July 5, 2026 • By Catherine Chen
The Second Staircase Costs $200,000 and Adds Zero Apartments. The Code Requiring It Predates Sprinklers.
In 1860, a fire tore through a Manhattan tenement and killed twenty people in their beds. The city responded by mandating two staircases in every multifamily building above three stories. A reasonable reaction, given the circumstances: no fire alarms, no sprinkler system, no fire-rated construction materials, and only one unenclosed wooden stairway that functioned as a chimney once the flames reached it. Everything about it was combustible.
One hundred sixty-six years later, every new apartment building in most of the United States still follows that same dual-stairway logic, even though the buildings themselves have changed beyond recognition: sprinklered, alarmed, constructed with fire-rated assemblies, served by pressurized stairwells that actively resist smoke infiltration. A Pew Charitable Trusts study found that 99 percent of residential fire deaths in the United States occur in buildings without sprinkler systems. That second staircase is solving a problem that sprinklers already solved.
But creating a new one in the process.
What the Second Staircase Actually Costs
A second staircase in a five-story apartment building consumes roughly 9 percent of the total floor area on every floor: the stairwell itself, plus the connecting corridor required to meet travel-distance limits between the two exits. That's floor area that produces no rent, houses no one, and exists solely to satisfy a code provision whose underlying hazard has been engineered away by other means. Construction cost for that stairwell and corridor runs approximately $200,000 per building, according to Pew's analysis with the Center for Building in North America.
On a 24-unit building, $200,000 divided by 24 is $8,333 per apartment in pure dead-weight construction cost. On smaller infill projects of 12 to 16 units built on the 3,000- to 5,000-square-foot urban lots where housing is most needed, Boston-area developers report that figure balloons to $200,000 to $500,000 because the second stairwell forces a larger building footprint, which means more foundation, more structural framing, and more exterior cladding to enclose what is functionally a fire exit that nobody uses as long as the sprinklers work.
Then there's the revenue it destroys. A 2024 study commissioned by Boston Indicators found that dual-stairway buildings have efficiency ratios roughly ten percentage points lower than comparable single-stair buildings. Efficiency ratio is the share of a building's floor area that sits inside apartments rather than hallways, stairs, and lobbies. Dual-stair mid-rise buildings typically land around 82 percent. Single-stair buildings hit 92. I ran the numbers on a hypothetical five-story building with a 5,000-square-foot footprint and 25,000 total square feet: that ten-point gap means 2,500 square feet of livable space that doesn't exist because a corridor connects two staircases. At a median asking rent of $3.00 per square foot per month in a mid-tier metro, that's $7,500 in monthly rent that the building can never collect. Over thirty years, the combined cost of the stairwell construction and the lost rental income approaches $2.9 million per building, all of it dead money that produces nothing for anyone.
Nineteen States Just Decided to Stop Paying It
Since 2022, nineteen states and Washington, D.C., have introduced legislation to study or allow single-stairway apartment buildings. In 2025 alone, seven states signed bipartisan bills into law. Colorado, Montana, New Hampshire, and Texas now explicitly allow single-stair construction above three stories, with height caps ranging from four stories in New Hampshire to six in Montana and Texas. Maine is in the process. Hawaii and Maryland convened study groups.
Politically, the coalition behind these bills is unusual enough to warrant attention. In Colorado, the bill (HB 25-1273) was co-sponsored by legislators from both parties and requires every municipality with a population over 100,000 to adopt building code language allowing single-stair buildings up to five stories by December 2027. In Texas, SB 2835 passed through a legislature that agrees on almost nothing else and allows buildings up to six stories and 24 units statewide. Montana's law is the strongest of the cohort: it applies to all jurisdictions, permits six stories, and carries fewer additional restrictions than any other state's version.
Tennessee enacted its reform in 2024 and offers a natural experiment in adoption speed. Rather than rewriting the statewide building code, Tennessee created single-stair code language that municipalities could opt into voluntarily. Within a year, the state's four largest cities adopted it — Chattanooga, Knoxville, Memphis, and Nashville — and Jackson followed shortly after. Tennessee's experience suggests that once a state removes the legal barrier, cities move fast because the development economics are too compelling to ignore.
