Gentle Density Study

Replicable Strategies to Allow Gentle Density on Properties in Historic Areas without Rezoning



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.


To develop strategies to designate historic or historic-eligible single-family residential properties as candidates for the addition of gentle density. 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.

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. 

One historic neighborhood will be selected as the pilot for this proposal. An inventory of the neighborhood will be conducted to:

  • determine and map location of existing ADUs; 

  • identify and illustrate parcel characteristics (e.g., parcel size, lot coverage, building setbacks, building height, parking, and access patterns); and 

  • identify site design criteria [e.g., parking availability, available space for ADUs (attached or detached), and alley adjacency.

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.



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


A report outlining design guidelines and strategies to add gentle density to historic or historic-eligible single family neighborhoods. The report will document and map every property in the pilot neighborhood. Each property will be evaluated against design criteria established as a result of this grant-funded research and ranked for ADU feasibility. 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 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. 


  • Dec. 2020 – City of Phoenix "Housing Phoenix Plan"

  • July 30, 2021 – "Gentle Density" Project Application

  • Oct. 7, 2021 – Moe Family Fund Grants Awarded to Address “Preservation Priorities”

  • Oct. 2021 – City of Phoenix "Preservation Phoenix Style" Report

For More Information

Jim McPherson

Chair, Homelessness & Affordability Subcommittee

Downtown Voices Coalition

“Replicable Strategies to Allow Gentle Density on Properties in Historic Areas without Rez