How Town Planning Works | TPW-0100 — The Single-Stair Building: How Egress Rules Quietly Decide Which Mid-Rise Homes Fit on Small Urban Lots

Some planning constraints are printed on zoning maps.

Others are hidden inside a building core.

A requirement for two independent exit stairs can sound like a narrow fire-code rule. On a large apartment site, two stairs may fit naturally into the building. On a small or irregular urban lot, the second stair can change almost everything: the width of the floorplate, the length of the corridor, the number of apartments, the amount of exterior wall, the ability to put family-sized homes on corners, the feasibility of an elevator, and whether a modest infill project works at all.

That is why the “single-stair” debate has moved from specialist code circles into housing policy and town planning.

It is also why the subject must be handled carefully. Fire safety is not an aesthetic preference. Egress rules exist because buildings fail under emergency conditions, smoke moves, occupants make mistakes and firefighters need to reach the incident. A planning argument that treats the second stair only as “wasted space” is incomplete.

At the same time, a rule that changes which buildings can physically exist is not merely a technical detail. It shapes the housing stock.

In June 2026, the International Code Council described single-exit middle housing as an active issue in the 2027 I-Code development cycle. Governmental members had voted 86–34 in favour of an egress proposal that would permit qualifying single-exit residential buildings up to four storeys, subject to a package of limitations and protections. The proposal’s code-development status matters, but the larger planning lesson is broader: one means-of-egress rule can determine which parcel geometries can support mid-rise housing.

The reader job: see the building-code rule as urban geometry without reducing safety to geometry

This article has one narrow job.

It explains how the number and arrangement of exit stairs changes the feasible form of small and medium apartment buildings, and how a planner should evaluate a single-stair proposal as a complete life-safety and urban-delivery system.

Neighbouring mechanisms already have owners. The Missing Middle owns the broader problem of housing forms between detached houses and large apartment blocks. The Objective Code owns predictable design standards. The Permit-Ready Pattern Book owns pre-approved housing plans as a route to faster delivery. The Building Edge owns setbacks, frontage and ground-floor relationship.

The Single-Stair Building owns one narrower question: how does egress geometry change the housing that can fit on a parcel, and what compensating safety conditions are necessary when a code permits one protected exit rather than two?

A stair is not only a stair

An exit stair arrives with walls, doors, landings, structural openings, fire-resistance requirements and a route that has to reach the public way or another approved discharge.

In many apartment buildings, two stairs are connected by a corridor. That corridor must be wide enough, long enough and protected appropriately. The stairs need separation. The building’s units arrange themselves around this circulation system.

On a large site, the core can be absorbed into a broad floorplate. On a small site, circulation can consume a much larger share of each storey. The rule therefore has a nonlinear effect: the smaller the buildable floorplate, the larger the proportional cost of duplicate vertical circulation.

The second stair changes the minimum viable building

A small apartment project has fixed costs. It needs foundations, structure, utilities, fire protection, access, professional design and usually a vertical core. If the code requires two full stairs and a long corridor, the project needs enough apartments to carry those costs.

That can push developers toward wider lots, larger land assemblies and bigger buildings.

The result is a planning paradox. A zoning code may legalise modest mid-rise housing, but the building code may make the modest version uneconomic or physically impossible, leaving only a much larger project capable of absorbing the required circulation.

Small lots are where the argument becomes visible

Established neighbourhoods are often divided into narrow parcels. They were not created for contemporary double-loaded apartment corridors.

If redevelopment requires several lots to be assembled before a compliant apartment building works, land acquisition becomes slower and more expensive. One holdout parcel can stop the project. Fine-grained ownership begins to favour either no change or very large assemblages.

A compact single-stair core can, in some code systems, allow a smaller building to fit within one or two ordinary parcels. That changes not merely the floor plan but the grain of redevelopment.

Point access replaces the long corridor with a short route to the core

Many single-stair apartment buildings use a point-access arrangement: a small number of homes open onto a compact landing or short lobby around the stair and lift.

This can reduce internal corridor length. More of the building perimeter can belong to apartments rather than circulation. Units can wrap around corners and gain windows on two or more sides.

The architectural benefit is real, but it is not automatically a safety benefit. The safety case depends on the stair being protected, travel distances being short, occupant load being limited and the surrounding fire-protection system working as designed.

More perimeter can change the apartment mix

Long double-loaded corridors tend to favour apartments placed on either side of a central hall. Many units then have windows on only one external face.

