How Much Space Do Building Cores Take Up? Elevators, Stairs, Shafts, and Services Explained

There is no universal percentage of a building that should be occupied by its core. The amount of space required depends on building height, use, population, floor-plate dimensions, elevator strategy, egress requirements, structural system, mechanical distribution, plumbing, electrical infrastructure, and the way these systems are organized.

A building core commonly contains elevators and elevator lobbies, exit stairs, mechanical and electrical shafts, plumbing risers, utility spaces, and other support functions. In some buildings it also includes restrooms, service corridors, janitor rooms, telecommunications rooms, and substantial structural walls. The U.S. General Services Administration identifies elevators and their lobbies, stairs, toilets, janitor closets, mechanical and electrical spaces, and shafts among the typical core elements of an office floor.

For owners and developers, core size matters because these spaces can occupy a significant portion of every floor while enabling the rest of the building to function. A core that is unnecessarily large can reduce usable or rentable area repeatedly across dozens of floors. A core that is compressed too aggressively can create problems with elevator performance, egress, building services, structure, maintenance, and future flexibility.

Published research also shows why generic percentages should be treated carefully. One study of 42 high-rise buildings in Türkiye found that core areas increased as building height increased and that greater elevator density was associated with reduced space efficiency. A separate study of Asian supertall towers found an average core-to-gross-floor-area ratio of 29.5 percent within its specific sample, with substantial variation among the buildings studied. Those numbers describe particular datasets, not targets for another project.

The useful question during development planning is therefore not simply, “How much space does the core take?” It is why the core requires that space, whether its components are coordinated efficiently, and what the resulting core does to the rest of the floor plate.

A Building Core Is a Collection of Systems

A core may appear as a single block on an architectural plan, but internally it is an assembly of different systems with different requirements.

Elevators move occupants vertically. Stairs provide required egress and sometimes everyday circulation. Mechanical shafts distribute air and building services. Plumbing risers connect stacked fixtures. Electrical and telecommunications spaces distribute power and data. Fire-protection infrastructure may require additional risers and equipment. Walls surrounding these components can also form an important part of the building’s structural system.

Research on mixed-use high-rise buildings defines the typical-floor core similarly, including elevator shafts, stairs, ventilation shafts, cable shafts, public restrooms, and certain equipment rooms. The same research identifies core size as an important factor in determining how much of the floor remains available for other uses.

These systems cannot be reduced independently without consequences. Removing elevator capacity can affect vertical transportation. Moving a stair can alter egress and circulation. Reducing shaft space can constrain mechanical distribution. Relocating restrooms can complicate plumbing. Moving structural core walls can change the building’s lateral system.

For that reason, core planning is fundamentally a coordination problem. The objective is to organize all required components into a coherent vertical system while preserving as much useful floor plate as reasonably possible.

Elevators Can Become a Major Driver of Core Area

Elevators are often among the most visible components of a core because each elevator requires a hoistway, and groups of elevators generally require lobby and circulation space around them.

The number of elevators cannot be selected from floor area alone. Building population, number of floors, travel distance, use, peak traffic patterns, elevator capacity, speed, zoning strategy, service requirements, and expected performance all influence the vertical transportation system.

As buildings become taller, one elevator serving every floor can become increasingly inefficient. Tall office towers may divide elevators into low-, mid-, and high-rise zones or use other strategies to prevent every shaft from consuming space throughout the entire height of the tower.

This relationship between height, elevators, and usable area appears repeatedly in high-rise research. The study of 42 towers in Türkiye found that elevator and core areas tended to expand as building height increased. It also found that increasing elevator density reduced measured space efficiency within the sample. The development consequence can be substantial.

Suppose an additional elevator requires another shaft through many occupied floors. The impact is not confined to the lobby. That shaft can remove usable area repeatedly through much of the building, potentially alter the elevator lobby, change circulation, and require adjustments to the surrounding tenant or residential layouts. This is why elevator planning should begin during feasibility for projects where height and population make vertical transportation a significant issue.

Elevator Zoning Can Change the Core as a Building Rises

A tall-building core does not necessarily remain identical from bottom to top. Elevators serving upper zones may pass through lower floors without opening there. Some elevator shafts may terminate at intermediate levels. Mechanical floors can provide opportunities for transferring or reorganizing building systems. Service elevators may follow different patterns from passenger elevators.

As elevators terminate, the floor area previously occupied by those shafts can sometimes be reclaimed or reorganized, subject to structural, mechanical, life-safety, and other constraints. This means that core efficiency should be studied vertically as well as horizontally.

