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Seattle “C-Box” Backyard Cottage from a Shipping Container

Case Study by Chris van Daalen
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Seattle “C-Box” Backyard Cottage from a Shipping Container
"Bad ass" C-Box DADU in Seattle
Land Use and Development
Urban Infill
Knife Plate Anchor for Shipping Container
Seattle, WA
Melissa Aaron
Accessory Structure
Sage Saskill, Sage Designs NW
Mike Vacirca, Lastingnest
Swenson Say Faget
C-Box structural calculations


Built with a re-purposed blue shipping container by hip Seattle EcoBuilders Sage Saskill and Mike Vacirca, the C-Box was designed as a Detached Accessory Dwelling Unit (DADU), known in Seattle as a “backyard cottage”.  Since it would be used as a commercial kitchen to house the owner’s small culinary business, it was permitted as a “shed” i.e. accessory structure (non-dwelling). With characteristic grace and style, they unveiled their project at the Northwest EcoBuilding Guild’s 2015 Green Building Slam.

Permitting Process

The project is a 160 square-foot backyard accessory structure built using a recycled shipping container with interior insulative wall assembly, storage/sleeping loft, bathroom and a commercial kitchen to house a growing small culinary business. By using the loft for storage and not for sleeping (for now), they could permit it as a “shed” and save on permit fees. Later when the business outgrows the space, they plan to convert it to an accessory dwelling. So they built it to meet all requirements for a “backyard cottage” DADU as well as a commercial kitchen.

Instead of hiring a welder to connect the box to the foundation at the construction site, they devised a simple knife-plate steel connector to fit into the container’s existing corner castings, attached to four foundation plinths, at about 1/4th the cost of on-site welding.

Code RequirementCompliance Path
City of Seattle DCI Directors Rule 7-83 requires a backyard cottage (i.e. detached accessory dwelling unit or DADU) to comply with the 2012 Seattle Residential Code, Chapter 3 Sec. R301 Design Criteria. Designers submitted a structural engineering analysis to show the proposed structure was equivalent or better than light frame wood construction, “to ensure force transfer, continuity and compatible deformations.”
Customer Assistance Memo CAM 116b specifies the permitting process for DADU’s under the Director’s Rule Although they decided not to permit it as a DADU, the designers met all code requirements under SRC Section R301 so it can later be converted and permitted as such.

Project Details

When the architect submitted the project to land use code officials as a DADU, the planners bounced it back said “No Bed?  It’s a Shed.” So it was permitted as a detached accessory structure with a commercial kitchen in it.

Although the designers had been looking for a container project to work on for years, this was their first.  So they tapped the EcoBuilding network and consulted with fellow design-builder Jon Alexander, who was wrapping up work on another permitted container building at the time, to understand some of the challenges they might face. One of the key lessons Jon had learned was that on-site welding to connect the container to the footings was very expensive.

The project had a tight budget, so they had an idea: instead of hiring an expensive welder to work on site, why not utilize what shipping containers already had going for them? Built to be stacked like Legos and attached to trucks, trains and ships, containers have a “corner casting” standardized fitting at each bottom corner. They designed a simple knife plate connector which would fit into the casting, and be bolted to concrete footings set in the ground. As EcoBuilders, they happily shared their innovative approach at the annual Green Building Slam in Seattle (see the video).

The knife plate connectors were fabricated by a welder who worked at the same yard where they bought the container; for $100 each, four for $400. That’s less than 25% of what they would have paid to bring a welder on site.

Design / Build Process

To comply with structural design requirements, Saskill commissioned a structural engineer to produce structural calculations.

For the insulative wall assembly, they consulted with colleague Joe Lstiburek a Passive House Consultant who usually recommends insulating on the outside of container projects. But he knows a lot of EcoBuilders think shipping containers are pretty groovy just they way they are with the corrugated metal showing, so they decided to go with 2” of spray foam insulation on the inside, plus batt insulation covered with wall board.

The C-Box uses salvaged triple pane windows, doors and cabinets reclaimed from another kitchen remodel project being done by contractor.

Working with a level, plumb steel box made things go up fast. The whole building was constructed in about a week.  After installing the footings and knife plates, they flew in the box with a crane which took 2-1/2 hours crane time.  Once it was attached, they foamed the interior, framed up the pony walls on top, and buttoned up the roof within 3 days.  Interior framing commenced and completed quickly afterwards before they raised a glass and turned the keys over to the satisfied owner!

Note:  Sage has developed a related design for a 3-container building with containers spaced about 20’ apart with a roof over that to contain the living space, and use the containers for secondary uses and for the structure (Learn More).

To use it for food processing, owner Melissa Aaron was able to get her Washington State Department of Agriculture Food Processing and Storage license through the standard permitting process.  Interior fixtures and finishes had to meet food standards and be inspected, including commercial grade dishwasher, vented, sealed bathroom, and sealed, washable counters and washable floors and walls. Her business does not require a health department permit.  Culinary spices and other products of the owner’s business are stored in the loft above the kitchen space.  Later the upper space can be converted into a sleeping loft so the building can be used as a dwelling.

Cost / Benefit

In the meantime, Aaron figures her business will save enough, money which she would otherwise be spending on commissary service (i.e. renting commercial kitchen space certified by Department of Health), that the building will pay for itself in 4-5 years, just about the time she thinks her business will grow to needing an upgrade to a larger commercial space.

“Now that it's (mostly) done and I'm working in the space, I'm even more sure it was the right choice. It's an efficient space & a serene work environment.”

Hard costs for the container were: The container itself was $2500, then fabrication including the knife plate connectors was another $2500.

Lessons Learned

Sage and Mike think containers are cool, and there’s plenty of them, there’s a need and opportunity to use them for beneficial buildings!  But to do so required these innovators to bring a passion for seeing the challenge in a clear light, bringing smart solutions to the table for the “inconvenient truths floating all around us, understanding that those solutions are rarely found alone.  They are most likely found in unlikely combinations of ideas, putting things together that have all too often been in conflict, like design and build, development and sustainability, buildings and science, technology and craftsmanship!”


Project Contacts
Designer: Sage Saskill Sage Designs NW (206) 963-1420 Builder: Michael Vacirca Lastingnest Construciton (206) 550-0326
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