A compact point-access building can make corner units more common. That can make it easier to design larger apartments with bedrooms on different façades, better daylight distribution and, where climate and code permit, opportunities for cross-ventilation.

This is one reason the single-stair discussion has attracted planners concerned with family housing. The egress core affects which apartment geometries are economical, not just how residents leave in an emergency.

But no plan should assume a single stair is safe simply because other countries use one

International comparisons are useful only when the rest of the safety system is compared too.

Different jurisdictions use different combinations of sprinklers, compartmentation, travel-distance limits, unit counts, stair enclosure, smoke control, façade access, firefighter equipment, water supply and building height.

Importing the headline “one stair” while dropping the conditions that make the system work is not regulatory learning. It is selective copying.

The correct comparison is system against system

Fire safety is layered.

The building tries to prevent ignition, detect fire, suppress it, contain it, keep smoke away from escape routes, notify occupants, maintain tenable conditions, support evacuation and allow firefighters to operate.

Two stairs provide redundancy, but redundancy is one layer among several. A single protected stair may be allowed only when other layers are strong enough and the building remains within strict size, height, travel and occupancy limits.

The planning conversation should therefore never be “one stair versus two stairs” in isolation. It should be “which complete package of protections applies to this building type?”

Sprinklers change the fire before the stair is needed

Automatic sprinklers are a central part of modern multifamily fire safety. Their job is to control or suppress a developing fire near its origin, reducing the chance that conditions become severe enough to threaten the means of egress.

That does not make sprinklers infallible. Systems need water, inspection and maintenance. Obstructions and failures remain possible.

But a code that permits a single stair in a sprinklered building is making a different risk judgement from a code that permitted the same geometry in an unsprinklered building. The active protection is part of the permission.

Compartmentation buys time

Apartment walls, floors, doors and shafts can be designed to resist fire and smoke for defined periods.

This creates compartments. A fire beginning in one dwelling should not immediately make every other dwelling and the exit stair untenable.

Self-closing fire doors matter because an excellent wall cannot protect a corridor if the door is left open. Penetrations matter because pipes and cables can breach rated assemblies. Construction quality matters because the design rating exists only if the assembly is built and maintained correctly.

The stair itself must remain a protected route

A single stair concentrates dependence on one vertical path. That makes protection of the path especially important.

Relevant codes can require enclosure, fire-resistance, protected openings, smoke-control measures, direct discharge and restrictions on what can open into the stair. Some systems use pressurisation or natural ventilation approaches under defined conditions.

The lesson for a planner is simple: when there is only one stair, the quality and maintainability of that stair cannot be treated as a minor specification.

Travel distance is another form of redundancy

A second stair provides an alternative destination. A short travel distance reduces the distance an occupant must cover before reaching a protected route.

Single-stair codes therefore commonly pair one exit with small floorplates, few units per floor or tight travel-distance limits. The building remains deliberately compact.

This is important because “single stair” should not become permission for an enormous floorplate with a long corridor feeding one door. The geometry that makes the building attractive for infill is often also part of the risk limitation.

The International Code Council’s 2026 debate is a live example of risk calibration

The ICC’s June 2026 update described governmental members voting 86–34 in favour of a proposal that would permit single exit up to four storeys under the 2027 I-Code development process. The proposal had moved through an approved-as-modified stage pending the code-development process described by ICC.

The underlying proposal does not simply delete a second stair. It contains conditions involving residential occupancy, floor area, unit counts, travel distances and additional protection requirements.

That is what responsible code change looks like: define the building class, define the compensating measures, define the limits, then test whether the combined system achieves an acceptable level of safety.

The evidence base should include both fire outcomes and operational concerns

A 2025 report by The Pew Charitable Trusts examined modern single-stair apartment buildings in New York City and Seattle and international evidence. It reported no higher fire-death rate in the modern New York single-stair cohort than in other residential buildings and argued that modern safeguards such as sprinklers, rated construction and short travel distances matter to the result.

The same report also records a serious counterargument from fire-service organisations: a second stair can help separate occupant evacuation from firefighter attack and provide an alternative route if one path becomes compromised.

A world-class planning decision keeps both pieces of evidence in view. It does not turn a code question into a housing slogan or a fire-service slogan.

Firefighter access is not identical to occupant egress

Residents want to move out. Firefighters may need to move in, carry equipment, deploy hose lines, search, rescue and operate under worsening conditions.

