What Is Net-to-Gross Efficiency in Architecture and Why Does It Matter to Developers?

Net-to-Gross Efficiency = Net Area ÷ Gross Area

The calculation is simple. Deciding what belongs in the numerator and denominator, and understanding what the resulting percentage actually means for a development, is more complicated.

A building’s gross area must accommodate far more than apartments, offices, hotel rooms, classrooms, or other primary spaces. It also contains stairs, elevators, corridors, walls, structural elements, mechanical and electrical rooms, shafts, toilets, lobbies, and other support areas. The University of New Hampshire’s planning standards, for example, describe the difference between net assignable and gross area as consisting of circulation, building service, mechanical, and structural area.

For a developer, net-to-gross efficiency matters because it helps reveal how much building must be constructed to produce a given amount of useful program area. It can affect development yield, unit planning, leasing assumptions, construction budgeting, and comparisons between design options. It should not, however, be treated as a score that determines whether a building is good or bad.

A high ratio can indicate an efficient plan. It can also indicate that corridors, lobbies, service spaces, structure, or other necessary components have been compressed too aggressively. A lower ratio can indicate waste, or it can reflect a legitimate building requirement, amenity strategy, unusual geometry, or program that requires more support space.

The useful question for a development team is therefore not simply, “How high is the net-to-gross ratio?” It is what is causing the ratio, what is included in it, and whether the resulting building supports the project’s larger objectives.

Net and Gross Area Must Be Defined Before the Ratio Means Anything

The terms “net” and “gross” sound universal, but building area can be measured differently depending on the purpose of the calculation. During architectural programming, the Whole Building Design Guide describes net assignable square feet as space assigned directly to an activity. Additional “tare” space includes circulation, walls, mechanical and electrical equipment, wall thicknesses, and public toilets. Building efficiency is then calculated by dividing net assignable area by gross square feet. That approach is useful for programming, but it should not automatically be substituted for a leasing or property measurement standard.

BOMA International’s floor measurement standards, for example, include separate standards for office, industrial, gross-area, multifamily and hospitality, retail, and mixed-use properties. BOMA’s current office standard is ANSI/BOMA Z65.1-2024, whose primary purpose is calculating rentable area for office properties. BOMA also publishes a 2023 multifamily and hospitality standard with both gross and net measurement methods. This distinction is consequential.

A developer may encounter gross floor area used for zoning, gross building area used for construction budgeting, rentable area used for leasing, net assignable area used for programming, and net residential area used in a development model. These figures can answer different questions and may follow different boundary rules.

Two consultants can therefore produce different “efficiency” percentages for the same building without either calculation necessarily being wrong if they are measuring different things.

Before comparing ratios, the project team should establish:

• the measurement standard being used;

• what constitutes net area;

• what constitutes gross area;

• whether exterior walls are included;

• how shafts and vertical penetrations are treated;

• how common areas are treated;

• whether amenities are considered net program or support area;

• whether the ratio is calculated by floor, use, or whole building.

Without that information, the percentage can be misleading.

Net-to-Gross Efficiency Connects Building Design to Development Yield

A development model may begin with a target amount of residential, office, hotel, institutional, or other usable program. That program cannot simply be converted into a building of the same size. Additional area is required to support it.

The relationship can be understood conceptually as:

NET PROGRAM + SUPPORTING BUILDING AREA = GROSS BUILDING AREA

or, when the efficiency ratio has already been established:

Gross Area = Net Area ÷ Net-to-Gross Efficiency

This relationship is why net-to-gross assumptions matter during feasibility. The Whole Building Design Guide incorporates building efficiency into architectural programming precisely because the net program must be translated into the gross area necessary to accommodate it.

The consequences can extend into the development model. If two schemes contain the same gross area but one requires substantially more circulation or core space, less of that gross area remains available for the primary program. Conversely, a more compact circulation and service strategy may allow a greater proportion of the building to support apartments, workplaces, hotel rooms, or other intended uses. That does not mean every non-net square foot is economically unproductive.

A residential lobby can contribute to market positioning. An amenity lounge can support leasing. Wider circulation may improve accessibility and daily use. A mechanical room supports systems that make the building occupiable. A stair may not generate rent directly, but it provides required egress. Net-to-gross analysis is most useful when it reveals these tradeoffs rather than labeling all non-net area as waste.