What 130,000 Apartments Look Like on a Map
Harvard's Joint Center for Housing Studies and the design firm Utile produced the most granular analysis of what single-stair reform actually unlocks. They identified 4,955 undeveloped or underutilized parcels in Greater Boston located within three-quarters of a mile of rapid transit stations: parking lots, vacant land, single-story retail buildings. Under current dual-stairway codes, most of these parcels are too small for an economically viable apartment building because the second staircase eats so much floor area that developers can't hit the unit count needed to justify the land cost.
Remove the second staircase requirement, allow single-stair buildings up to six stories and 24 units, and those same parcels can produce an estimated 130,000 new housing units through urban infill, not on greenfield land, not in the exurbs, but on lots that already exist, already have infrastructure, and already sit within walking distance of transit that taxpayers have already paid for.
Boston is not unique in having transit-adjacent infill parcels. If a comparable density of opportunity exists in the metro areas covered by the seven states that have already enacted reforms, which include four of the nation's ten largest metros in Texas alone plus Denver, Nashville, and the full state of Montana, the national potential is a multiple of Boston's 130,000. I am deliberately not extrapolating a national figure from one city's parcel inventory because the housing math varies enormously by market, and responsible analysis does not pretend Denver's lot sizes, land costs, and transit coverage replicate Boston's. What the Boston study demonstrates is a methodology: identify transit-adjacent parcels, model the development economics with and without the second staircase, and quantify the gap. Every metro in a reform state should be running that analysis right now, and the overwhelming majority are not.
The Safety Argument That Isn't One
Fire officials have pushed back, and their concerns deserve a direct response rather than dismissal.
Officials at the International Fire Code Council have questioned whether single-stair buildings create evacuation bottlenecks for wheelchair users and elderly residents who cannot descend stairs quickly. In a building with one stairway and a fire blocking the stairwell entrance on a middle floor, those residents have no alternative egress path. This is a real scenario, and it deserves a serious answer rather than deflection. It is also a scenario that the dual-stairway code does not solve as cleanly as its advocates suggest, because in a five-story building, two staircases at opposite ends of a double-loaded corridor still require a mobility-impaired resident to traverse the full length of that corridor under smoke conditions to reach the farther exit. Staircase count isn't the problem. It's the absence of areas of refuge, which European codes address directly and American codes largely don't.
Safety data on the macro question is unambiguous. New York City has allowed single-stair buildings up to six stories for decades. Seattle has done the same since the 1970s. Honolulu since 2012. Pew examined twelve years of fire incident data in those cities and found that fire death rates in sprinklered single-stairway buildings of four to six stories were comparable to other residential building types. Four fatalities occurred in single-stair buildings over those twelve years. In none of them was the absence of a second stairway a contributing factor.
Most of Europe never adopted the dual-stairway requirement in the first place, relying instead on fire-resistant materials, compartmentation, and sprinklers, and their fire death rates are consistently lower than those in the United States and Canada.
What This Means If You're Building in a Reform State
If you're a developer or architect working in Colorado, Montana, New Hampshire, Texas, or Tennessee, the immediate implication is floor-plan redesign. A single-stair building doesn't look like a dual-stair building with one staircase removed, and anyone who approaches it that way will design a worse building than what the old code produced. It's a fundamentally different building type: a point-access block where apartments cluster around a central stair and elevator core, with units getting windows on two or three sides instead of one. European and Asian cities have built millions of these. According to the Architect's Newspaper, point-access block designs can achieve floor-plate efficiencies of up to 95 percent, compared to the 82 to 85 percent typical of double-loaded corridor buildings.
Design competitions drive the point home. Denver ran a Single-Stair Housing Challenge that produced winning designs for buildings on infill lots as small as 3,500 square feet, with unit mixes ranging from studios to three-bedroom family apartments, each with cross-ventilation and natural light on at least two sides. Those are the kinds of apartments that don't exist in most American cities, not because nobody wants to live in them, but because the building code made them uneconomic to build.
California is watching. Culver City became the first jurisdiction in the state to legalize six-story single-stair apartments in September 2025, positioning itself as a pilot for statewide reform. Meanwhile, the International Code Council is considering an exception for four-story single-stairway buildings in the 2027 edition of the International Building Code, which would establish a national baseline that states could then build on.
The code doesn't care about your timeline. But for the first time in 166 years, it might stop eating your floor plan.