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D4COST Software


Square Footage 

Measuring the Area of Buildings.
A Simple Task. Or Is It?

Arthur O'Leary, FAIA, MRIAI

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Inconsistency and confusion.
Although a square foot is a square foot in all buildings, the total square footage in a building can differ widely. Why? Simply because there are a number of different rules and methods in common use for calculating building areas. They are different because of the variety of needs and applications for the information. So, it is important that one knows the objective before calculating building areas or relying on quoted square footage.

As a result of the various business and personal interests affected by the method of calculating building area, a number of different standard systems are in use across the country. Most of these systems have been developed by the national professional societies that represent the interests and needs of their members.

Governments and code writing agencies have also defined square footage of buildings in various ways for the purpose of administering certain building and planning laws.

Square footage needed for the programming, design, and construction of buildings.
Square footage is an accurate and convenient way of expressing space requirements and for measuring the size of a space. This is the basis of all design programs. The same information is directly related to estimating the cost of a building or meeting a cost budget.

Construction cost statistics are generally expressed in terms of cost per unit of area, usually as dollars per square foot. These are average costs of high and low cost areas of a building and therefore all areas of the building must be included in the square footage. The cost per area system is only relevant, however, if everyone is using the same system for computing the area. Area-based cost calculations are used primarily for estimating the costs of buildings that are going to be built and therefore must include all elements of the building.

The American Institute of Architects has developed and published a standard method of calculating building areas. This system is normally used by all elements of the construction industry.

The generally accepted method for calculating building areas among architects and contractors is described in AIA Document D101-1995, “Method of Calculating Areas and Volumes of Buildings.”

This is the method by which the Design Cost Data case study cost breakdowns are predicated and construction cost statistics are developed.

Square footage needed for compliance with planning and building laws.
Many planning and building laws provide maximum building areas for certain types of buildings for certain specified uses and for determining parking requirements. When the code does not specify otherwise, most users interpret that to mean clear inside areas, excluding exterior walls. Some users go so far as to also exclude toilet rooms, machine rooms, corridors, stairways, and other adjunct spaces.

Some codes are explicit in their floor area calculation method, thereby leaving no room for expanding or limiting the code limitations or any other misunderstandings or self-serving manipulations.

Square footage needed for the sale of buildings.
In the real property field, real estate brokers often compare the value of properties and offer them for sale on the basis of their relative floor areas. Frequently, sellers tell their real estate sales people their understanding of the square footage and this, in turn, is quoted to prospective buyers, often without confirmation. The seller’s understanding of the building area may not always be realistic and in some cases may be intended to deceive an unwary buyer.

Sometimes, owners or their brokers attempt a field measuring of the building and, not always knowing the proper measuring method or appropriate geometry, they arrive at erroneous or questionable results. Also, there is no consistency in knowing what areas to include or exclude in the calculation.

Land area, on the other hand, is usually fairly easy to calculate except when the shape of the parcel involves complex geometry. But with buildings, buyers often become dissatisfied with their purchases when they cannot later reconcile the area of the building by simply measuring and calculating. Some buyers will go for years not knowing of the great bargain that they made, predicated on erroneously understated square footage information. Or, it could as well have been that they paid too much for less square footage than they were led to believe they were getting.

Square footage needed for the leasing of office and retail sales spaces.
Offices, retail sales spaces, factories, and warehouses are generally rented or leased on the basis of building area, expressed in dollars per square foot per month or year. The lessee would like to pay only for the useable space while lessors would prefer to predicate rent on the gross building area including walls, columns, and shafts. There is much negotiation and manipulating of area figures on both sides of the transaction.

Some lessors market their space on the usable space basis, sometimes adding in the lessee’s proportionate share of common areas such as corridors, stairways, elevators, toilet rooms, and machine rooms. 

In office building rental and leasing activity, a standard method has been developed that is widely used by owners and managers of office buildings. The method is based on the lessee being charged rent for exclusive use areas as well as a proportionate share for spaces used in common with other lessees. The common use areas would include such areas as building corridors, stairways, elevators, toilet rooms, machine rooms, and structural elements. 

The most commonly used method, called the BOMA Standard, has been developed and is published by the Building Owners and Managers Association. “Standard Method for Measuring Floor Area in Office Buildings.” Copies may be purchased from BOMA Publications, TASCO, Inc., P.O. Box 753, Waldorf MD 20604. 

The BOMA standard gives definitions for such arcane terms as Gross Building Area, Gross Measured Area, Building Measured Area, Floor Rentable Area, Floor Usable Area, Usable Area, Floor Common Area, Basic Rentable Area, Building Common Area, Rentable Area, Office Area, and Store Area. 

Since the system is complex, and should be accurately applied, some architectural firms provide a service to lessees and lessors of field measuring existing office buildings and calculating the square footage. They also provide the same service for proposed buildings, working from the construction drawings. 

Square footage needed for the marketing of new single family dwellings. 
The competition among merchant home builders is very keen and it is in their interest that they and their competitors use a uniform method of computing floor areas in their advertising and marketing. 

The American National Standards Institute (ANSI) has developed a standard method for calculating floor areas in single family dwellings, “Square Footage - Method for Calculating,” ANSI Z765-2003. Copies may be purchased from National Association of Home Building, Research Center, 400 Prince George’s Boulevard, Upper Marlboro, Maryland 20774. This standard is not suitable for use in apartment buildings or multiple dwellings. 

The National Association of Home Builders has supported this voluntary standard among its members since its inception in 1996. The second edition was promulgated in 2003. Although some of the largest home builders have adopted the standard, many have not. Some builders publish and use their own standard. 

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