The biggest move in climate action might not be a wind farm or a solar array. It might be a duplex.
That’s the takeaway from a sweeping new report commissioned by the Colorado Energy Office, which finds that legalizing infill housing—homes built in already-developed areas, near jobs and transit—could drastically cut greenhouse gas emissions while helping to solve the state’s growing housing crisis.
The Colorado Land Use Policy & Greenhouse Gas Co-benefits Study, released in late 2024, models how changing zoning laws can reduce emissions from both buildings and cars. The results are striking: compact, transit-oriented development doesn’t just make housing more affordable—it’s a climate solution hiding in plain sight.
The study modeled three land use scenarios to evaluate how zoning reforms might shape future housing growth through 2050. The baseline scenario assumed no changes. Scenario A included reforms like legalizing accessory dwelling units (ADUs) and promoting transit-oriented development (TOD). Scenario B went further, adding multi-family housing along commercial corridors, middle housing (such as fourplexes), and parking reform.
Under Scenario B, nearly 80% of new housing would be built in already-developed areas—a dramatic shift from the baseline’s 48%. Housing near transit stations would more than double, and compact housing types like apartments and townhomes would dominate the new housing stock.
This shift has major climate implications:
- Building emissions fall by 31%
- Transportation emissions fall by 15%
Perhaps most revealing: parking reform—eliminating costly and unnecessary minimum parking mandates—turned out to be the most effective single policy, enabling two to three times more new housing than ADU or TOD reforms alone.
“Colorado is facing a large housing supply deficit today and anticipating significant future growth,” said Will Toor, Executive Director of the Colorado Energy Office. “By enacting policies that encourage growth near transit and in infill areas, the state can significantly lower the emissions of new development compared to the status quo.”
Those words reflect a growing national recognition: housing policy is climate policy. Suburban sprawl—with its long commutes, large homes, and infrastructure demands—is a massive driver of emissions. But by allowing more homes to be built where people already live and work, emissions can be dramatically curbed.
Colorado isn’t just studying the issue—it’s acting on it. In 2024, the state passed three key land use bills that reflect the study’s priorities:
- HB24-1152: Legalizes ADUs statewide
- HB24-1313: Promotes transit-oriented communities
- HB24-1304: Ends minimum parking requirements
Together, these bills begin to chip away at the single-family-only zoning laws and car-centric development patterns that have locked many cities into high-emissions growth.
But advocates say Colorado must go further to achieve the full vision of Scenario B.
California YIMBY, a pro-housing group, praised the study, calling it a model for the rest of the country. “The report underscores what we’ve been saying for years: legalizing infill housing is one of the most powerful tools we have to fight climate change,” the group wrote in a recent blog post.
Indeed, Colorado’s approach mirrors similar reforms gaining traction in California, Oregon, and Washington. But where many states frame these moves around affordability, Colorado’s study offers hard data linking housing location and form to emissions outcomes—making the climate case explicit.
While legalizing housing is critical, implementation matters too. Infrastructure, local planning capacity, and transit investment will determine whether the promise of infill zoning is realized.
Still, the message is clear: climate action doesn’t just happen at summits in Paris or Washington. It happens in city council meetings, zoning boards, and statehouses.
The duplex—or fourplex—might just be the unsung hero of the climate movement. All we have to do is legalize it.
