ADUS (ACCESSORY DWELLING UNITS) IN DENVER
TABLE OF CONTENTS
1. Intro
2. Why Now?
3. Process and Timeline
4. History of ADUs
5. What is an ADU Today?
6.. ADU Concerns
7. ADU Benefi ts
7. Current ADU Regulations
8. Peer Cities
1. INTRO
What will this project include?
In this project, city planners will look at how the
Denver Zoning Code regulates ADUs (accessory
dwelling units). This project is not expected to change
where in the city ADUs are allowed but will look at
how they are designed, how they fi t in with diff erent
types of neighborhoods and block patterns, and how
updates to the zoning code may reduce barriers to
creating ADUs.
Why do this work?
As our city grows and changes, the way people live is
changing too. Many people want…
• a separate space where elderly parents or kids
living at home can still have independence,
• a space that can be rented out to generate
income,
• or just to rent a smaller, more aff ordable space
Creating an ADU can have a big impact helping
residents grow their home equity and earn new
income to remain comfortably in their homes longterm.
These smaller dwelling units are also a lowimpact
way for neighborhoods to expand their range
of housing choices and add some gentle density.
2. WHY NOW?
Denver’s population is rapidly growing and we need to
make it easier to provide a range of housing options,
and ADUs are part of the solution. ADUs are becoming
increasingly popular throughout Denver, as rezoning
requests suggest, and more and more folks are
interested in fi guring out ways to add housing to their
property. Whether it’s for distant in-laws to visit, as a
rental property, or an expanded family arrangement;
the need for accessible housing is more pressing than
ever. With the plan guidance of Blueprint Denver 2019,
many single-unit residential properties are now eligible
for a rezoning to allow ADUs on their property. But the
rezoning is only the fi rst hurdle. Once your property
is zoned for an ADU, you then must go through the
process of design, permitting, and construction. This
project is intended to make the processes as clear and
as cost eff ective as zoning will allow. The building form
standards for a Detached ADU are quite restrictive
and can impose many cost implications that make
for diffi cult design decisions in building an ADU. This
project will look at which restrictions can be lifted
or alleviated, while still creating outcomes that are
sensitive to the neighborhood context.
3. PROCESS AND TIMELINE
The ADUs in Denver project will remain relatively small
in scope, focusing on removing barriers to ADUs where
they are currently allowed. By focusing exclusively
on the built outcomes of ADUs, we are able quickly
implement and accomplish some of our blueprint
goals, with emphasis on a one-year timeline.
4. HISTORY OF ADUS
ADUs are not at all that new to Denver. They have
been around since the origins of Denver, in the form
of a ‘carriage house’. A carriage house was a separate
living quarters for the storage of horse drawn carriages
and for servants of the main house to live, some date
back as far as the1880’s. They were small scale separate
buildings at the rear of the property, usually accessed
from the alley, in order to easily pull in a carriage.
Servants of the main house would live above the
carriage storage, in a loft or a full second story. Many
of Denver’s more historic and affl uent neighborhoods
have existing carriage houses on their property and
over time many have been converted into separate
sell-able parcels. You can fi nd many historic carriage
homes in Denver’s older neighborhoods like Baker,
Capitol Hill, Curtis Park, City Park West, Congress Park,
Cole, Whittier, Speer, Country Club, West Washington
Park, and Platt Park.
What happened to the ADUs?
It’s diffi cult to pinpoint exactly when ADUs fell out of
favor in the eyes of Denver’s zoning. In 1925 Denver
adopted its fi rst zoning code, which only consisted of
3 districts: residential, hotel, and offi ce. The residential
district didn’t have any limitations to the number of
units that were allowed on a property. But in 1954, a
new zoning code introduced Single-Unit (SU) only
districts, and began to limit the number of dwelling
units on a property to just one in these districts and
began stipulating that ‘a family’ could only live in a
single dwelling.
The project will consist of an advisory committee made
up of a diverse group of community representatives
and knowledgeable ADU experts to help guide staff
in making decisions on zoning code updates to ADUs.
Focus groups will be established to tackle specifi c
issues such as suburban context ADUs and shortterm
rental concerns. Public meetings will invite a
larger discussion and help inform communities of the
changes proposed.
