Finding Philadelphia’s Missing Middle- How a Design Competition is Reimagining Housing Affordability

By Tya Winn and Rebecca Johnson 

Philadelphia’s housing story is a familiar one written across many American cities: rising prices, stagnant wages, and dwindling affordable units. From 2000 to 2018, the city lost more than 53,000 affordable homes across nearly every neighborhood. By 2018, 90 percent of census tracts experienced a net loss in low-cost units. The pandemic only amplified this crisis, deepening racial inequities and exposing the fragility of many households on the economic margins. 

But a new initiative, Finding Philadelphia’s Missing Middle, led by the Community Design Collaborative (CDC) with support from AIA Philadelphia and AARP, is enlisting communities and designers to co-create the city’s next generation of affordable housing.  

The Missing Middle: A Typology for Today  

Philadelphia is a city of rowhomes. We have more than any other city in the country. The rowhome — in particular the “Trinity,” which houses three rooms on three floors — was an early iteration of the “tiny house” that still defines the urban fabric. But today’s regulatory frameworks, rising construction costs, and energy efficiency standards make delivering new rowhomes at an affordable price nearly impossible without subsidies. Meanwhile, zoning restrictions limit the flexibility of adding density in ways that support community needs. 

Missing middle housing can offer a way forward. This type of housing bridges the gap between single family housing and large-scale apartment buildings. AARP has been a leader in advocating for missing middle housing. They published a terrific primer, entitled “Discovering and Developing Missing Middle Housing: What it is, Where it went, and why it is a needed housing option for people of all ages.” Examples of missing middle housing typologies include: 

• Accessory Dwelling Unit 

• Cottage Court 

• Duplex 

• Fourplex 

• Mansion Apartment 

• Townhouse (i.e. Rowhome) 

• Live/Work property 

• Courtyard Property 

• Multigenerational House 

Civic Leaders and Residents Explore the Possibilities  

In the fall 2024, the CDC convened a steering committee of over 40 civic leaders, elected officials, housing experts, designers, and community leaders to discuss the barriers to creating missing middle typologies. The Steering Committee helped to shape the ideal housing typologies that would work in various Philadelphia neighborhoods and identified regulations that would need to change in order to make these typologies legal, and ideally, as a matter of right, streamline zoning approvals.  

Additionally, the CDC engaged community-based organizations across Philadelphia, asking them to weigh in on the types of housing that they think would work well in their communities. Those partners included Mount Vernon Manor CDC, Philly House, North City Congress, and The Welcome Table (St. James School). Overall, 120 community members offered critical feedback.  

Accessory Dwelling Units (ADUs), compact dwellings located on the same lot as a primary residence, were particularly identified. ADUs can house aging relatives, provide affordable rentals for low-income families, or create multi-generational living arrangements. They also bring density to neighborhoods without altering their character. 

ADUs are legal in some districts in Philadelphia. Many people are familiar with the form, however not necessarily enamored with it. This became the charge of the John Stewardson Memorial Fellowship in Architecture’s 2025 Design Competition: Reimagine the Accessory Dwelling Unit (ADU) as a tool for inclusion, resilience, and affordability.  

ADUs: A Legal and Regulatory Framework  

Despite their potential, ADUs face significant regulatory restrictions in the city: 

• Limited Zoning Permission: ADUs are allowed only on lots with single-family detached or semi-detached homes in specific districts (RSA-5 and CMX-1), or within historic structures. 

• Restrictions on New Construction: ADUs must be created inside existing principal or accessory buildings; no new detached ADU structures are permitted by-right. 

• Owner-Occupancy Requirement: If the primary dwelling is rented, the ADU must be occupied by the owner. 

• Basement Limitations: Bedrooms cannot be located in basements unless there is direct exterior access. 

• Overlay Constraints: Additional district overlays and zoning rules continue to apply, adding complexity to approvals. 

Award-Winning Designs  

The Steering Committee, community residents, and CDC staff shaped the competition abstract based on common conditions found in Philadelphia. Those conditions include an intermediate (mid-block) vacant lot, a corner vacant lot, an intermediate lot with an existing building, a corner lot with an existing building, and five contiguous vacant lots. The 2025 Stewardson ADU Design Competition winners showcase how diverse strategies can respond to the city’s housing needs: 

Planting A Seed — By Drexel alumni Adeniyi Onanuga, Alexander Puerto, and student Colin Cooper. This proposal envisions ADUs as hubs of agricultural and social stewardship, pairing rowhouses with ADUs and shared green commons. It weaves food production and aging-in-place strategies into regenerative urban systems. 