The Core Is One of the Largest Drivers of Floor-Plate Efficiency

In a multistory building, the core typically concentrates several systems that repeat vertically: elevators, stairs, mechanical and plumbing risers, electrical infrastructure, service spaces, and associated circulation. Its area therefore has an outsized influence on net-to-gross efficiency.

Recent tall-building research reinforces this relationship. A 2026 peer-reviewed study of 43 high-rise buildings in Shanghai analyzed net floor area, gross floor area, core type, building form, structural system, and other design characteristics. The authors found that space efficiency declined with increasing building height in their sample, in part because taller buildings required larger service cores.

A 2025 study examining 42 high-rise buildings in Türkiye reached a related conclusion: increasing elevator density reduced space efficiency, and elevator and core areas expanded as building height increased.

The architectural consequence is important for developers considering additional height. Increasing the number of floors can create additional development area, but the amount of usable area gained does not necessarily increase at the same rate. Taller buildings may require additional elevators, larger structural components, expanded life-safety systems, mechanical distribution, and other infrastructure. This is why a development study should not evaluate height only through zoning floor area or gross square footage. The changing core should be tested simultaneously.

A Smaller Core Is Not Automatically a Better Core

Because the core reduces the area available for primary program, it can be tempting to treat core minimization as an objective in itself. That can create a false economy.

Elevator quantity and configuration affect vertical transportation performance. Stair number, width, enclosure, and separation affect egress. Shafts need adequate space for mechanical, electrical, plumbing, fire protection, and other building systems. Structural core walls may play a critical role in lateral stability. Compressing those systems may increase the net-to-gross percentage on a spreadsheet while creating operational or technical problems elsewhere.

The opposite can also occur. A core can become unnecessarily large because elevator banks, corridors, shafts, stairs, and service rooms were developed independently rather than coordinated as a system. The architectural task is therefore not to produce the smallest core. It is to determine the amount and configuration of core area necessary to make the building work while limiting unnecessary duplication and poorly used residual space.

Research into tall-building space efficiency supports this more nuanced view. A 2026 study specifically cautions that space efficiency is a method-dependent construct rather than an absolute measure of design performance. Comparisons can become unreliable when projects use different definitions or methodologies. For development decisions, that is an important warning. A one-point increase in an efficiency ratio has little value if it was achieved by changing what was counted rather than improving the building.

Circulation Can Consume Area Quickly

Corridors are another major component of the difference between net and gross area. Their impact depends heavily on floor-plate geometry.

A compact plan with units or tenant spaces efficiently distributed around a central circulation system may require relatively little corridor per unit of usable area. A long, narrow, irregular, or highly articulated building can require substantially more circulation to reach the same amount of program.

The number alone does not determine whether that circulation is excessive. A corridor may be required to connect multiple exits. It may serve apartment entrances, elevators, service spaces, accessible routes, or amenities. In some building types, circulation also provides social, visual, or environmental value. The University of New Hampshire’s planning framework categorizes circulation explicitly as part of the difference between net assignable and gross building area.

For developers, the useful analysis is therefore circulation per unit of program, combined with a review of what the circulation accomplishes. A plan that reduces corridor area but produces awkward apartment entrances, poor wayfinding, excessive travel distances, or inefficient unit shapes may have improved one metric while weakening the project.

Floor-Plate Shape Can Be as Important as Floor-Plate Size

Two buildings with identical gross floor area can have very different planning efficiencies. A simple rectangular floor plate usually provides different planning opportunities from an L-shaped, triangular, curved, deeply stepped, or otherwise irregular plan. Site geometry, setbacks, zoning envelopes, view corridors, neighboring buildings, and street-wall requirements can all influence the shape available to the architect. Irregularity often creates residual areas where the structural grid, façade, circulation, and program do not align cleanly.

In residential buildings, those areas can appear as awkward apartment corners, oversized circulation, unusable niches, or difficult furniture layouts. In office buildings, they can affect planning modules and tenant subdivision. In hotels, they can disrupt the repetition of rooms and back-of-house systems.

Tall-building research consistently identifies building form alongside core configuration and structural system as one of the variables relevant to space efficiency. A global study of 166 tall and supertall case studies published in 2025 specifically examined how architectural form, programmatic function, structural system, and regional context interact to influence net-to-gross performance.

