How Building Code Shapes Pathways and Experience in High Rise Office to Residential Conversion

Why Code Determines Conversion Feasibility

In a previous AIA Philadelphia advocacy series on Point Access Buildings, we examined how building code and financial structures have reinforced a dominant model of large‑scale multifamily development—the five‑over‑one podium building. That discussion revealed how regulatory frameworks quietly shape not just building form, but the range of housing types a city can realistically produce. Jurisdictions such as Seattle have demonstrated that alternative code pathways can safely enable missing‑middle housing and small‑scale urban infill common in many international contexts.

A similar regulatory dynamic is now playing out in the conversion of high‑rise office buildings to residential use. While tax abatements, Keystone Opportunity Zones, and by‑right residential permissions in commercial districts are all relevant to office‑to‑residential (OTR) conversions, this article focuses on a more decisive and less visible factor: building code. Requirements governing daylight, ventilation, and habitability often determine whether conversions are incremental and widespread—or rare, extreme, and capital‑intensive.

Philadelphia offers a useful case study. The city does not require parking for large office buildings in CMX‑4 and CMX‑5 districts, and many existing buildings are grandfathered under pre‑modern zoning provisions. More importantly, Philadelphia largely adheres to the base International Building Code (IBC), a choice that materially affects which buildings can convert and how.

New York City: When Code Forces Extremes

Some of the most technically ambitious adaptive reuse projects underway today are occurring in New York City. In response to local code requirements for natural light and ventilation in habitable residential spaces, deep‑floor‑plate office towers are being radically reconfigured. Entire floor slabs are removed to create interior lightwells, effectively carving new courtyards into the cores of existing towers.

These interventions are structurally consequential. Floor diaphragms play a critical role in lateral stability, and their removal requires extensive re‑engineering of the structural system—often within the constraints of existing foundations. The costs associated with slab removal, façade reconstruction, and system replacement are substantial. To recapture lost square footage, zoning strategies are typically employed to add new area back to the building, maximizing allowable Floor Area Ratio (FAR).

It is important to distinguish between these regulatory layers. Building code drives the need for interior lightwells and unit depth limitations, while zoning governs whether lost area can be recaptured through additions. These projects succeed only when both layers can be optimized simultaneously. The result is a small number of highly capitalized, technically sophisticated conversions—impressive, but difficult to replicate at scale.

The IBC Baseline and What It Allows

Under Chapter 12 of the International Building Code, natural light and ventilation are not universally required in habitable spaces if artificial lighting and mechanical ventilation systems meet prescribed performance standards. This is not a loophole; it is a deliberate baseline assumption of the model code. Local jurisdictions may, and often do, amend these provisions.

The flexibility of the base IBC allows for design strategies such as borrowed light through adjoining rooms. Bedrooms may meet daylight requirements via sufficiently sized openings—windows, glazed doors, sliders, or transoms—into adjacent spaces that themselves have access to exterior light. This provision enables deeper unit plans and greater adaptability in existing buildings.

New York City, Ohio, and other jurisdictions have modified the IBC to require natural light and air in residential sleeping spaces. These amendments reflect legitimate concerns about occupant health, circadian rhythm regulation, and resilience during mechanical system failures as well as IEQ, safety, and comfort. They also raise the threshold for conversion feasibility, particularly in buildings originally designed with deep floor plates and large service cores and have added impact to the ability to utilize the existing envelope of an existing building.

Philadelphia: A Case Study in Regulatory Flexibility

Philadelphia has emerged as a national leader in high‑rise office‑to‑residential conversions not because of a single incentive, but because several structural conditions align. The city’s historic street grid produces smaller block sizes and, by extension, narrower building footprints. Many downtown office buildings are therefore closer to residentially viable proportions.

Equally important is Philadelphia’s adherence to the base IBC. By allowing mechanically ventilated interior bedrooms and by not mandating façade reconstruction to introduce new windows, the city enables conversions that would be infeasible elsewhere. Buildings with aging curtainwalls or complex façade systems can be reused without wholesale replacement, dramatically reducing cost and embodied carbon impacts.

The presence of bedrooms without direct access to daylight is often framed as a design failure. It results from how code requirements structured and regulatory priorities are balanced. The relevant policy question is not whether a single unit achieves an idealized standard of habitability, but whether the city benefits from keeping large buildings occupied, adaptable, and contributing to street‑level vitality.

Urban Vitality and Mixed‑Use Occupancy

Office‑to‑residential conversions play a significant role in sustaining urban life, particularly in the wake of post‑pandemic shifts in office demand. Residential occupancy stabilizes buildings and extends activity beyond the workday. Mixed‑use buildings—whether vertically integrated or distributed across districts—support ground‑floor retail by diversifying daily rhythms.

Office workers typically arrive, eat, and depart in concentrated time windows during the workweek, with limited evening or weekend presence. Residential occupants exhibit more varied and continuous patterns of use, generating steadier foot traffic and supporting a broader range of neighborhood businesses. At the district scale, these differences materially affect street life, safety, and economic resilience.

Large-scale adaptive reuse of commercial buildings can reasonably be understood as one of the most environmentally responsible development strategies available. Existing structures contain substantial embodied carbon—the cumulative emissions associated with the extraction, manufacture, transport, and assembly of materials already in place. Demolition not only wastes that embedded investment, it triggers a second wave of emissions through debris processing and new construction. Adaptive reuse retains the building’s primary structural and envelope components, conserving the embodied emissions embedded within them.

What Should Change—and What Should Not

The question is not whether Philadelphia should adopt stricter daylight and ventilation requirements, but how such requirements would affect conversion feasibility, housing supply, and reuse potential.

Several targeted policy approaches merit consideration:

• Borrowed Light Standards: Adopting International Property Maintenance Code language without exception to permit modest glazed transoms into bedrooms could establish a baseline visual and daylight connection without mandating full exterior exposure.
• IEBC Conversion Thresholds: The International Existing Building Code could require enhanced daylight provisions when an entire building—or a defined percentage, such as 50 percent—converts to residential use, while preserving flexibility for partial conversions.
• Incentive‑Based Improvements: Rather than imposing universal mandates, the city could condition tax abatements or other financial incentives on voluntary daylight or ventilation upgrades, aligning public benefit with private feasibility.

Each approach balances habitability, cost, and adaptability differently. What matters is recognizing that building code is not neutral. It actively shapes the human environment by determining which buildings can change, how cities absorb economic shocks, and whether reuse is incremental or exceptional. At the same time, our role as architects extends beyond meeting code requirements. We can contribute by identifying ways to measure long-term environmental and social impacts, clarifying the broader benefits of adaptive reuse, and thoughtfully applying our expertise to enhance livability within these conversion projects. By engaging these questions, we help ensure that regulatory compliance is accompanied by meaningful improvements to quality of life.

Philadelphia’s current framework reflects a conscious—if often unspoken—policy choice: prioritizing adaptability and occupancy over prescriptive uniformity. Any future code evolution should evaluate proposed standards not only at the unit level, but at the scale of the city itself.