Planting a Seed is a distributed housing and farming strategy that reimagines Accessory Dwelling Units as nodes of agricultural and social stewardship. Located on a CMX-1 parcel, each site pairs a mixed-use rowhouse with an ADU, connected by a productive commons that transforms the yard between buildings into an active green space. 

Residential Cooperative Mixed-Use (RCX) — By Jefferson alumni Drake Schaefer and Wyatt Korb. This proposal introduces a new zoning classification to accommodate diverse housing types within existing blocks, modeling scalable affordability. 

Residential Cooperative Mixed-Use (RCX) proposes a new classification system for residential development that liberates housing from conventional zoning constraints. By allowing vague or combined lot lines and removing the requirement for each dwelling to have a street-front entry, it creates the conditions for unprecedented housing diversity within Philadelphia’s established urban fabric. 

Pitch and Porch — By Varsha Iyer (UT Austin), Yalan Zhang (University of Michigan), and Janani Suriyanarayanan (Syracuse University). A “village of roofs and roots,” the design clusters homes and ADUs around shared courtyards, integrating solar power and rainwater harvesting. 

Imagine a new kind of Philadelphia neighborhood: angular roofs tilt toward the sky and porches face one another to spark connection. It’s a place built not just for density, but for dignity and care across generations. At its heart are five single-family townhomes framed by courtyards and softened by landscape. Woven seamlessly around the townhomes are five ADUs extending each home’s reach.  

Lean Loft — By Drexel alumna Rachel Sasson. This design reimagines Philadelphia’s narrow backyard lots, using modular construction to transform tight spaces into light-filled, adaptable ADUs. 

Tucked behind a standard Philadelphia rowhome, this compact ADU transforms a rear yard into a light-filled, dignified living space. The semi-prefabricated structure quietly challenges current zoning limitations, proposing a new model for housing density within the city’s tightest infill conditions. Designed for one or two occupants — such as a young adult seeking independence, an aging family member, or a dependent in need of proximity — the unit balances autonomy with connection. 

The House at Third and Narrow — By Penn alumni María Jose Fuentes and Eric Anderson. Inspired by historic trinity houses, this design integrates vertical living with modern detailing in a compact, sustainable footprint.

This ADU is attached to the back of a Philadelphia rowhome. Its design draws inspiration from the city’s rich legacy of Trinity house architecture, known for compact footprints and efficient use of vertical space. In a similar spirit, this new structure is arranged across three thoughtfully designed levels, each serving a distinct function. Spatial planning began with a careful assessment of the existing rowhome, identifying opportunities to “reclaim” rooms and integrate a new ADU seamlessly into the site. A balance of historic reference and modern detail results in a home that is both rooted in context and forward-looking in its design.  

Honorable mentions included “Corner Bloom” (Penn team) and “We’re Halfway There” (Drexel team). 

Corner Bloom, This project presents a speculative ADU design for a corner lot in Philadelphia that challenges the limitations of current zoning regulations by proposing multiple accessory dwelling units on a CMX-2 parcel. The design explores how small-scale urban infill can support more flexible, adaptable living arrangements while enhancing neighborhood life. Its layout is designed for multiple occupancy scenarios. It can accommodate two separate tenants, function as a guest or rental suite, or integrate with the corner rowhouse’s commercial space to act as an extension of the owner’s business and domestic life. 

We’re Halfway There, This project is a multifamily building in Philadelphia that combines sustainability, affordability, and accessibility with building generational wealth through home ownership. The units prioritize low-income residents, with special considerations for seniors who are aging in place. Each primary dwelling unit has an adjacent ADU, giving residents the flexibility to live in either unit while renting the other for additional income.  

Outcomes and Next Steps  

The most challenging site to design for is arguably the most common application in the City: an intermediate, infill block that has an existing building on it. Because of the potential to become a model for thousands of lots, our next steps for this initiative is to engage the professional community on these difficult site(s) and to formulate demonstration projects to advance this conversation. 

REBECCA JOHNSON is executive director at AIA Philadelphia and DesignPhiladelphia. 

TYA WINN is executive director at Community Design Collaborative.