This is why the efficiency of a zoning envelope should be tested, not assumed. The maximum buildable envelope may not be the most effective architectural envelope if filling every available corner produces disproportionately inefficient floor plates.

Structure Has an Area Cost

Columns, shear walls, bracing, transfer structures, and other structural components occupy physical space. Their effect on net-to-gross efficiency varies with building type, height, structural system, span, geometry, and material.

In a straightforward low-rise building, the structural footprint may be relatively modest. In a tall tower, lateral loads can require substantial core walls, perimeter columns, outriggers, or other systems. Transfers may be necessary where residential, hotel, office, retail, or parking grids do not align.

Research into North American skyscrapers illustrates the relationship. A 2024 study of 31 towers evaluated building form, core design, structural system, and structural material together with space efficiency. The research found substantial variation among the case studies rather than a single efficiency level applicable to every tower. A 2023 study of supertall mixed-use buildings similarly found that space efficiency tended to decrease as building height increased within its dataset.

For a developer, the implication is that structural design should enter efficiency studies earlier than a simple floor-area model might suggest. Moving a column, changing a span, aligning vertical elements, or avoiding an unnecessary transfer can affect both usable area and construction complexity.

Building Type Changes What “Efficient” Means

A residential building, office tower, hotel, hospital, school, and laboratory should not be expected to produce the same net-to-gross ratio. Their support-space requirements are fundamentally different.

A hotel needs guest corridors, elevators, housekeeping, service circulation, back-of-house spaces, and other infrastructure. An office may require large flexible tenant plates around a central core. A hospital needs substantial clinical support, vertical distribution, service spaces, and specialized systems. A residential building balances apartments against corridors, stairs, elevators, shafts, amenities, and service functions. Even within one building type, project objectives vary.

BOMA’s decision to maintain separate measurement standards for office, industrial, retail, multifamily and hospitality, mixed-use, and gross areas is one indication of how different property types require different measurement frameworks.

Academic research shows the same problem from another direction. A 2024 study comparing high-rise buildings by function found different relationships among efficiency, core design, structural systems, and use. This makes universal efficiency benchmarks dangerous.

A developer should compare a project with genuinely comparable buildings and use the same measurement methodology. Comparing a luxury residential tower with extensive amenities against a speculative office building using a different area standard can produce a percentage comparison with little practical meaning.

Amenities Complicate the Definition of Efficiency

Amenities reveal one of the limitations of a purely mathematical approach. Consider a residential building with a fitness center, coworking lounge, roof terrace, pool, package room, and generous lobby. Depending on the development model and measurement methodology, these spaces may not be counted as net residential unit area. Yet they may be central to the building’s positioning and operation. Removing them could increase one version of the efficiency ratio while reducing the product the developer intends to offer.

The same issue exists in office buildings. Shared conference facilities, tenant lounges, terraces, wellness spaces, and other common amenities may influence leasing even though their treatment in area calculations differs from private tenant space. BOMA’s 2024 office standard specifically addresses evolving building designs and tenant amenities while producing area figures useful for leasing, valuation, benchmarking, and space-use analysis. The developer therefore needs to distinguish between space that is inefficiently planned and space that is intentionally shared. Those are not the same thing.

Efficiency Should Be Studied Floor by Floor

A whole-building ratio can conceal important differences within the building. The ground floor may contain a lobby, loading, service spaces, utilities, and retail. Typical floors may be highly repetitive and efficient. Setback floors may become smaller and require unusual unit layouts. Mechanical floors can contain little or no primary program. A rooftop amenity level may function differently again. A single whole-building percentage averages all of those conditions together.

For architectural decision-making, it is often more informative to calculate efficiency at multiple scales:

Typical Floor Efficiency
How effectively does the repetitive floor plate convert gross area into the intended program?

Special Floor Efficiency
What happens at setbacks, transfers, amenities, mechanical levels, and other atypical conditions?

Whole-Building Efficiency
What proportion of the complete building supports the selected definition of net area?

This distinction becomes particularly important in towers. A typical floor may perform well while the total building becomes less efficient because of large lobby levels, mechanical floors, transfer zones, crown conditions, or below-grade infrastructure. Development models should understand both.

