Accessory Dwelling Units in Historic Districts: A Denver Case Study

By Special Contributor posted 10-18-2018 15:39


By Jessica Hanlon

Accessory dwelling units (ADUs)—also known as carriage houses, granny flats, or even garage apartments—are an important and effective strategy for accommodating density without demolition. When thoughtfully sited, scaled, and designed, ADUs can be neatly nestled into the historical fabric of any neighborhood.

Cities with a high demand for housing—like Portland, Oregon, or Seattle—have seen a recent rise in ADU development. And just a few miles over the Canadian border, Vancouver has led the way: 35 percent of its single-family homes have an ADU on the lot. This shows the potential for other U.S. cities to embrace this form of development.

Façade of a home in historic Curtis Park with a matching ADU in the back left of the lot. | Credit: Photo courtesy of Jessica Hanlon

The Benefits of ADUs

ADUs serve a multitude of community functions, all while helping to stabilize neighborhood character. Their many benefits include:

  • Accommodating new residents;
  • Diverse, smaller, and affordable housing;
  • Supplemental income for primary dwelling owners through short- or long-term rentals;
  • Multigenerational housing; and
  • Aging in place.

Resident homeowners do sometimes voice concerns about issues such as increased congestion, parking requirements, and an influx of short-term renters, but policies promoting ADUs can be adapted to fit the needs of a particular city or neighborhood. In Denver, for instance, a single-unit zoned lot requires that the primary resident live on the property and have a license for short-term rentals. In Los Angeles, on the other hand, there are no requirements regarding owner occupancy. So long as policies reflect the attitudes and needs of residents, ADU development can thrive, revitalizing neighborhoods or even underserved communities by aiding social cohesion and increasing the sense of place.


An ADU-converted barn with reclaimed material and original shingles next to the property’s two-door garage. | Credit: Photo courtesy of Jessica Hanlon

Accommodating Growth in Denver

Denver is recognizing the promise of ADUs. Widely regarded as one of the nation’s most livable cities, Denver is projected to grow by 200,000 residents, or 21 percent, by 2040. As the population increases, the city and county will need to provide new housing opportunities—including in previously undeveloped areas as well as older neighborhoods and brownfields. If this rapid growth is not carefully managed with strategies such as infill development and building reuse, it could threaten the city’s character and history through demolition and displacement.

While Denver’s 2010 citywide rezoning effort allowed roughly 25,000 parcels to permit ADUs, only about 200 units have been constructed in the eight years since rezoning. Recently, however, the city has seen renewed efforts to advocate ADUs, most notably through Denveright’s Comprehensive Plan 2040. This citywide plan for Denver promotes a 20-year vision of healthy and complete neighborhoods through connected infrastructure and strategic investments. To provide a more diverse housing stock, both in terms of size and price, Denveright suggests zoning all residential neighborhoods for ADUs. Furthermore, it encourages streamlining the application process and reducing fees to promote construction and ensure that standards are suited to specific neighborhoods.

The West Denver Single Family Plus (WDSF+) pilot program also promotes ADUs in Denver, focusing primarily on strengthening and developing at-risk neighborhoods in West Denver. WDSF+ aims to prevent involuntary displacement and ensure affordable housing for existing residents through educational resources and ADU development assistance.


Alley view of a partially constructed and newly constructed ADU in historic Curtis Park. | Credit: Photo courtesy of Jessica Hanlon

ADUs in the Curtis Park Historic District

ADUs are typically sited to minimize visibility from the street so as not to disrupt community character in historic districts like Denver’s Curtis Park. Full of carriage houses from the late 19th and early 20th centuries, many of which could accommodate dwelling units or expansions, Curtis Park is well situated for ADUs. And, indeed, the Curtis Park Conservation Overlay District permits ADUs that are designed and sited appropriately for the district’s long, narrow lots—they may be up to two stories high on a typical 25-foot wide lot. This zoning gives homeowners more flexibility to develop their property than the broader Denver zoning code, which only allows property owners to build a hallway over the garage. Zoning codes that incentivize infill and allow residents more freedom with construction help communities grow.

Sue Glassmacher, chair of the Curtis Park Neighbors’ Design Review Committee (DRC) says, “We believe that ADUs provide a tool to preserve the historic district and keep it vibrant. It provides density and additional housing in the neighborhood. … Curtis Park Historic District is not a museum. It is a living district.”


View of an ADU from the alley, its dormer windows facing the street. | Credit: Photo courtesy of Jessica Hanlon

While creating ADUs in historic districts is quite similar to doing so in Denver’s other neighborhoods, the process does require a pre-application that is often referred to a Registered Neighborhood Organization before being approved by the Landmark Preservation Commission (LPC). The additional review and applications required for a historic district usually add only about six weeks to a construction schedule. Adding an ADU, from planning to construction, tends to take around a year and cost about $200,000. Additional costs for constructing an ADU within a historic district tend to result from permits and applications; otherwise ADUs fall within a similar price range regardless of district.

The LPC is most concerned about how structures complement the design of the neighborhood and contribute to the character of the streetscape. The most common issues it raises pertain to the massing, building material, and historic detailing on the elevations that face the street; the secondary elevations that face away from the street only need to complement the primary structure. In some cases, the roof treatment of an ADU’s primary elevation is reminiscent of the primary building, while its secondary elevations, which face the alley, may be flat to maximize interior space. This allows homeowners the flexibility to create more usable spaces while still blending with historical forms and traditions.

View of a primary structure with a matching ADU in the lot behind it. | Credit: Photo courtesy of Jessica Hanlon

Curtis Park’s DRC exists to help applicants with the LPC review process. Well versed in the specific neighborhood design characteristics, the DRC helps ensure the community’s cohesive aesthetic by providing suggestions to applicants and offering recommendations to the LPC, thus speeding up the process at the city level. The LPC is quite flexible and understanding when it comes to ADUs, often adding applications to a consent agenda that it votes on as a slate.

The successful integration of ADUs in Curtis Park can serve as a model for other historic districts and older neighborhoods. It is better to provide these communities with the tools for growth and stabilization than to scrape them clean and build new. ADUs allow historic districts to grow and adapt without disrupting their character or tradition. Density without demolition is healthier and more sustainable for a growing city like Denver.

Jessica Hanlon interned with the Research & Policy Lab team in Denver, CO this summer. She is currently finishing up her Environmental and Urban Studies degree at the University of Chicago.