5. WHAT IS AN ADU TODAY?
Accessory Dwelling Units (ADUs) are secondary and
subordinate units to a primary dwelling unit. They are
secondary in their use classifi cation, as they must be
secondary to a ‘primary dwelling use’. They may not be
sold off as a separate property, unlike a duplex which is
a ‘two-unit dwelling use’ or two separate primary uses.
Currently, ADUs are only allowed with a single-unit
primary use, meaning that you cannot have an ADU
with a duplex or a row-home.
ADUs may be attached or detached. Detached
ADUs are the most thought of type of ADU. They
are a separate building at the rear of the lot which
usually provides living space above a garage, similar
to Denver’s historic carriage houses. Detached ADUs
are regulated by the ‘Detached ADU’ building form
standards and regulate building size by lot size. New
attached ADUs are often less common, but can be a
viable solution for smaller lot sizes or where existing
structures already fi ll their lots. Attached ADUs are
often found in basements with separate entries, or on
a second or third level with exterior stairways. They can
also be built as additions to the main house. See the
graphic at right for examples of each.
6. ADU CONCERNS
Many residents are worried that a nearby ADU could
impact their quality of life. Some of these concerns are
summarized below:
a. Short-term Rentals
• ADUs may be rented out as a short-term rental as
long as the owner of the ADU lives on the property
and maintains permanent residence there.
• Short-term rental income can help off set
construction costs of an ADU. But a shortterm
rental is sometimes seen as a negative by
neighbors.
• In our brief surveys of ADU owners, many of them
did short-term rent out their unit for a few years,
just after construction, but eventually became tired
of the requirements and turned their unit into a
long-term rental.
b. Safety Concerns
• Some neighbors fear that adding ADUs to their
alley will increase automobile traffi c and crime.
• In reality, the additional traffi c is minimal. Adding
ADUs, and the gentle density they bring to alleys
can increase their safety by adding more eyes
on the alley which helps to prevent crimes of
opportunity and deter loitering.
c. ADUs Without Alley Access
• Concerns about allowing ADUs on lots without
alley access are growing as ADU interest begins to
spread away from our urban center.
• Most of Denver’s urban neighborhoods have
alley access to the rear of the lot, but in our more
suburban neighborhoods, alleys are less common.
• Special consideration should be given to the
zoning regulations in this context, especially rear
setbacks (how close you can build to the rearlot
line of a property) which abut a neighboring
property.
d. Displacement
There is some concern that ADU construction will
increase property values and taxes throughout a
neighborhood, causing displacement, but offi cials in
Denver, Portland and San Diego note that ‘property
values depend on many factors and properties with
ADUs have little if no impact on the value of properties
without ADUs.’
e. Not Enough Parking
• Some residents are concerned about ADUs
exacerbating parking demand.
• Currently there is no off -street parking requirement
for single unit dwellings or ADUs.
f. Loss of Privacy
• Many neighbors fear the loss of privacy in their
backyards due to an ADU next door.
• The zoning code regulates side setbacks and a
‘bulk plane’ that help encourage windows to face
the alley and the primary structure to prevent
infringement on neighboring properties.
7. ADU BENEFITS
Many organizations have researched the benefi ts of
ADUs, some of which are provided below from AARP:
ADUs are able to house people of all ages
• An individual’s housing needs change over time,
and an ADU’s use can be adapted for diff erent
household types, income levels, employment
situations and stages of life.
• ADUs off er young people entry-level housing
choices.
• ADUs enable families to expand beyond their
primary home.
• ADUs provide empty nesters and others with
the option of moving into a smaller space while
renting out their larger house or letting an adult
child and his or her family reside in it.
ADUs are community compatible
• ADUs off er a way to include smaller, relatively
aff ordable homes in established neighborhoods
with minimal visual impact and without adding to
an area’s sprawl.
ADUs provide gentle density, a more dispersed and
incremental way of adding homes to a neighborhood
than other options, such as multi-story apartment
buildings. As a result, it’s often easier to get community
support for ADUs than for other housing types.
ADUs are just the right size
• Generally measuring between 600 and 1,000
square feet, ADUs work well for the one and
two-bedroom homes needed by today’s smaller,
childless households, which now account for nearly
two-thirds of all households in the United States.
ADUs are good for the environment
• ADUs require fewer resources to build and
maintain than full-sized homes.