Zoning Floor Area and Net-to-Gross Efficiency Answer Different Questions

In New York development, zoning floor area and architectural net-to-gross efficiency should not be treated as interchangeable metrics. Zoning determines how floor area is counted for regulatory purposes under the applicable zoning framework. Net-to-gross analysis asks how the building’s physical area is distributed between the selected definition of usable program and the supporting spaces required to make it work.

Those calculations can have different boundaries and exclusions. A space excluded from zoning floor area is not necessarily physically absent from the building. Conversely, an area that counts toward a regulatory floor-area calculation may be treated differently in a leasing, programming, or development-efficiency model.

This distinction can materially affect feasibility. A project may appear attractive when evaluated only through allowable zoning floor area but produce less net program than expected after stairs, elevators, corridors, structure, shafts, walls, and mechanical spaces are resolved. The development team should therefore track regulatory area and architectural area separately rather than trying to force them into one number.

Why Chasing the Highest Percentage Can Damage a Project

A higher net-to-gross ratio is generally attractive when the additional net area is genuinely useful and the rest of the building remains functional. The problem begins when the percentage becomes the objective instead of the diagnostic.

Professor Roger K. Lewis argued in The Washington Post that overly tight grossing assumptions can reduce design flexibility and compress common spaces to the point that circulation, movement of furniture or equipment, and maintenance become more difficult.

The same principle applies to development planning. A corridor that is technically sufficient may still be poorly proportioned for the number of apartments it serves. A lobby can be reduced until it undermines arrival and package operations. Mechanical and service spaces can be squeezed until maintenance becomes unnecessarily difficult. A core can be compressed until elevator or stair planning becomes problematic.

At the other extreme, generous circulation and oversized support areas can consume buildable area without providing commensurate value. Neither extreme represents good efficiency. The strongest plan uses the necessary amount of non-net area deliberately.

Net-to-Gross Efficiency Should Be Tested Before the Building Is Fixed

Net-to-gross analysis is most valuable when the design team can still change the variables that produce it.

During feasibility and schematic design, alternatives can be compared by changing:

• core position and configuration;

• elevator quantity and arrangement;

• stair location;

• corridor organization;

• floor-plate dimensions;

• unit mix;

• structural grid;

• shaft distribution;

• building form;

• setbacks;

• amenity placement;

• service strategy.

The goal is not to manipulate the percentage upward. It is to understand why one scheme uses area differently from another.

For example, Scheme A may have a slightly higher net-to-gross ratio but poor apartment proportions and limited premium frontage. Scheme B may sacrifice some nominal efficiency while creating better unit layouts, more corner exposure, simpler structure, or a more coherent service strategy.

The percentage alone cannot choose between them. This aligns with the broader direction of recent research. Studies of tall buildings increasingly analyze efficiency as the product of interacting variables rather than one isolated design feature. The 2025 global study of 166 towers explicitly evaluates architectural form, program, structure, and regional context together. That is the appropriate mindset during development feasibility as well.

What Developers Should Compare Between Design Options

When comparing two architectural schemes, the development team should first ensure that the area calculations use the same methodology. Then the ratio can become one part of a larger comparison.

The team should understand the gross area, net program area, core area, circulation area, service and mechanical area, structural implications, unit or tenant configuration, façade exposure, and any unusual floor conditions. The comparison should also ask why the ratios differ.

If one scheme is more efficient because its core is smaller, what changed in the core? If the corridor is shorter, did the unit configuration improve or become less flexible? If more area reaches the perimeter, does that produce useful apartments or tenant space? If the floor plate became deeper, what happened to daylight and planning quality? These questions convert net-to-gross efficiency from a percentage into a design tool. They also reduce the risk of optimizing one metric while unintentionally damaging another.

The Best Efficiency Ratio Is Project-Specific

There is no defensible universal net-to-gross percentage that every building should achieve. Published high-rise research illustrates why. A study of 31 North American skyscrapers reported net-to-gross efficiencies ranging from 62 to 84 percent. A study of 63 tall buildings in Singapore reported a range from 68 to 91 percent. A 2026 study of 43 Shanghai high-rises found an even wider range, from 52 to 93 percent. These datasets use defined research methodologies and specific building samples, so they should be treated as evidence of variation rather than universal benchmarks.

A newer methodological study goes further, arguing that reported space efficiency is sensitive to how the researcher defines and measures net and gross area. That makes context essential.

