Gentle Density, Big Impact: How Multi-Unit ADUs Are Shaping Urban Housing

ADUs—accessory dwelling units—are on a roll, and for a few reasons. Legislation in multiple states has smoothed the permitting process for the “gentle density” ADU-driven urban infill creates. High housing costs are motivating local governments to not just allow, but to promote ADUs (a.k.a. granny flats, in-law apartments, cottages) as smart ways to boost housing stock and temper prices while bringing in tax revenue from landlords and new residents alike.

The governmental embrace increasingly extends to multiple ADUs built on land parcels shared by condominium complexes and apartment buildings. At the same time, changing generational tastes have for years been boosting interest in living in walkable, vibrant, higher-density communities. Sustainability is also a factor in the increased ADU interest by buyers, renters, and governments—and, ultimately, for builders.

There are various facets to ADU sustainability. One that gets less attention than big-picture environmental sustainability has to do with aesthetics. ADUs provide low-profile, distributed housing, quietly preserving a street’s character and avoiding the ire of neighborhood associations irked by the prospect of ungainly multifamily buildings or slot houses, both of which can also run into permitting problems.

ADUs Are Built Tight and Run Lean

For buyers, the key sustainability features mainly have to do with efficiency of the finished product. ADUs typically boast tankless water heaters as well as highly efficient mini-split HVAC systems that handle both heating and cooling. Appliances tend to be both right-sized and Energy Star rated. Taken together, a well-built ADU is built tight and runs lean. That translates into lower emissions and utility bills.

Also, modern ADUs use electricity for heating, cooling, cooking, and heating water. Electrical HVAC systems and appliances can be powered renewably, and ADU roofs are typically designed to accommodate solar.

Freestanding ADUs also tend to be more efficient than the single-family homes they augment. ADU building envelopes typically include above-code insulation and weatherizing with such features as Zip System wall sheathing, OSB with rubberized membranes, and ultra-high-efficiency doors and windows. They comply with strict building-code standards such as California’s Title 24 Building Efficiency Standards, which, among other requirements, demand that additions and new homes—including ADUs—include enough solar panels to achieve net-zero energy consumption. That makes going with an ADU typically easier than retrofitting or adding to an existing structure.

ADU Sustainability Starts in the Factory

The energy and resource efficiency associated with prefab construction techniques also apply to ADUs. High-volume, factory-based production based on standardized inputs such as Forest Stewardship Council-certified dimensional lumber enables the precise matching of input materials to finished outputs. In fact, the waste from the on-site build of a modular ADU is measured in garbage cans, not dumpsters. That same precision and scale also enables the efficient recycling of metals and the composting of what little wood waste the build generates.

Modular ADU efficiencies extend to shipping and the speed of onsite construction. ADU makers typically ship their products palletized on a single truck. Once the pallets arrive, crews can wrap up construction in weeks rather than the months it takes for traditional stick builds, and that includes site prep and utility connections. With multi-unit ADUs, crews’ experience with standard plans and assembly approaches further improves efficiency. Substantially fewer worker commutes to the jobsite cut down on the embodied emissions of the finished product even as shorter construction timelines enable more projects and open doors to predictable, quick ADU builds as gap work between bigger jobs.

Gentle Density Delivers Long-Term Sustainability

Bigger picture, though, perhaps the greatest sustainability impacts of one-off or multi-unit ADUs hark back to gentle density. Green buildings are about more than building green, and builders should keep that in mind as they consider new projects and markets.

Development on greenfield urban fringes consumes open space, paves over wildlife habitat, and increases air and water pollution. Particularly in the western United States, pushing development deeper into the wildland urban interface brings greater risk of mass destruction from wildfires and higher insurance premiums. The perceived abundance of exurban space leads to bigger dwellings and greater direct resource use and long-term operational costs. New communities need new public infrastructure to be built and maintained. Great distances to job hubs bake in long commutes.

Given all that, it’s no surprise that one study found that doubling urban density by population reduces emissions from household travel by 48% and residential energy use by 35%. Those are big numbers, and all the more so if you consider that household travel and residential energy use account for about 42% of total U.S. carbon emissions.

Individual and multi-unit ADUs close to urban centers mean less need for cars and more opportunities for public transit. Car trips give way to bike jaunts and foot traffic. It means more people living closer to jobs, schools, shopping, restaurants, and urban cultural amenities such as museums, concert venues, and sports stadiums. They are, from economic, political, quality-of-life, and—not least—sustainability perspectives, a gentler way to boost density.

Prefab ADU construction at scale is sustainable across many dimensions and time horizons. Builders should follow governments’ lead in considering ADUs as key elements in solving housing challenges in ways that are at once more livable and more sustainable.