Frequent misunderstandings arise when hospital leaders, department staff, planners, and architects confuse net square feet with gross square feet (or net square meters and gross square meters). It is particularly disturbing when facility planners and architects simply specify “square feet” in their documents.
Space in “square feet” may be any one of the following:
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- Net square feet (NSF) is typically used in a room-specific space program and refers to the usable or assignable square footage within a room or area (inside wall-to-wall dimensions). Department net square feet (DNSF) refers to the sum of the net square footage of all rooms or assignable areas within the department.
- Department gross square feet (DGSF), typically used in the preparation of block schematic drawings, represents the actual footprint of a specific department or functional area. This includes the net square footage of all rooms/areas within the department plus the space occupied by intradepartmental circulation and the walls and partitions within the department. DGSF excludes shared public corridors and atriums, elevators, stairs, space occupied by the building’s exterior wall, and major mechanical spaces. The term component gross square feet (CGSF) is interchangeable with DGSF.
- Floor gross square feet (FGSF) and building gross square feet (BGSF) represent the overall footprint of a floor or building, respectively, and include shared public corridors and atriums, elevators, stairs, the space occupied by the building’s exterior wall, and major mechanical spaces.
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SPACE CONVERSION FACTORS
During the pre-design planning process, space conversion factors are used to translate the space program into meaningful block schematic drawings as location issues are being resolved, and to estimate renovation or construction costs before design. To convert the sum of all net spaces to an estimate of the actual department footprint, a net-to-department gross space conversion factor is used. Net-to-department gross space conversion factors generally range from 1.20 to 1.50. Large open spaces that do not involve maneuvering patients on stretchers, such as those usually planned for building support services, have a lower factor since there are a limited number of small rooms and minimal access corridors. A surgery suite would have a larger factor to account for numerous and variously sized rooms that must be connected by eight-foot-wide corridors to accommodate patient stretchers. The factor for a specific functional area also varies depending on whether new construction is planned (lower factor) or if the function is to be retrofitted into existing space with specific design constraints (higher factor). Physical constraints — such as the shape of the existing building envelope, minimal bay width, unusual column spacing, and fixed mechanical spaces and shafts — may necessitate a larger DGSF (DGSM) to accommodate the same amount of NSF (NSM) than in new construction.
Additional factors are used to estimate the overall size of the floor or building footprint to prepare pre-design construction or renovation cost estimates. An additional 20-30 percent (or a factor of 1.20 to 1.30) is generally used to estimate the floor gross square feet (floor gross square meters). This factor accounts for shared public corridors, elevators, stairs, and the space occupied by the building’s exterior wall. To estimate the total building gross square feet (building gross square meters), an additional 8 to 12 percent (or a factor of 1.08 to 1.12) may be required to allow for major mechanical spaces and a central power plant depending on the scope of the project and existing capacity. These factors do not account for unique design features such as atriums and interior courtyards. Ultimately, the actual design will determine the final space requirements. However, if the eventual design affords a very large net-to-gross space conversion factor and there are no special architectural features, the overall efficiency of the design should be questioned.
ACADEMIC INSTITUTIONS
To confuse the matter further, universities often use the term “assignable square feet” or ASF to describe rooms and areas available for assignment to an individual program, department, or user. They then use the term “net assignable (or usable) area” to describe the combined total of the ASF and the common or shared areas.
COMMERCIAL REALTORS
Commercial realtors use a slightly different set of terms that need to be understood if a healthcare organization wishes to lease space off-campus. The Building Owners and Managers Association International (BOMA) uses the term “usable square feet” to define the footprint of the space that is under the tenant’s direct control and then applies a factor for the tenant’s share of the common areas to arrive at the “rentable square feet” on which the tenant will pay rent. In typical multi-tenant, multi-story buildings, the common area factor can range from 14 percent to 16 percent. In smaller buildings with fewer amenities and smaller lobbies, the factor ranges from 10 percent to 12 percent. The International Facilities Management Association (IFMA) has developed its own definition of rentable square feet, which facility managers also use to allocate or charge-back square footage to specific departments.
COST ESTIMATING
It is important to always define how square feet (meters) are calculated when comparing or estimating construction costs. Department gross square feet are used in the preparation of renovation costs for individual departments. Floor gross square feet or building gross square feet are generally used in the preparation of predesign construction cost estimates for new construction or major renovation projects.
CONCLUSION
Misunderstandings among members of the planning team can be disastrous. For example, confusing department net space with department gross space can deem certain facility configuration options feasible when they are not or can result in inaccurate early cost estimates. Physicians and clinical department managers may ask their peers at other institutions for comparative space information, receive “square feet” with no indication of how it was calculated, and then use this information to demand that their existing space be enlarged. Knowing how space allocation is calculated also eliminates surprises when leasing space off-site. In addition, net and department gross square feet (meters) used for facility planning should not be confused with other methods of space measurement used by finance to account for charge-backs to individual departments, cost reimbursement, and asset tracking.
This article is an update of a previous post.