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ADU Update

ADU Update

Accessory Dwelling Unit Update

The city launched an update to the Accessory Dwelling Unit (ADU) regulations with two open houses, one in north Boulder and one in south Boulder, to solicit community input prior to crafting a proposal.

Open Houses

South Boulder - Nov. 27, 2017

Location: St Paul's United Methodist Church
4215 Grinnell Ave, Boulder, CO 80305
Date: Monday, Nov. 27 - COMPLETED - View Comments From Event pdf
Time: 5:00 – 7:00 p.m. 
(drop in any time, an optional presentation will start promptly at 6pm) 

North Boulder - Dec. 11, 2017

Location: Shinning Mountain Waldorf School
1179 Union Ave., Boulder, CO 80304 
Date: Monday, Dec. 11 -  COMPLETED  - View Comments From Event pdf
Time: 5:00 – 7:00 p.m. 
(drop in any time, an optional presentation will start promptly at 6pm) 


Feedback from the Public 

What we've heard during the open houses:

What we've heard online through the Share your ADU experience online form:

How does the ADU Update process relate to the nine-step decision-making process adopted by council on Nov. 21 as part of the Engagement Strategic Framework? View memo here pdf.

City staff is working on one additional document: an explanation of how community comments, by theme, will be used to inform next steps. This will be posted as soon as completed.

Did you miss the open houses? Share your thoughts and ADU experiences with us here and learn more about the project online at Materials available at the open houses will also be available online.  

DRAFT Why Statement 

Increasing the diversity of housing opportunities is a city goal and priority as identified in the recent Boulder Valley Comprehensive Plan update, discussed during a 2012 Study of ADUs, and as part of community discussions regarding housing over the past decade or more. The current regulations regarding accessory dwelling units have resulted in a relatively small number of legal accessory dwelling units (205 as of January 2017) being constructed since the first ordinance was adopted in 1983.

DRAFT Purpose Statement   

The city, with the community, will craft a proposal for incremental changes to the relevant regulations addressing accessory dwelling units (i.e., Accessory Dwelling Units (ADUs) and Owner’s Accessory Units (OAUs)) to simplify the regulations and remove apparent barriers to the construction of this housing type in ways that are compatible with neighborhoods.

The update is intended to achieve the following:

  • Provide additional flexibility to homeowners to stay in their homes by allowing for options that may either create supplemental revenue sources or allow for aging in place on the property.
  • Increase workforce and long-term rental housing opportunities while addressing potential impacts to existing neighborhoods.

Proposed List of Focused Code Changes

The following options were identified based on the issues identified from the 2012 Accessory Dwelling Unit Study, specific cases brought before the Board of Zoning Adjustments, preliminary guidance from City Council and staff experience in working with property owners who would like to add accessory units. It is important to note that additional analysis will be required to understand the potential impacts of these changes prior to any formal proposal to change the regulations.  

1. Simplify Rules

A few minor amendments would simplify the current regulations to make implementation easier for both staff and the applicant. These amendments may not create a significant number of additional units, but would remove some regulatory barriers and create a more predictable and streamlined process for building an accessory unit.


a) Remove the five-year requirement. Currently, a home must be at least 5 years old to create an accessory unit. The original intent was to help existing property owners remain in their homes who may have challenges meeting mortgage and other costs. However, it is not clear today this is the only policy objective. In recent years, council members have discussed the desire to provide more flexibility for additional housing opportunities in general.

b)   Automatically transfer an accessory unit permit when a home changes ownership. All units are required to obtain an accessory unit permit. There are specific code requirements to obtain a permit and the number of permits that may be issued for a given area is limited based on the concentration rule (see #2 below). The permit may be transferred from one property owner to the next, but only if there is not currently someone on the waiting list for the area. The intent was to provide fairness for other property owners, that may be limited by the concentration rule, to have an ADU when a property changes hands. However, the current permit system is confusing to many in the community and creates a burden on current ADU owners and future homebuyers if they do not transfer the permit within 30 days as it requires the removal of the kitchen or the separation between the house and the accessory unit.

c) Remove the neighborhood notice requirement. Currently, directly adjacent property owners are notified of an accessory unit application by mail and a notice is posted on the site. The original intent of this requirement was to create transparency in the regulatory process by providing neighbors with notice of creation of an additional legal unit in zoning districts where duplexes are otherwise prohibited. Although notice is a good idea in theory, it tends to create expectations on the part of neighbors that they can influence the review process or stop it entirely. Accessory units are reviewed through an administrative process. If the proposal meets specific objective criteria, the application is approved.

2. Concentration

No more than ten percent of properties within a 300-foot radius of the applicant’s property may have an accessory dwelling unit in Residential – Low zone districts. In Residential – Estate and Residential – Rural zone districts this provision applies to properties within a 600-foot radius of the applicant’s property. An additional barrier is the requirement to include existing nonconforming structures in calculating the ten percent limitation factor. These are typically duplexes, but sometime include apartments. The intent of these provisions was to prevent an overabundance of non-single-family units in predominantly single-family neighborhoods. This provision is unique among accessory unit ordinances across the nation. As of January 2017, there were three people on the waiting list in three different neighborhood areas. It is difficult to estimate how many people consider building an accessory unit, but don’t pursue it because others already exist in their neighborhood, and the neighborhood area is at maximum saturation.


a) Increase the saturation rule to 20 percent. This would be a relatively small incremental change and would reduce the barrier for existing homeowners who have been prevented from adding an accessory unit.

b) Remove nonconforming structures in the saturation rule. Duplexes are different from a principal dwelling unit with an accessory unit as they are two self-contained units often of equal size. The neighborhoods with grandfathered duplexes, such as the Hill, are constrained from creating accessory units by counting nonconforming uses in the neighborhood area. The original rational for counting these structures within the neighborhood areas is because it was thought that they would have similar or greater impacts than an ADU.

c) Create different saturation rates based upon geographic areas of the city. Given that some neighborhoods have been downzoned over the years, they have a greater degree of non-conforming uses and the impacts related to these non-conforming uses. It is possible to consider greater saturation rates based on existing neighborhood characteristics.

