The Tape Measure Decides: Why Only Some Empty Offices Can Become Apartments

The 90,000-Unit Moment

There has never been a year like 2026 for turning empty offices into homes. Roughly 90,300 apartments now sit in the United States conversion pipeline, a jump of about 28 percent over the previous year, and adaptive reuse has become one of the defining development stories of the decade. New York leads with more than 16,000 units in progress, followed by Washington and Chicago. In February, Los Angeles passed a citywide adaptive reuse ordinance that rewrote its zoning to make the switch far less painful. The pressure behind all of it is blunt: office vacancy sits stubbornly high, housing remains scarce, and a half-empty tower earns almost nothing while a full apartment building collects rent every month.

Yet the headline number hides a truth architects learn on the first site visit. Most office buildings cannot become apartments at any sensible price. Whether a given tower is a candidate is decided less by its address or its zoning than by a single measurement taken on the plan: the distance from the exterior window to the central service core.

Why the Floor Plate Decides Everything

An office wants a deep floor. Rows of desks do not care about daylight, so developers in the postwar decades pushed the glass ever farther from the elevators and risers to maximise rentable area. A typical tower built between 1960 and 1990 runs 50 to 70 feet from window wall to core. A home wants the opposite. People need daylight and fresh air in every bedroom, and no one wants to live 40 feet from the nearest window in permanent shadow.

The comfortable working range for a residential floor is roughly 30 to 45 feet of depth, glass to core. Push past that and the middle of the plate becomes space that can hold a corridor, a bathroom or a closet, but never a legal bedroom. The building itself is fixed; the code is fixed; only the layout can flex, and past a certain depth even the cleverest layout runs out of room.

The 30-Foot Rule

The constraint is not a matter of taste. Building codes require habitable rooms to draw natural light and ventilation from a window opening onto a street, yard or court. New York, for instance, holds that no part of a habitable room may sit more than 30 feet from such a window, and its residential provisions require every legal bedroom to have an exterior window for light and air. A bedroom buried deep inside a 65-foot plate simply cannot comply.

That single rule sorts the entire national office stock into winners and losers. Slender pre-war towers, built before air conditioning and fluorescent light when every worker needed a window, tend to have shallow plates and convert almost gracefully. The fat postwar slab, engineered around mechanical systems and artificial light, fights back.

Cutting Light Into the Middle

When the plate is too deep, there is one architectural move that rescues it: carve a hole. Designers remove a vertical slot through the centre of the building, floor after floor, creating an interior light court or atrium that brings a second source of window to units that would otherwise face only a dark core. One recent conversion cut a courtyard of roughly 1,200 square feet out of the middle of the structure so that apartments could line both the outer wall and the new inner court.

It works, but it is expensive. Every square foot of removed floor is rentable area erased, and cutting a clean shaft through an occupied structural grid means reinforcing the edges the demolition creates. The light court is the reason a deep building can be done at all, and also the reason many deep buildings never pencil out.

The Wet-Stack Problem

Below the floor lies the second hidden obstacle. An office serves an entire floor from a handful of shared restrooms, so its slabs were poured with only a couple of plumbing risers. An apartment floor needs a kitchen and a bathroom, sometimes several, in every unit, each demanding its own supply and drainage. Threading dozens of new wet stacks down through a slab never designed for them is slow, disruptive and costly, and it often decides the budget as firmly as the floor plate decides the plan.

The Glass That Has to Go

Then there is the skin. Many postwar and later towers are wrapped in sealed curtain walls of single-pane glass that never open, with thin insulation at the spandrels. Homes need operable windows and a thermal envelope that holds heat. Where the existing glass cannot meet residential standards, the whole curtain wall may have to come off and be rebuilt, an enormous cost that also carries a heavy load of embodied carbon, undercutting the green argument that adaptive reuse is supposed to make.

Which Buildings Win

The math explains the projects making headlines. In Manhattan, the former 1960s office tower at 25 Water Street reopened as roughly 1,300 apartments, the largest office-to-residential conversion in United States history, with ceilings reaching about 12 feet. Nearby, Pearl House turned a 1970s office building into 588 rental homes wrapped in resort-grade amenities. These succeed because their owners could absorb the courtyard cuts, the new risers and the reglazing, and because location made the rents worth it.

What It Means for the City

Conversion will not empty the vacancy rolls on its own; the buildings that convert easily are a minority, and the rest need deep intervention or nothing at all. But the ones that do work are quietly powerful. They add housing without pouring a new foundation, reuse millions of tons of existing structure, and bring residents into downtowns that empty at six each evening. The lesson for 2026 is that the future of the empty office is not decided in a marketing brochure. It is decided by a tape measure, held from the glass to the core.