• ADUs use signifi cantly less energy for heating and
cooling. (Of all the ADU types, internal ones tend to
have the lowest building and operating costs.)
8. CURRENT ADU ALLOWANCES
The Denver Zoning Code regulates where detached
and attached ADUs are allowed and includes building
form standards regarding height, minimum lot size
and depth, setbacks, parking, fl oor area, and structure
length. The zoning code also includes limitations
on ADUs that further defi ne the circumstances
under which they can be built. The regulations are
summarized below and should be re-evaluated to
consider more context sensitive standards that could
help reduce barriers to ADU construction.
Besides zoning requirements, ADUs must meet all building and
fi re code standards for new construction.
In order to rezone a property to allow an ADU, a
property owner must submit an application and pay
a minimum $1,000 review fee. Some property owners,
intimidated by the process, pay additional fees to hire a
consultant to administer the application for them. The
rezoning process takes 4-6 months and includes a staff
analysis of the rezoning’s consistency with zoning code
criteria and involves public hearings.
Zone DistrictsZone Districts
The zoning code defi nes numerous residential zones
specifying building forms that are allowed and
minimum lot sizes, which are categorized based on
their neighborhood context (urban, urban edge,
and suburban). Many of these residential districts,
particularly single-unit and two-unit districts, allow
urban and suburban house forms and have a
counterpart district that also allows ADUs. For example,
the U-SU-A district allows urban houses on a minimum
zone lot area of 3,000 square feet, while the U-SU-A1
district includes similar standards as U-SU-A but also
allows ADUs. A few row-house and multi-unit zone
districts also allow ADUs, however, an ADU is permitted
as ‘accessory only to a primary single unit dwelling’.
Additionally, no more than one ADU is allowed per
zone lot and ADUs are not permitted when there is
more than one primary structure on the same zone lot.
Height & Bulk Plane
Detached ADUs are limited to a maximum height
of 1.5 stories and 24’. A bulk plane that rises 10 feet
vertically from the side interior or side street zone lot
line, then slopes 45 degrees, also applies to detached
ADUs, which essentially requires a sloped roof form on
smaller lots. The height limit allows ADUs to be built
above garages while the bulk plane limits their mass
and the amount of shade they cast. However, the bulk
plane requirements can also limit the design of ADUs
and can increase the cost of their construction
Lot Size & Depth
The minimum lot size and depth for ADUs are defi ned
by the zone districts where they are allowed. The
Urban and Urban Edge zone districts that allow
detached ADUs range in size from 3,000 to 10,000
square feet and do not specify a minimum lot depth
required for detached ADUs. However, the S-SU-F1
zone district, which is the only Suburban district that
allows ADUs, requires a minimum zone lot area of
8,500 square feet and only allows detached ADUs on
lots that are at least 150’ deep.
Furthermore, ADUs are only allowed on lots that meet the minimum lot size required by the detached ADU building form standards that apply to a particular zone
district. This means that a property owner can’t build a detached ADU if their property is smaller than the minimum lot size required even if they are in a zone district that allows ADUs. For example, if a property is zoned U-SU-H1, which requires a minimum lot size of
10,000 square feet, and the lot is 9,500 square feet, a detached ADU is not allowed.
Setbacks
The zoning code requires detached ADUs be located
in the rear 35% of the zone lot depth with 3- to 5-foot
side setbacks, depending on lot width, and 5-foot rear
setbacks. Increasing the setbacks on larger lots and/
or lots without alleys could help create a more context
sensitive ADU. However, increasing the setbacks could
reduce the develop-able area for a detached ADU to
less than the maximum size allowed.
Off -street Parking Requirements
Similar to single unit dwellings that don’t require a
minimum of off -street parking spaces, ADUs also don’t
require parking. However, if an alley is present, the
zoning code requires any off -street parking for an ADU
to be located off the alley. Street access (a driveway)
to parking is only allowed if there is no alley present.
However, an additional driveway from the street,
separate from a driveway serving the main house, is
not allowed. The driveway shall be shared between the
dwelling units. Alleys are prominent in many Denver
neighborhoods and the majority of ADU rezonings
thus far approved have been on alleys. While parking
availability is often a concern with added density,
requiring off -street parking for ADUs can result in
additional costs and site constraints that inhibit ADU
construction. ADUs are also subject to rules regarding
the maximum number of cars allowed per property.