3. Size

The size of an ADU is limited to either 1,000 square feet or 1/3 of the size of the principal dwelling unit (whichever is less). The original intent was to ensure that the accessory unit is smaller in size and therefore subordinate to the main home. This requirement presents challenges for people with smaller homes. The size restrictions in other cases can also lead to unnecessary remodel work such as walling off portions of a basement to meet the size regulations for ADUs. Additionally, there is a 450-square foot limitation for detached OAUs which is a barrier since existing accessory structures are often larger. Additionally, if the OAU is in a detached accessory structure, the building must have less than 500 square feet of building coverage. OAUs are also subject to design requirements for garage doors and roof pitches. OAU’s can go up to 25 feet in districts where accessory building height limits are 20 feet. These design requirements for OAU’s are intended to help the buildings be compatible with the existing character of the neighborhoods. The intent of these regulations was to create accessory buildings that look like traditional carriage houses in predominately older neighborhoods like Whittier. However, these restrictions on accessory unit size and design may no longer be necessary as they were put in place prior to the adoption of a suite of development regulations commonly known as Compatible Development. These regulations, found in sections 9-7, "Form and Bulk Standards" and 9-8, "Intensity Standards", B.R.C. 1981, include Side Yard Wall Articulation, Side Yard Bulk Plane, Building Coverage and Floor-Area Ratio requirements. These zoning requirements create regulations that ensure that the size, height and building design of additional development on a property is limited and sensitive to neighboring properties.


a) Change the maximum size of an ADU or OAU located within the principal structure to 1,000 sq. ft. or 50 percent of the principal dwelling unit, whichever is less. This minor change retains the intent to ensure the accessory unit is smaller and subordinate to the principal unit. Increasing the size limit from 1/3 to 1/2 of the principal unit provides additional flexibility, particularly for smaller homes.

b) Increase the maximum size of a detached OAU. Similar to a), this minor change retains the original intent to ensure the OAU is smaller and subordinate to the principal unit. Increasing the maximum size and removing the limit on building coverage provides more flexibility to build accessory units, while the Compatible Development standards ensure that the size, height and building design is limited and sensitive to neighboring properties.

c) Review design requirements such as single-car garage doors and specified roof pitches for detached OAUs.

d) Remove the five percent expansion limit for attached ADUs. Currently, expansion of the primary structure’s existing foundation area is limited to no more than five percent. Similar to a) and b), this minor change retains the original intent of the regulation while providing additional flexibility.

4. Parking

Parking has been cited as one of the obstacles to creating an accessory unit. Some properties cannot easily accommodate an additional off-street parking spot, while other areas are more conducive. The parking standard is generally to provide one additional off-street parking space for ADUs and OAUs in addition to the standard parking requirement for that property, while a minimum of three off-street parking spaces are required for LAUs. In line, tandem parking is not counted as two or more spaces. The original intent of the parking requirement was to reduce the potential impact of additional cars parked in the street as a result of the accessory unit. Few other cities require off-street parking. Furthermore, establishing an accessory unit on a property does not increase the occupancy limits for a property. In other words, the parking needs for the parcel are comparable to a home without an accessory unit. If there is a parking impact, it is minimal, given the tradeoff of providing more smaller units in the city.


Remove the requirement for additional parking in specific areas of the city or entirely.

5. Location-specific Implementation

Some zoning districts or areas of Boulder may have features (e.g., larger lot sizes, alley ways, annexations, etc.) that better position them for targeted expansion of accessory housing. Staff could explore location-specific rather than citywide implementation of an ordinance to further enable ADUs.


Explore location-specific implementation.

6. Short-term rentals

Short term rentals have become a more prevalent option for renting in recent years. In many instances, short-term rentals (e.g., AirBnB, VRBO, etc.) provide significantly more revenue for a homeowner than renting to a long-term tenant. Short-term rentals are counter to the city’s desire to provide affordable, work force rental opportunities in the community.


Permit only long-term Rentals in ADUs and OAUs.

7. Lot Size

A single-family home must be on a lot of 6,000 square foot or more to construct an ADU or OAU. This was intended to ensure that only lots meeting the minimum lots size of the typical zone can build accessory units because smaller lots (considered substandard) may not be appropriate to accommodate an additional complete housekeeping unit. However, it may be more appropriate to regulate the size and presence of an ADU or OAU based on the size of the existing home rather than the lot size.


Remove or reduce the lot size requirement.

8. Allowed Zones

Currently, OAUs are allowed in the RR, RE and RMX-1 zoning districts and ADUs are allowed in the RL-1, RL-2, RE, RR-1, RR-2, A or P zoning districts. Specifically, OAUs are not allowed in the RL-1 zoning district, which is one of the most prevalent zones in the city. These areas of the city are typically developed post World War II and do not have alley access and most garages are attached to the primary structure. However, detached garages do exist in these zones and there may be additional opportunities to create OAUs that are compatible with the existing neighborhood character.


Allow ADUs and OAUs in the same zones.

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