A typical-floor plan alone can conceal how much the core changes through the building. Developers evaluating tall buildings should therefore look at core diagrams by vertical zone. The questions include which elevators serve which floors, where shafts begin and terminate, how stairs continue, where mechanical systems transfer, and how usable floor area changes as the building rises. The effect becomes especially important in mixed-use towers, where residential, hotel, office, retail, and other uses may require different vertical transportation systems.

Stairs Take Space for Reasons That Extend Beyond the Stair Flights

An exit stair occupies more than the visible treads and landings. The complete assembly can include its enclosure, structural walls, doors, landings, vestibules where required, pressurization or other life-safety systems depending on the building, and circulation needed to reach the stair. Its position also affects the rest of the floor plate.

Exit requirements can influence how far apart stairs must be, how occupants reach them, and how corridors are organized. A floor plate that appears efficient before egress is studied can change considerably once credible exit paths are introduced.

Stair configuration can sometimes alter the amount of core area required. Architectural Record’s discussion of 111 West 57th Street, for example, describes how the tower’s floor-to-floor configuration allowed the two required exit routes to be nested in a scissor arrangement, helping minimize the floor area devoted to the core in that specific project.

That is a project-specific response rather than a universal solution. Applicable code, occupancy, construction type, building configuration, fire-resistance requirements, and local interpretations determine whether a particular stair arrangement is permissible. The broader lesson is that stairs should be studied as part of the complete core rather than inserted after elevators and floor plans have already been fixed.

Shafts May Look Small Individually but Become Significant Collectively

Mechanical, electrical, plumbing, fire-protection, and telecommunications systems need vertical routes through a multistory building. Those routes often appear as a collection of relatively small shafts on a plan. Collectively, they can consume substantial core area.

Mechanical systems may require supply and return air pathways, exhaust shafts, outside-air distribution, and other vertical connections depending on the system. Plumbing requires risers for water, sanitary, vent, and other services. Electrical and telecommunications infrastructure needs vertical distribution. Fire-protection systems require their own infrastructure.

Shaft dimensions are therefore not arbitrary architectural allowances. They depend on systems that need to be designed and coordinated by the appropriate disciplines. The GSA's building standards similarly recognize vertical utility distribution as part of the building core and shell, including water, sanitary, electrical, HVAC, fire protection, and communications infrastructure.

For developers, the timing of this coordination matters. If the architectural core is designed around undersized placeholder shafts, later engineering can force those shafts to grow into areas already allocated to tenant space, apartments, corridors, or other program. A slightly larger but credible preliminary core can therefore provide a more reliable feasibility model than a highly efficient diagram that depends on unrealistic service allowances.

Restrooms and Service Rooms Can Be Part of the Core

In many commercial buildings, common restrooms are grouped near the core. This allows plumbing fixtures and risers to stack vertically and keeps service infrastructure concentrated rather than distributed throughout the floor.

Electrical rooms, telecommunications rooms, janitor closets, and other recurring service spaces may follow the same logic. The result is an important distinction between core area and vertical circulation area. A core should not automatically be understood as only elevators and stairs.

GSA guidance for office buildings explicitly describes typical core elements as including elevators, elevator lobbies, stairs, toilets, janitor closets, and mechanical and electrical equipment spaces and shafts.

That broader definition helps explain why two buildings with the same number of elevators and stairs can still have very different core sizes. One may concentrate restrooms and building services inside a central core. Another may distribute some of those functions around the floor plate.

The percentage attributed to the “core” therefore depends partly on what the project considers part of it. Any comparison between buildings should first confirm that the areas are being measured consistently.

Structure Can Add Area to the Core

In many multistory and tall buildings, the core performs two jobs simultaneously. It organizes circulation and services, and it contributes to the building's structural stability.

Reinforced-concrete shear walls around elevators and stairs can form a stiff vertical structure that helps resist wind and seismic forces. Depending on the structural system, those walls may become thicker or more substantial as loads increase. The relationship between structure and core can become especially important in tall towers.

Architectural Record's discussion of several New York residential supertalls demonstrates how structural strategy and core planning can interact differently from project to project. At 111 West 57th Street, structural walls and deep beams work with the core. At 432 Park Avenue, the structural exterior tube permits a column-free zone between the façade and central core. At 53 West 53, the stiff exterior diagrid reduces reliance on the core for lateral resistance.