A stair wide enough for normal evacuation may behave differently when bidirectional movement begins. A code review therefore needs fire-service input on stair width, standpipes, staging, exterior access, apparatus positioning and local operational capability.

That requirement also explains why rules cannot be copied between cities solely from architectural drawings. Fire-service capacity is part of the operating environment.

Maintenance can erase the safety assumed at approval

At completion, a building may have working sprinklers, clear corridors, self-closing doors and a protected stair.

Ten years later, doors may be wedged open, storage may occupy the landing, penetrations may have been cut through rated walls, sprinklers may be poorly maintained or an e-bike may be charging in an inappropriate location.

Single-stair policy therefore requires an inspection and enforcement culture, not just an elegant approval standard. The more the safety strategy depends on several coordinated protections, the more important long-term compliance becomes.

A second stair can consume more than its own footprint

The spatial penalty is not simply the square metres inside Stair B.

Two stairs often need to be separated so that one incident is less likely to disable both. Connecting them can require a corridor. Unit entrances move to that corridor. The building may become deeper. More façade may belong to circulation-adjacent spaces rather than rooms.

This is why cost studies that count only the second stair enclosure can understate the effect. The entire plan reorganises around two remote exits.

The circulation ratio becomes a housing-policy variable

Suppose two buildings contain the same gross floor area. One dedicates 10 per cent to shared circulation. Another dedicates 18 per cent.

The difference can become apartment area, fewer total apartments, or a larger building needed to achieve the same housing yield. The effect then reaches land value, construction cost and embodied carbon.

Circulation is necessary infrastructure inside the building. The planning question is not how to eliminate it, but how much is required for the safety and accessibility job being performed.

Single-stair buildings can support fine-grained redevelopment

When a project can fit on a smaller parcel, more landowners can participate without assembling an entire block.

That can preserve a finer rhythm of buildings, ownership and entrances. Redevelopment can happen incrementally rather than through one massive project.

But fine grain is not automatically good. Small projects can still produce poor façades, inaccessible homes or weak public space. The benefit is optionality: the code allows a smaller development actor and smaller parcel to participate.

The elevator question can move in the opposite direction

If two stairs consume too much area on a small lot, a project may stay lower and avoid an elevator under local rules, or may not be built at all.

A compact core can sometimes make it viable to include both a stair and lift in a modest building. That can improve step-free access even though the building has fewer stairs.

This illustrates why code trade-offs should be evaluated across the whole building. One requirement can indirectly determine whether another accessibility feature is financially feasible.

The climate effect is mixed, not automatic

Reducing duplicate stairs and long corridors can reduce constructed area for a given number of homes. Smaller buildings on infill sites can also use existing streets and utilities and may place residents closer to daily destinations.

But compactness alone does not guarantee low carbon. Structure, façade, materials, operational energy, parking and location all matter.

The relevant climate claim is therefore modest: egress geometry can change material efficiency and make infill housing feasible, which may improve some carbon outcomes. It does not turn every single-stair building into a climate solution.

Single-stair reform should be written around a building class, not a slogan

A safe reform starts by defining the precise buildings eligible for the alternative.

Maximum building height and number of storeys.
Maximum floor area per storey or total building area.
Maximum number of dwellings served by the stair.
Maximum travel distance from each dwelling to the protected stair.
Automatic sprinkler requirements.
Fire-resistance and compartmentation requirements.
Stair enclosure, smoke protection and door requirements.
Firefighter access, standpipe and water-supply requirements.
Alarm and detection requirements.
Restrictions on hazardous or incompatible occupancies.
Inspection and maintenance responsibilities after occupation.

If the eligibility boundary is vague, the reform can spread beyond the evidence used to justify it.

Do not use planning approval to improvise fire engineering case by case

Town planning is poorly suited to inventing bespoke life-safety standards for every project.

If a jurisdiction wants single-stair mid-rise buildings, the safest route is normally a clear building-code pathway developed with fire, engineering and enforcement expertise. Planning can then coordinate parcel form, density, frontage and housing outcomes around a known technical standard.

Discretionary planning negotiations should not substitute for a proper code where life safety is concerned.

A pilot can answer local questions before a city scales the rule

Where evidence is incomplete or local operating conditions are unusual, a city can begin with a tightly defined pilot or code alternative.

Track design outcomes, approval times, fire-protection maintenance, incident data, firefighter feedback, dwelling mix, construction cost and whether the reform actually unlocks smaller sites.