Building type, height, geometry, structure, core configuration, services, amenities, regulations, market positioning, and measurement method can all affect the result.

For developers, net-to-gross efficiency is most useful as a comparative tool within a clearly defined project framework. It can reveal where area is being consumed, help test competing schemes, expose the consequences of core and circulation decisions, and translate architectural planning into information that can be incorporated into feasibility analysis.It should not replace architectural judgment.

The most effective scheme is not necessarily the one with the highest percentage. It is the one that uses its gross area deliberately while producing the amount, quality, and type of usable space the development requires. Daniel Inocente Architecture D.P.C. can assist owners and development teams in evaluating floor plates, cores, circulation, unit planning, and net-to-gross efficiency during feasibility and schematic design.

Sources

Whole Building Design Guide, National Institute of Building Sciences
Architectural Programming
WBDG Architectural Programming

Building Owners and Managers Association International
Floor Measurement Standards
BOMA Floor Measurement Standards

Building Owners and Managers Association International
BOMA 2024 for Office Buildings: Standard Methods of Measurement
BOMA 2024 Office Standard Overview

University of New Hampshire Facilities
Chapter 3: University Planning Standards
UNH Building Area and Net-to-Gross Standards

Tampere University / Journal of Asian Architecture and Building Engineering
Shanghai’s High-Rise Buildings: Exploring Space Efficiency, Structural Systems, Forms, Materials and Core Designs
Tampere University Research Portal

Journal of Asian Architecture and Building Engineering
Evaluation of Space Efficiency Criteria in High-Rise Buildings Based on Functions: A Case Study of Türkiye
High-Rise Space Efficiency Study

Buildings
Space Efficiency in North American Skyscrapers
North American Skyscraper Space Efficiency Study

Applied Sciences
Space Efficiency of Tall Buildings in Singapore
Singapore Tall-Building Space Efficiency Study

Journal of Building Engineering
A Study on Space Efficiency in Contemporary Supertall Mixed-Use Buildings
Supertall Mixed-Use Space Efficiency Study

Tampere University
Toward Context-Aware Vertical Urbanism: A Multidimensional Synthesis of Space Efficiency in Functionally and Formally Diverse Tall and Supertall Buildings
Global Tall-Building Space Efficiency Study

Journal of Sustainability Research
Space Efficiency as a Method-Dependent Construct: Evidence from High-Rise Timber Buildings
Space Efficiency Methodology Study

The Washington Post
Factoring in Floor Footage Plays Big Role in Building Design
Washington Post Net-to-Gross Discussion

FAQ

What is net-to-gross efficiency in architecture?

Net-to-gross efficiency compares a defined usable or assignable area with the gross area required to contain it. In architectural programming, the Whole Building Design Guide expresses building efficiency as net assignable square feet divided by gross square feet. Circulation, walls, mechanical and electrical spaces, and other support areas account for part of the difference.

What is a good net-to-gross ratio?

There is no universal percentage that defines a good building. The appropriate ratio depends on building type, measurement method, height, core requirements, structure, circulation, services, amenities, and program. Published high-rise studies show substantial variation between buildings and regions, which is why comparable methodology and building type matter when benchmarking.

How can an architect improve net-to-gross efficiency?

Efficiency can potentially be improved by coordinating the core, elevators, stairs, circulation, structural grid, shafts, floor-plate geometry, and program rather than optimizing each independently. The correct strategy depends on the project. A smaller core or shorter corridor should not automatically be considered an improvement if it creates problems with egress, vertical transportation, services, structure, accessibility, or usable-space quality.

Why does net-to-gross efficiency matter to developers?

It helps a developer understand how much gross building area is required to produce a particular amount of usable program. That relationship can influence feasibility, yield, construction budgeting, leasing assumptions, unit planning, and comparisons between architectural schemes. Building-area measurement is consequently used during planning and design for program control, cost analysis, and efficiency evaluation.

Is net-to-gross efficiency the same as rentable area?

No. Net-to-gross efficiency is a relationship between selected definitions of net and gross area. Rentable area is a property measurement concept calculated under an applicable methodology. BOMA maintains different standards for office, multifamily and hospitality, retail, industrial, mixed-use, and gross building areas, so the measurement basis should always be identified before comparing figures.