These examples show why a core should not be evaluated solely by its architectural contents. Changing the structural system can change what the core must do. That can affect wall thicknesses, openings, elevator and stair placement, and the relationship between the core and perimeter structure.

Core Placement Can Matter as Much as Core Size

Two cores with identical area can produce very different buildings depending on where they are located. A central core creates relatively consistent zones between the core and façade and is common in many office and tall-building configurations. An offset core can free larger contiguous areas on one side of the floor. A perimeter core can place service functions against an exterior or party-wall condition. Distributed cores can serve long or irregular floor plates.

GSA planning guidance identifies centralized, distributed, exterior-perimeter, and modular core arrangements and notes that each has different implications for building footprint size and configuration. The appropriate configuration depends on the project.

Trinity Tower in Paris demonstrates an intentionally different approach. Cro&Co Architecture moved the core away from the conventional central position and placed glass elevators along the façade. According to the project description, the off-center core helps connect vertical circulation with views and shared spaces.

Another project, One on One by Moreno Architecture, uses a reinforced-concrete central core to consolidate stairs and elevators while the steel structure is pushed toward the façades, freeing the surrounding office floor. Neither strategy is universally preferable. The point is that core area alone does not describe its architectural impact. Location determines what shapes remain around the core.

The Space Between the Core and Façade Determines Whether the Remaining Area Is Useful

After the core is established, the remaining floor plate still needs to function. A developer can calculate a favorable gross-to-core ratio and still end up with inefficient usable space if the geometry surrounding the core is poor.

The distance between core and façade affects office planning, residential unit depth, circulation, daylight, structural spans, and flexibility. GSA planning guidance specifically identifies footprint size as a major factor in determining building configuration and core placement and connects core location, occupiable depth, planning modules, and circulation efficiency. This is particularly important on irregular sites.

If a core is placed in the middle of a narrow floor plate, the remaining spaces on either side may become too shallow or fragmented for the intended program. Moving the core toward a less valuable edge could improve planning, but it might introduce structural, egress, service, or façade consequences.

Core planning therefore needs to happen with actual test fits. For residential development, that means apartments. For office development, it means realistic tenant layouts. For hotels, it means room modules, housekeeping, service circulation, and back-of-house functions. The most efficient core is the one that allows the rest of the floor plate to become efficient as well.

Building Height Can Increase the Core's Share of the Floor Plate

Height changes the demands placed on a core. More floors can mean more occupants traveling vertically, longer elevator journeys, greater structural demands, additional mechanical distribution, and different life-safety requirements.

Research on high-rise buildings consistently identifies this relationship. The Türkiye study found that elevator and core areas expanded with increasing building height.

Research into Asian supertall towers found an average space efficiency of 67.5 percent in its sample and an average core-area-to-GFA ratio of 29.5 percent. The authors also observed that space efficiency tended to decline as tower height increased.

The important point for development feasibility is not the specific average. Supertall towers are a specialized building category and their ratios should not be applied to ordinary residential or commercial projects. The useful finding is the direction of the relationship.

Additional height can produce additional gross floor area while simultaneously increasing the amount of infrastructure required to serve that area. A development model that assumes every new floor provides the same proportion of usable area may therefore become increasingly inaccurate as the building changes in scale and complexity.

Building Use Changes Core Requirements

An office tower, apartment building, hotel, hospital, school, and mixed-use tower should not be expected to have identical cores. Their populations and operational patterns are different.

An office building can experience concentrated arrival and departure periods. A residential building has a different traffic profile. Hotels combine guest elevators with housekeeping and service requirements. Hospitals require specialized vertical transportation and extensive building systems. Mixed-use towers may need separate elevator groups and lobbies for different occupancies.

Academic research confirms that building function is one of the variables affecting high-rise space efficiency. The study of 42 high-rise buildings in Türkiye compared office, residential, and mixed-use projects and found differences in architectural and structural characteristics associated with their functions.

Research into tall hotel towers likewise found considerable variation in core-to-gross ratios across a sample of 31 projects, reinforcing that even within one building type there is no single core percentage that can be universally applied. For developers, benchmarks are therefore most useful when the comparison building has a similar use, height, scale, structural approach, and measurement methodology.

Mixed-Use Buildings Can Require Multiple Vertical Systems

Mixed-use development introduces another layer of complexity. Consider a building containing retail at its base, offices in the middle, and residences above.

