So you think you want to build an ADU in Denver? Part 2 – A deeper look at the Denver Zoning Standards & Requirements


In our prior article, we introduced the discussion and interest in ADUs (Accessory Dwelling Units) within the City of Denver. We started with basic zoning elements that first need to be checked to determine if your property is eligible to construct an ADU. These included:

  • Zone Lot District
  • Lot Size
  • Lot Coverage

Now that we have hopefully verified an ADU is allowed for your property, there are additional requirements that must be considered as we continue to explore the feasibility of an ADU addition. These zoning standards will apply:

  • Location on the lot
  • Building footprint
  • Overall structure length
  • Habitable space (1 ½ story)
  • 2nd story/rooftop decks


The proposed ADU must be positioned in the rear 35% of the eligible zone lot. Subtle consideration is that you will most likely need to maintain 15 ft of separation between the primary house and the proposed accessory structure in order to utilize the 50% exemption for the area of the footprint counting towards the maximum allowed lot coverage. In conjunction with the 15 ft separation, at least 80% of the street level Gross Floor Area (GFA) of the garage portion of the structure shall be used for vehicle parking.

Additionally, the City of Denver issued a clarification last year regarding another requirement for location of the ADU. The clarification explains that the mass of a proposed ADU shall be sited such that it abuts the southernmost property setback line. “If exceeding one story or 17’ shall be located adjoining the southernmost side setback line.” The reason is to preserve backyard privacy and access to sunlight for the adjoining neighbor to the north.

Building Footprint

Depending on the size of the zone lot, the zoning requirements indicate the maximum footprint that will be permitted. A few examples include:

  • If you are developing an ADU project on a property zoned U-SU-C1, which is less than 6,000 square feet (SF), then the maximum allowed size of the ADU footprint is 650 SF.
  • If the lot is between 6,000 SF and 7,000 SF then you are permitted an 864 SF building footprint.
  • Finally, for a zoned lot larger than 7,000 SF, the ADU footprint ultimately cannot exceed 1,000 SF.

There are some zone districts (e.g., U-TU-B) that outline the maximum building footprint to be 1,000 SF regardless of the lot size. Keep in mind as well that other zoning considerations such as permitted lot coverage may influence the ultimate size of the ADU building footprint.

Length of Structure

The uninterrupted length of a building structure cannot be any greater than 36 ft. If you consider that you might be building an ADU on a 6,250 SF lot, which is fairly typical, then the building dimensions could be 36 ft across the width of the lot, and potentially 24 ft deep for a total building area of 864 SF. The intent with this constraint is to prevent buildings from having large blank uninterrupted, un-articulated wall planes. Visually this will promote a more aesthetically pleasing building form.

The depth of the ADU could be 36 ft in length in the long direction of the property, and 27 ft wide for a 1,000 SF building footprint. Most projects ArcWest has had the opportunity to work on, the rectangular shape of the ADU has typically aligned the long face of the ADU with the width of the property. Every property and design are unique.

Maximum Floor Area and Habitable Space

The ADU building allows for the construction of a 1.5-story structure. The constraint within this definition limits the amount of habitable space of the living unit on the second level to 75% of the gross floor area or building coverage below. Per Chapter 13 of the Denver Zoning Code, “For the Accessory Dwelling Unit building form, a half story is calculated based on the Gross Floor Area of the floor below.” This can get a little confusing. Using the example of an 864 SF building area described earlier, the maximum floor area of the dwelling unit above cannot be any larger than 648 SF.

Habitable space is defined as living area with a clear ceiling height greater than 4 ft minimum, but where more than half of the space is 7 ft or greater. The City of Denver issued another clarification regarding this: “A story that has at least 4 feet between the ground level and the ceiling joists and which has enough area to provide a room with net floor-to-ceiling distance of 7 feet over half the area of 3 the room. A “habitable space” may or may not constitute a habitable room.”

Read the city’s clarification published June of 2018

Another way to look at this, in a structure with a sloped gable roof similar to the traditional 1.5-story Victorians that are characteristic throughout the city, there are potential floor areas below the 4-foot mark that do not count towards the maximum floor area.

2nd Story / Rooftop Decks

One of the questions we often field when first meeting with clients centers on the desire to have a rooftop deck. As we talk about building areas, they often ask, “Can we have a roof deck in that 25% area of the upper floor that is not considered part of the living / dwelling space?”  Per Denver Zoning Section, “…rooftop or 2nd story decks are simply not permitted in the rear 35% of the zoned lot.” The intent with this restriction is once again in consideration of being a good neighbor. The goal is to preserve the sense of privacy between neighbors. If you think about being the neighbor trying to enjoy a quiet summer evening in your backyard, adjacent to a roof top deck occupied with your neighbors doing the same, you may not feel as much solitude as they look over and down into your “private” oasis. Now, as with many rules there are exceptions. There are one or two districts (e.g., General Urban “G” Zone District) that do permit rooftop decks in the rear 35% of the property.

What’s Next?

Now that we’ve covered these topics, the next step in a potential project is to consider the ADU building architecture that will provide you with the living and utility spaces you have been dreaming of, while at the same time satisfying Denver zoning requirements.

We encourage feedback and questions on the zoning considerations outlined and discussed in this article.

Additional Resources

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