A pilot should have a learning plan before the first building opens. Otherwise “pilot” becomes a label for permanent policy without evidence.

The reform can fail if zoning still makes the building impossible

Removing a second-stair requirement does nothing if the site still cannot support enough floor area, if setbacks consume the parcel, if parking rules require a basement, or if only detached houses are permitted.

Building-code reform and zoning reform therefore need to be checked together.

This is another reason The Floor Area Ratio belongs beside this article. Legal floor area and egress geometry jointly determine what the lot can actually carry.

The reform can also fail if every project needs a bespoke approval

A technically legal building type can remain commercially unusable if each project must spend months proving that the ordinary rule should not apply.

Predictability matters because land is purchased and design teams are appointed before the final permit. If the alternative pathway is objective and clear, it can be priced. If it depends on uncertain negotiation, the risk premium can erase much of the intended benefit.

Watch the apartment mix after reform

One claimed benefit of compact point-access buildings is the ability to create more corner and multi-aspect homes.

Do not assume the market will automatically use that freedom to produce family-sized apartments. Developers may still choose the most profitable unit mix.

Measure what happens: average unit size, bedroom count, window orientation, corridor length, accessible units and floorplate efficiency. A successful code reform should be judged by delivered buildings, not only by theoretical diagrams.

Watch fire incidents without waiting for a catastrophe

Rare catastrophic events are a poor sole feedback mechanism.

Track sprinkler activations, smoke migration, door failures, blocked egress, alarm performance, firefighter access problems and maintenance violations. Near misses reveal weaknesses before mortality statistics do.

Data should be linked to building typology so future code decisions can distinguish a four-storey six-unit point-access building from a much larger apartment block.

Do not let a successful exception quietly become an unlimited principle

If evidence supports one protected stair for a tightly constrained four-storey building, that does not automatically prove the same arrangement is appropriate at eight, twelve or twenty storeys.

Risk changes with height, population, evacuation time, firefighter operations and rescue difficulty.

Planning systems are vulnerable to policy creep because successful reforms create pressure for extension. Each extension should have its own evidence and compensating safeguards.

A single-stair planning and code audit

Building class: Exactly which residential occupancies are eligible?
Height: What is the maximum storey count and height?
Floorplate: What is the maximum area served by the stair?
Unit count: How many dwelling units can open onto each level?
Travel distance: How far can the most remote occupant be from the protected stair?
Sprinklers: What system is required, and how is long-term maintenance enforced?
Compartmentation: What fire-resistance ratings protect dwellings, corridors and shafts?
Doors: Are self-closing and smoke-control requirements part of the system?
Stair protection: Is the stair enclosed, pressurised, naturally ventilated or otherwise protected under a defined standard?
Firefighter operations: Can responders enter, stage equipment, deploy hose and pass evacuating occupants?
Water: Is reliable firefighting water available?
Accessibility: Does the building remain accessible during ordinary operation, and what refuge or evacuation provisions apply?
Parcel effect: Which lot sizes become feasible that were not feasible before?
Housing yield: Does the rule actually add homes or mainly increase developer efficiency?
Apartment quality: Does the new geometry improve daylight, orientation or family-unit possibilities?
Maintenance: Who inspects the safety layers after occupation?
Incident data: How will the jurisdiction learn from fires and near misses?
Scaling rule: What evidence would be required before increasing height, area or unit limits?

The stair count is a proxy for a bigger question

How much redundancy should a building carry, and where should that redundancy live?

It can live in two exits. It can live partly in suppression, compartmentation, protected travel, limited size and short distances. It can live in firefighter capability and maintenance. Different codes combine these protections differently.

The correct answer is not found by maximising one layer while ignoring every other layer. Nor is it found by stripping layers away because housing is expensive.

It is found by designing a complete safety system whose performance is credible and whose geometry does not unnecessarily prevent the urban form the city is trying to create.

A building code can shape the city without ever drawing a street

This is the larger town-planning lesson.

Zoning may say that five storeys are allowed. A housing strategy may say small infill apartment buildings are desirable. A transit plan may say more people should live near a station.

Then an egress rule can determine whether the building needs three parcels instead of one, whether it has eight homes or twenty, whether family apartments fit on corners, and whether the project can afford an elevator.

That does not mean the egress rule is wrong. It means planners need to see it.

The single-stair question is valuable precisely because it reveals how the hidden technical rules of buildings become the visible form of towns.