Making it easier for homeowners to add mother-in-law suites, basement apartments and backyard cottages is not just a good idea for housing choices and affordability — it’s also green. Tucking these modest homes into existing neighborhoods helps prevent sprawl, cuts traffic and commutes, and fights climate change.

On Monday, the Seattle City Council has a chance to vote on new rules making it easier to build accessory dwelling units (ADUs) in single-family neighborhoods. As champions for a sustainable city and region, we strongly support the key reforms in Councilmember Mike O’Brien’s proposal: Allow two ADUs per lot, don’t require parking, don’t force the owner to live on site, raise the limit on residents per lot from 8 to 12, and cap the size of new houses at 2,500 square feet.

When it comes to energy use and the climate pollution it creates, smaller dwellings are better. And ADUs are small — on average, a third the size of Seattle’s newly built detached houses. This means an ADU home consumes about half the energy of the typical new house. Over a home’s 70-year life span, that could reduce carbon dioxide emissions as much as taking nearly 100 cars off the road for a year.

Even more important than ADU efficiency, though, are the far-reaching environmental benefits. Adding homes to existing communities helps rein in our region’s single biggest global warming culprit: the tailpipe. Studies of cities across the globe consistently show that as population density rises, driving declines.

Seattle’s rapid growth has come almost entirely in the form of apartments. Meanwhile, on the city’s residential land where apartments are banned, population has flatlined. Relaxing restrictions on ADUs will let these communities do their part to help Seattle cut its carbon emissions. The Sightline Institute estimated that if a third of the houses in typical Seattle neighborhoods added an ADU, the average household would drive 1,000 fewer miles per year.

It’s a virtuous circle. When a neighborhood welcomes more residents, local businesses gain new, more loyal customers. More people can meet daily needs by walking or biking. Population-rich neighborhoods also support better transit service, making it even easier for people to choose not to drive. Newcomers and long-timers alike lighten their carbon footprints.


What’s more, the green benefits of more ADUs in Seattle spread beyond the city. Making room for more people in Seattle reduces pressure to build new homes on the urban fringe — the kind of sprawling expansion that paves over farms and forests and destroys salmon spawning habitat. Sprawl also stokes a vicious circle, forcing more and more of the region into car-dependence and locking in excessive climate pollution for decades.

Make practical changes to backyard-cottage rules to preserve Seattle neighborhoods

ADUs particularly expand opportunity for working families to live in neighborhoods from which they would otherwise be shut out because they can’t afford a stand-alone house in a city where the median home price has ballooned beyond $700,000. These are the folks who make our city tick — the teachers and service workers who, when pushed outside Seattle to find cheaper housing, end up trapped in long, miserable, carbon-spewing car commutes.

That’s why Washington’s Growth Management Act first mandated that larger cities permit ADUs a quarter century ago. The problem is, most cities — just like Seattle — imposed a slew of restrictions that have held back ADU construction to a trickle.

By loosening rules for backyard cottages, granny flats and garage apartments, Seattle will advance equitable access to housing and move closer to its climate goals — an overwhelmingly urgent task that we’re currently failing on and one that also has profound ties to equity, since low income and communities of color are being impacted first and worst. ADUs are a key ingredient to a greener and fairer Seattle. It’s way past time to let them grow.