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A private study commissioned by the Downtown Voices Coalition Homelessness & Affordability Subcommittee funded by the Arizona Preservation Foundation and Richard and Julia Moe Family Fund of the National Trust for Historic Preservation in conjunction with the Coronado Neighborhood Association




Phoenix, AZ is a young city with limited historic and historic-eligible housing. Most of these homes are in single-family neighborhoods and some have Accessory Dwelling Units (ADUs) built prior to current zoning laws. There is no mechanism to add affordable ADUs for a gentle density increase in historic neighborhoods without dramatic entitlement changes through rezoning. This proposal, under the auspices of the Downtown Voices Coalition Homelessness & Affordability Subcommittee and with the financial support of the National Trust for Historic Preservation and Arizona Preservation Foundation, will develop replicable strategies to allow gentle density on properties in historic areas without rezoning.


What are Accessory Dwelling Units (ADU)?


As per Fannie Mae, an ADU is an additional [secondary] living area independent of the primary dwelling that may have been added to, created within, or detached from a primary one-unit dwelling. The ADU must provide for living, sleeping, cooking, and bathroom facilities and be on the same parcel as the primary one-unit dwelling. The ADU must be subordinate in size to the primary residential dwelling. Selling Guide: Fannie Mae Single Family,” October 6, 2021. Section B2-3-04, Special Property Eligibility Considerations, December 16, 2020: Accessory Dwelling Units, pages 270-71


Types of ADUS


ADUs are known by many names: Guest houses, casitas, granny flats, guest cottages, carriage houses, garage or basement flats. ADUs can be converted from an existing structure such as a garage or attic, they can be attached to the main residence (extension), or they can be a stand-alone detached ADU. All ADUs must meet local zoning, historic preservation, and building code requirements, including setbacks.

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“The ABCs of ADUs: A guide to Accessory Dwelling Units and how they expand housing options for people of all ages,” AARP.


Why is the study needed?


Over the past ten years, in response to the growing need for additional and affordable housing, many cities have amended their zoning ordinances to allow ADUs (with cooking facilities) in single-family zoning districts. One of the concerns is that single-family zoning standards are being applied to historic districts as well, without research on the possible benefits or impacts those standards could pose to Historic Districts. 


Phoenix historic districts are some of the ‘youngest’ in the nation, with limited historic and historic- eligible housing, and mostly located in areas of urban growth and near light rail lines or future rail expansion. 


There are barriers (real or perceived) to adding ADUs on existing properties, such as: zoning and permitting requirements, cost, parking availability, loss of neighborhood stability and value, and inappropriate scale of ADUs. Without careful consideration and implementation of appropriate guidelines for historic properties and districts, allowing ADUs could open the door to potentially devastating speculative development. It is the intent of this proposal to demonstrate how gentle density can be achieved and benefit historic neighborhoods while addressing barriers and promoting affordability. 


Historic neighborhoods in Phoenix often have many ADUs, however most were not established legally. The Zoning Ordinance allows only one kitchen per single-family parcel, thereby excluding the opportunity for ADUs to add gentle density in historic preservation areas. Many properties have existing accessory buildings that could potentially be converted to ADUs. Other properties would need to build an ADU from the ground up.


In the 2020 Housing Phoenix Plan, the City of Phoenix included language to amend the zoning ordinance to allow for ADUs in targeted areas, but research and the adoption of an ADU amendment is scheduled only for the long term.  Therefore, this private study provides an ideal opportunity to research if there are specific ADU guidelines needed for historic districts, and to outline potential strategies that may provide guidance to the City of Phoenix and other Arizona municipalities. 


Study Area


The Coronado Historic District will be the study’s area of interest. Coronado is a large historic district with mostly single-family dwellings and some existing guest houses (ADUS), built prior to current zoning laws. As in many older and historic areas in Phoenix, Coronado is a mix of single-family (R1-6) and multifamily zoning districts (R-2, R-3, R-4).

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Community outreach will be one of the core elements of the study. The Coronado Historic District Accessory Dwelling Unit (ADU) Survey (link to survey upon approval) was developed so residents will have the opportunity to be part of the process by providing vital input, and by voicing their support, concerns, and suggestions regarding their historic district and ADUs.


Purpose of the Study


To develop strategies to designate historic or historic-eligible single-family residential properties as candidates for the addition of ADUs. Gentle density guidance in the form of accessory dwelling units (ADUs) provides flexible housing options for residents, their extended families and members of the community and allows addition of ADUs which are smaller than the primary unit on the property. Whether the ADUs are attached or detached from the main home, they present abundant opportunities for inclusion and affordability, as well as adding to the population base for neighborhood commerce. 


ADUs can facilitate aging in place by providing housing for caregivers, housing for adult children and other family members, and in addition, may provide a supplementary source of income (contributing to affordability and property maintenance). Community benefits include: preservation/stability of historic properties and neighborhood fabric or character, use of existing infrastructure and community services, and the increased community interaction that occurs with more people living in an area.




Research, inventory, community and stakeholder outreach, feedback/discussion, strategy formulation, mapping, illustrating, and report development is anticipated to take up to 12 months.


Anticipated Outcomes


  • Final Report outlining design guidelines and strategies to add gentle density to historic or historic-eligible single-family neighborhoods in a manner and scale that is appropriate to the historic district. Tools to preserve or achieve affordability will be identified. 

  • Recommendations for opportunities to achieve code compliance will be provided, and the feasibility of developing pre-approved building plans for ADUs will be investigated. 


Future Plans


The purpose of this grant-funded effort is to devise a toolkit for use in historic preservation areas in Phoenix and other Arizona cities and towns, which can be adjusted to the needs of individual communities, while also allowing and encouraging the addition of gentle density. As cities look to increase density, the needs of historic districts must be acknowledged and protected. This report will guide local jurisdictions so they can achieve gentle density in their historic preservation areas in a manner and scale appropriate to individual properties. 

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