18 1
Photos: Mark Woods

In metropolitan areas with decreasing availability of land, homeowners are looking to new alternatives to improve housing affordability. Seattle is taking the lead on the trend of detached accessory dwelling units (DADU) and attached accessory dwelling units (AADU). The dwellings are great for homeowners who want to earn additional income by renting them out or provide space for a family member such as an aging parent.

Seattle-based architect Robert Hutchison completed this DADU in the northwest neighborhood of Fremont in February 2018. The principal of Robert Hutchison Architecture says, “We would like to do more. We have the Fremont one, one in design right now, and we might have one or two down the pike.”

The project was for a young couple. The husband had purchased the home five years previously, and he wanted the separate dwelling so he could live in that and rent out the home. “During design, he got engaged,” says Hutchison. “They live in the DADU and rent out the house to friends.”

Tight Quarters

DADUs are driven by zoning and code restrictions. For example, total building lot coverage (including primary residence, garage, DADU and other accessory structures) cannot cover more than 35 percent of a lot, and the DADU structure must be under 16 feet in height as measured from average grade to the eave. Unless the residence is near a frequent transit stop, it must include a parking space, and the total floor area of a DADU cannot exceed 800 square feet. Hutchison’s design totals 799 square feet.

When you combine these zoning requirements with building code requirements for stairway width, landing space, exterior door openings, and riser and tread minimums, you end up with a severely restricted jigsaw puzzle.

You would think that those restrictions would result in DADUs that are all identical, but Hutchison says, “What’s amazing is we keep coming up with various solutions for each site. We developed 15 schemes for this site alone. You would think you would not be able to come up with just a few schemes. But each site is unique and different, and if you think about how the building sits on the site, you continually find new ideas.”

Space for Living

To make everything work, Hutchison chose a pinwheel layout that places the stairway at the center of the DADU and the rooms radiate out from it. “When you come in through the entry door, you have four choices,” he says. “Go left into the kitchen, go right into the double-height reading room, go straight up the stairs or go back outside.” The same kind of decisions occur at the top of the steps, where the bedroom and office are located.

Also, with the pinwheel plan, “half of the façade looks out onto outdoor space,” says Hutchison. That means lots of room for large fenestration that opens the indoor to the outdoors and provides great views of the Seattle environment. “I particularly like standing in the double-height space and looking up through the large second-floor window and seeing nothing but sky,” he says.

Final Finishes

The metal cladding of both walls and roof gives the DADU a cohesive, singular form. The 7/8-inch corrugated siding from Taylor Metal Products, Auburn, Wash., provides texture to the exterior. The pre-weathered zinc finish “has a luster to it that is really beautiful in the Seattle climate,” says Hutchison. The original impetus for selecting metal exterior products came from the client. “The client preferred something that he didn’t have to refinish. If you use cedar siding, which is common here, he would have to deal with it every four years. So, we went toward a metal siding system.”

In keeping with the rigorous attention to detail, the trim and finishes on the exterior cladding are tightly done. The corner trim is flush with the siding and flashing at the eaves, and wall penetrations show no edges or protrusions. “It comes across as very refined,” Hutchison says. “Even the gutters are custom detailed and downspouts are flush with the walls.”

While the exterior clad in metal gives the DADU a monumental feel, the interior is finished all in wood, including using AC finish plywood for walls. “It has a nice face to it, but you can still see all the footballs,” says Hutchison. “We coated it with a clear finish. By finishing everything with wood, the walls and floors, the interior space is kind of like furniture placed inside the volume.”

“There are no enclosed rooms,” Hutchison says, “except for the bathroom.” The owners, while occupying less than 800 square feet of living space, feel as if their environment is much larger because of the openness and continuity of finishes.