HD Construction in Seattle is part of a wave of builders tackling the housing crisis with high performance ADUs

Take a look around and you may have noticed a new phenomenon in your neighbourhood. Miniature dwellings—sometimes called backyard cottages, garage apartments, pool houses, casitas, in-law suites, and guest houses—are cropping up in droves around cities and suburbs alike. More formally known as Accessory Dwelling Units (ADUs) or Detached Accessory Dwelling Units (DADUs), they’re an increasingly common solution for homeowners looking to house multigenerational family members or earn some extra rental income. The need for flex work, school, and living spaces has never been greater as the Covid-19 pandemic has shifted many towards remote lifestyles. With the affordable housing and climate crises intensifying, cities and states are re-examining current housing models and enacting code changes to build density, sustainably.

An ADU by Maydan Architects, based in Palo Alto, California. Source: Architectural Digest.

In the ADU hotbed of Seattle, HD Construction, now specializes almost exclusively in the design-builds of high performance backyard cottages and offices. For founder Barry Gibbons, the need is personal—the pandemic forced his wife, Sara, and their two sons to work and school from home. This personal element is evident in the company ethos: HD prioritizes the health and comfort of all their clients by carefully selecting quality products and processes that minimize their environmental footprint, while delivering a superior end result.

When building Sara’s detached home office (dubbed her “sanity shed”), Barry utilized 475’s Smart Enclosure System, just as you would with any full-size building project. He installed INTELLO PLUS air barrier and smart vapour control, sealing it at the joints with vapour-open TESCON VANA tape. To keep the assembly airtight, Roflex 150 gaskets were used around pipes and ducts and a LESSCO Utility box was used around the junction box and taped to the INTELLO. (Note: when possible, we recommend using a service cavity with INTELLO to greatly reduce the number of penetrations through the control layers. Otherwise, the LESSCO Utility box is a good alternative.) 

Sara’s detached home office/flex space is continuously ventilated with the Lunos e² HRV system. The gray outer hood can be seen to the upper left of the horizontal window.

The LESSCO Utility box around a junction box and taped to the INTELLO air barrier keeps the assembly airtight.

A subtle LUNOS e² HRV system with MERV13 filtration (top right) provides superb ventilation in the space.

The Rise of ADUs

It’s no wonder why ADUs have taken off in recent years. New laws across the country have made it easier to build them. In 2019, Seattle Mayor Jenny Durkan signed legislation to remove regulatory barriers and assist homeowners in the ADU design and permitting process. Up until then, existing regulations resulted in less than two percent of single family lots having ADUs despite its legalization for years. To help expedite the process, the City of Seattle has since pre-approved 10 detached accessory dwelling unit (DADU) construction plans to help cut through the red tape.

In states like California, ADUs are now permitted on multi-family lots, whereas they were once only allowed on single-family lots. In a related housing measure, California Governor Gavin Newsom recently signed bills ending a controversial 100-year single-family-only zoning history for the state that contributed to unsustainable sprawl and precluded marginalized communities from home ownership. In New York, a recently introduced bill would legalize ADUs statewide and limit their rent as part of an affordable housing measure. And many cities are following suit too. Urban enclaves of Denver are actively pursuing rezoning to allow ADUs in the 75% of lots that do not currently allow them; a Tuscon proposal to allow ADUs city-wide is similarly undergoing public review.

In fact, the growing popularity of the ADU seems to point to a new housing paradigm in the U.S.—one that recognizes the inherent sustainability of density. And a high performance ADU takes that several steps further: ensuring our living spaces minimize energy demand while remaining comfortable, regardless of the season. For Barry, his ethos of building is simple: “You can definitely build sustainably no matter your style, but the most sustainable practice is to build once and have it last.”

Guidance on Building Well While Building Small

Regardless of the size, form, use, or assembly type, we have a Smart Enclosure detail for you. We highly recommend that you use these free downloadable details as the foundation for your assembly design, adjusting wall thickness and insulation as needed for the job.

For any situation where details may diverge from standard construction, we recommend booking a consultation with Isabelle Nagel-Brice of A Tiny Good Thing. Isabelle has been a go-to resource for 475 for years when it comes to understanding the overlap of high performance details and tiny house needs. Isabelle was recently featured on an episode of the Tiny House Lifestyle Podcast, where she breaks down which materials will help your house breathe properly, ventilate well, prevent mold, and just be a healthy tiny house to live in.