The residential occupants may need a dedicated lobby and elevator bank. Office users may require another elevator system. Retail may need separate public and service access. Loading and back-of-house circulation must reach the appropriate portions of the building. Mechanical systems may also be divided by use.

These systems can overlap physically without being operationally interchangeable. As a result, the lower floors of a mixed-use tower can contain substantially more vertical infrastructure than upper floors. This is another reason whole-building core analysis is more useful than looking only at a typical tower plan. A core can widen, divide, consolidate, or release area as different uses begin and end. The development team should understand these transitions early because they can create some of the most complicated floors in the project.

Core Area and Usable Area Should Be Tracked Together

The development consequence of core planning becomes clearer when core area is studied alongside usable area.

Conceptually:

Gross Floor Area = Core + Circulation + Primary Program + Other Building Areas

The exact categories and measurement boundaries depend on the methodology being used, so this should not be treated as a regulatory area formula. It is a planning framework.

If the gross floor plate stays constant and the core grows, another portion of the plan must shrink. That reduction may come from office space, apartments, hotel rooms, amenities, or other program. The effect repeats vertically when the same core condition occurs across multiple floors. This makes seemingly small planning changes important in tall buildings.

It also explains why the core is closely connected to net-to-gross efficiency. Research on mixed-use high-rises describes core size as a fundamental variable in determining rentable-space ratios and notes that reorganizing functional components, core elements, and structure can improve space efficiency. The goal should not be to eliminate support area. The goal is to understand where gross area is going and whether each component has a defensible purpose.

A Smaller Core Is Not Automatically a Better Core

Core optimization can become misleading when it is reduced to a percentage target. An elevator bank can be reduced until vertical transportation becomes inadequate. Shafts can be compressed until engineering coordination forces them to expand later. Stairs can be positioned for geometric convenience rather than an effective egress strategy. Service rooms can be fragmented into leftover spaces. Structural walls can be treated as architectural obstacles rather than part of the building's lateral system.

These decisions may temporarily improve a spreadsheet. They do not necessarily improve the building. The opposite condition is also possible. An oversized or poorly coordinated core can contain redundant circulation, unnecessarily large lobbies, inefficient shaft layouts, or spaces that no longer serve their original purpose.

Handel Architects encountered a particularly clear example during the redevelopment of 111 West Wacker in Chicago. The existing core, inherited from a previous hotel scheme, measured nearly 58 by 71 feet and contained substantial unused space. Rather than treating all of that inherited area identically, the design team evaluated individual core and podium spaces for new mechanical, electrical, and support functions as part of converting the unfinished structure into a residential tower. That project demonstrates a useful principle: core optimization is about coordination and usefulness, not simply reduction.

Existing Buildings Make Core Decisions More Difficult

Core planning becomes particularly consequential in adaptive reuse and conversion projects. The existing building already has elevators, stairs, shafts, structural walls, and service spaces. Their locations may have been appropriate for the original use but poorly suited to the proposed one.

Changing them can be difficult because cores frequently intersect structure, foundations, floor openings, fire-rated construction, and major building services.

An existing office building converted to residential use, for example, may have more elevators than the new population requires but lack the plumbing distribution needed for apartments. An older building may have stairs or elevators that do not align with the intended planning strategy. New shafts may need to penetrate many existing floors.

The architectural analysis therefore becomes:

What can remain?

What must change?

What can be repurposed?

What does modifying the core do to the structure and surrounding floor plate?

Discovering these issues before acquisition or before a conversion scheme is substantially developed can materially change how the building is evaluated.

Core Design Can Affect Construction Strategy

The core is also a construction system. In conventional high-rise construction, reinforced-concrete cores can influence the sequence of the surrounding structure. Alternative systems can change that relationship.

Rainier Square in Seattle provides a useful example. Its composite steel-plate shear-wall system, commonly known as SpeedCore, was developed in part to reduce schedule lag associated with a conventional concrete core preceding the surrounding steel frame. Architectural Record describes how the composite core simultaneously provided structural strength during erection and permanent lateral resistance after concrete infill. This does not mean that one core construction system is appropriate for every project. It illustrates that the core influences more than floor efficiency.

Material, structural system, erection sequence, trade coordination, openings, and vertical service installation can all be connected to the way the core is designed. For a developer, core decisions can therefore have spatial, structural, operational, and constructability consequences simultaneously.