ADU Floor Area
The maximum fl oor area allowed for an ADU depends
on the size of the lot in addition to other standards.
Limiting the size of a detached ADU can help ensure it
is smaller than the main house and doesn’t overwhelm
or shadow neighboring houses, but it can also limit
the diversity of households that can live in an ADU.
Detached ADUs are also limited to a maximum
building length of 36 feet regardless of lot size.
Building Coverage
Building coverage per zone-lot is defi ned by the
primary building form standards and includes
all accessory structures. The smallest lots allow a
maximum 50% building coverage while most lots are
limited to 37.5% coverage. The detached ADU form
allows an exception from the 37.5% building coverage
standard allowing the lesser of 50% or 500 square feet
if a minimum of 80% of the street level gross fl oor area
of the detached ADU is used for vehicle storage and
there is a at least 15 feet of separation between the
ADU and the main house.
Owner-Occupancy Regulations
The zoning code requires owners of properties with
an ADU to maintain permanent residency in either
the primary or accessory dwelling unit. This prevents
property owners from renting out both units and can
help ensure a sense of ownership and maintenance,
but it can also be an infl exible constraint preventing
someone from making income from the property if
they move elsewhere or inherit the property while
living in a diff erent jurisdiction, for example.
Denver further requires a minimum of 200 square feet
of gross fl oor area per occupant in ADUs to prevent
overcrowding, which means 1-5 residents could
occupy an ADU, depending on its maximum allowed
size. While this may seem reasonable, Denver recently
amended the zoning code to increase the number
of people who could occupy a primary dwelling unit,
and now the city generally defi nes a Household as
any number of related persons; up to 5 adults of any
relation plus related children and domestic employees;
8 “handicapped” adults; or 8 adults 55+ years of age.
These numbers apply to all primary dwelling units,
including apartments, regardless of size. The perperson
square footage requirement for ADUs, vs.
primary residences, may unnecessarily restrict the
household types that can rent ADUs.
ADU and the main house.
9. PEER CITIES
California
A series of laws enacted over the past fi ve years have
systematically eliminated most explicit and implicit
legal barriers to ADU construction across the entire
state. Los Angeles is the epicenter of ADU production,
accounting for 11,500 of the 26,600 California ADUs
permitted from 2017-2019. The distribution of ADUs
are random across space, but fairly predictable across
time. Access to jobs, near employment centers and
transit options, is the most important predictor of ADU
production. If production continues at 2019 levels, one
in ten LA houses will have a modern ADU by 2026.
Some city characteristics, like job access and being in
the coastal region, help explain high ADU permitting
rates. But the majority of the variation across cities
cannot be easily explained. The diff erences may result
from city outreach eff orts, regulatory practices, or
be an artifact of city reporting practices. Alternately,
there could be strong “contagion” eff ects, with ADUs
catching on in some places a few years earlier than
others nearby.
Berkeley, California
Berkeley was one of the fi rst cities to pass supportive
ADU legislation, but over the years very little
construction resulted. To help spur ADU construction
locally, a Berkeley City Council Member established an
advisory ADU Task Force. The Task Force membership
includes local community members who are also
realtors, architects, planners, developers, mortgage
specialists, and ADU advocates. It has been working
collaboratively with the broader Berkeley community
and City Council to advance ADU policy and
construction in the city. Berkeley’s fi rst revisions to
its ADU ordinance were adopted in 2017, and, in
response to local advocacy eff orts, the City updated
its ordinance again in 2018. The amended ordinance
increased maximum ADU sizes from 750 to 850 sq.
ft., and eliminated the parking requirement. This
is because the 2016 state laws prohibit parking
requirements for ADUs within half a mile of public
transit, and all Berkeley homes are located within
others nearby.
half a mile of transit. Berkeley also allows 4’ side and
rear setbacks for ADUs, which is smaller than most
jurisdictions across the state. In addition, the City does
not have minimum lot size requirements and allows
ADUs on single family parcels in all zoning districts
with few exceptions. To clarify the development
process for applicants, the Berkeley Planning
Department posted ADU guidance documents on-line,
including a fl ow chart, table of development standards,
and responses to frequently asked questions.
San Diego, California
San Diego stands out for its eff orts to educate
homeowners and assist with fi nance, particularly
for low-income residents. Some of the important
features of its ADU (Companion Unit) program are
eliminating the owner occupancy requirement,
relaxing parking requirements, and expanding the
zones where ADUs are permitted. Additionally, the
City allows large maximum ADU sizes (up to 1,200
square feet), construction of Junior ADUs, and has
eliminated development impact fees. The City has
also conducted extensive community outreach to
describe the regulations and ADU development
process to homeowners and posted several on-line
resources for the public. These include a helpful 42page
ADU handbook and a fact sheet with information
about ADU development standards and responses to
frequently asked questions. Another important feature
is that San Diego has established subsidy programs to
assist homeowners with ADU development. During
the 2018-2019 fi scal year, the City set aside a $300,000
fund to help homeowners cover water and sewer fees.
The City has increased the amount of available funding
to $800,000 for the 2019-2020 fi scal year. In addition,
the City’s Housing Commission has launched a pilot
program to build 40 ADUs of varying sizes and designs
for low income tenants. The ADUs are being built
adjacent to single-family dwellings on land that the
Commission owns. The purpose of this program is to
identify costs, development timelines, the construction
process, and potential hurdles in advance of launching
their loan program in Spring 2020 to help low income
homeowners build ADUs on their property
Seattle, Washington
In 2019 City Council voted unanimously to enact
legislation that will make it easier for homeowners to
build in-law suites, garage apartments, and backyard
cottages or ADUs. The adopted changes amount to the
most progressive ADU policy in the US. By boosting
the addition of ADU homes in the three-quarters of
Seattle’s residential land reserved for expensive, singledetached
houses. Two ingredients set Seattle’s policy
apart: an allowance for not just one but two ADUs
per lot, and a size limit on new houses. The only other
major North American city that permits two ADUs in
single-dwelling zones is Vancouver, BC. This fl exibility
maximizes opportunities for smaller, less expensive
homes in neighborhoods otherwise fi lled with big,
stand-alone houses. The legislation’s house size limit
of 2,500 square feet on a typical lot will make it more
attractive to add ADUs to existing houses instead
of tearing those houses down. When teardowns
do inevitably occur, the size cap will provide a new
incentive to build ADUs along with the new house.
Seattle also got rid of their two biggest ADU barriers: parking quotas and the requirement for property owners to live on site. For cottages, the legislation bumps up allowed size, height, and backyard coverage,
making it easier to build family-friendly homes and lowers the minimum lot size to 3,200 square feet.
To avoid discriminating against non-traditional
family arrangements, it raises the limit on unrelated
occupants per lot from eight to twelve.
The council rejected an amendment that would
require a year of property ownership before granting
permit. Council also refrained from enacting a
proposed outright ban on using ADUs for shortterm
rentals. Seattle already caps all short-term
rentals at two per owner. Fully banning short term
rentals in ADUs would quash fl exibility and dissuade
homeowners from building them.
Portland, Oregon
Portland’s residential districts do not have alleys.
Although Portland has had an ADU program in place
for several years, ADU development was not eff ectively
promoted until 1998, when the city amended its
laws to relax the regulations governing ADUs. The
amendments eliminated the minimum square
footage and owner-occupancy requirements. ADUs
are now allowed in all residential zones with relaxed
development standards.
Portland’s regulations permit the construction of ADUs
on lots with a single-family home, as long as they
are smaller, supplementary to the primary residence,
and no more than 800 square feet. They can be
created by conversion of an existing structure or by
construction of a new building. An early assistance
process is available to help with project development
for ADUs created through the conversion of an
existing structure. ADUs that meet all the standards
are permitted by right and do not require a land use
review. No additional parking is required for accessory
units. Portland’s ADU program guide outlines ways to
bring existing nonconforming units into compliance.
The city considers ADUs to be more aff ordable than
other housing types because of the effi ciency of the units in using fewer resources and reducing housing costs. City planner Mark Bello notes that allowing more ADUs did increase the housing supply, and that city residents viewed ADUs positively and were satisfi ed with the changes made. He also added, “There were no
signifi cant negative issues that arose from liberalizing
Portland’s code.”
homeowners from building them.