Core Planning Should Begin During Feasibility

A preliminary massing model without a credible core can significantly overstate development potential. At the earliest stage, every elevator shaft, stair, mechanical riser, or service room does not need to be fully engineered. But the design should reserve realistic space for the systems the building is likely to require. As the project develops, those assumptions should be tested with the appropriate consultants.

Elevator planning should reflect building population and traffic. Egress should be evaluated against the applicable code. Structural engineers should determine what the core needs to contribute to the lateral system. Mechanical, electrical, plumbing, and fire-protection engineers should confirm vertical distribution requirements.

The core can then evolve while the surrounding floor plate is still flexible enough to respond. Waiting until the apartments, offices, façade, and structural grid have been substantially fixed makes that coordination much harder. For an owner comparing early design schemes, the core should therefore be visible rather than hidden inside a generic rectangle. The comparison should show what is actually inside it.

The Useful Question Is What the Core Allows the Building to Do

A building core will always consume space because a multistory building requires vertical circulation, life safety, structure, and building services. The development objective is not to drive that area toward zero. It is to make sure the space is doing necessary work.

A well-resolved core coordinates elevators, stairs, shafts, structure, services, and circulation while allowing the surrounding floor plate to support the building’s intended program. Its size should be evaluated in relation to height, use, population, geometry, structural strategy, engineering systems, and operational requirements.

This is why generic core percentages have limited value during feasibility. Research can provide useful benchmarks, but the range among actual buildings is substantial, particularly as height and program change.

For developers, the more useful analysis is project-specific: determine what the building needs, test how those systems can be organized, measure their effect on usable area, and compare alternative core strategies before the surrounding architecture becomes difficult to change. Daniel Inocente Architecture D.P.C. can assist owners and development teams with building-core studies, floor-plate planning, massing, circulation, and early development feasibility.

Sources

U.S. General Services Administration
Facilities Standards for the Public Buildings Service (P100)
GSA Facilities Standards

Buildings, MDPI
Hierarchical Quantification of Utilization Rate and Related Indicators of Mixed-Use High-Rise Buildings
Mixed-Use High-Rise Core and Space Efficiency Study

Architectural Engineering and Design Management
Evaluation of Space Efficiency Criteria in High-Rise Buildings Based on Functions: A Case Study of Türkiye
High-Rise Space Efficiency Study

International Journal of Building Pathology and Adaptation
An Analysis of Space Efficiency in Asian Supertall Towers
Asian Supertall Tower Space Efficiency Study

Tampere University
Space Efficiency in Tall Hotel Towers
Tall Hotel Tower Space Efficiency Research

Architectural Record
Moving Up in the World
Architectural Record: New York Supertall Structural and Core Strategies

Architectural Record
Rainier Square by NBBJ
Architectural Record: Rainier Square

Handel Architects
Rags to Riches: How Chicago's 111 West Wacker Was Rescued
Handel Architects: 111 West Wacker Core Strategy

ArchDaily / Cro&Co Architecture
Trinity Tower
Trinity Tower

ArchDaily / Moreno Architecture
One on One
One on One Office Building

Bertrand Goldberg Archive
Astor Tower
Astor Tower

FAQ

What percentage of a building is typically the core?

There is no reliable universal percentage. Core area varies with use, height, population, floor-plate geometry, elevator quantity, egress, structural system, and building services. Research samples illustrate the variation. A study of Asian supertall towers reported core ratios ranging from 14 to 38 percent of gross floor area, but those buildings are specialized supertalls and the range should not be used as a general design target.

What is usually included in a building core?

A typical core can contain elevators, elevator lobbies, exit stairs, restrooms, mechanical and electrical rooms, plumbing and utility risers, janitor spaces, telecommunications infrastructure, and structural walls. The exact contents depend on building type and how the project defines its core.

Why do taller buildings often need larger cores?

Additional height can increase elevator requirements, vertical service distribution, structural demands, and life-safety complexity. Research on high-rise buildings has found that elevator and core areas tend to increase with height, which can reduce the proportion of each floor available for primary program.

Can a building core be too small?

Yes. Compressing a core without resolving its systems can leave inadequate space for elevators, egress, mechanical and electrical distribution, plumbing, structure, maintenance, or circulation. Core efficiency should therefore be evaluated through coordinated planning rather than area reduction alone.

When should architects determine the core size?

A credible preliminary core should be included during feasibility and early schematic design. The design can then be refined as elevator, structural, life-safety, mechanical, electrical, plumbing, and fire-protection requirements are developed. This is particularly important because core placement and configuration can influence the entire building footprint and circulation system.