Programming and Analysis: How to Size a Building

With the construction of increasingly taller buildings, how does one determine how large a building can be? That comes down to the codes. There is not one but many codes to reference for the design and construction of a building. For sizing a building, there are a few main codes to reference- zoning code, the IBC building code, and local codes/deeds that may include provisions for easements and the like.

Easements determine if there are any utilities or other services for the public or special site conditions that necessitate the dedication of a portion of private land for its function. These often restrict blocking, removing, or building on certain areas.

Zoning codes are municipal codes that determine the density and character of cities-usually more urban areas. Zoning codes spell out the ability to build on a lot including the floor area ratio (FAR) and building height limitations. FAR determines how much a site can be built upon, which can start to determine the number of stories, etc. For example, in an R-1 residential zone, there may be a FAR of 0.5. R-1 zones are low-density, typically detached single-family homes. A FAR of 0.5 indicates that the building can only occupy 0.5 or 50% of the lot square footage. So, if there is a lot of 2,400 square feet, the allowed floor area of the building is 1,200 square feet. Zoning also indicates how much of the lot can be developed, including the width of front, rear, and side yards, etc. The FAR is different from the footprint of the building. If the footprint of the building is restricted to 600 square feet (as an extreme example, but it makes the math easy) and the building is allowed 1,200 square feet, does that mean I have to give up 600 square feet? No. It means that the building can be two stories (or more) with a total of 1,200 square feet for the building. What counts toward FAR is defined in the definitions of the zoning code, since spaces like mechanical rooms are typically not counted toward the FAR.

However, there are often height limitations. Those are included in the zoning code along with restrictions due to the sky-exposure plane, height of street walls to maintain, occupancy classification, types of construction, building frontage, the requirement of sprinklers, parking requirements, etc.

The IBC building code also includes provisions for height and area limitations due to the type of construction, occupancy, and sprinklers.

These two codes work hand in hand to define the buildable area for a structure and should be referenced at the beginning of the process. The most stringent code is the one that takes precedence and codes-especially zoning codes-change. It is very common practice for areas of a municipality to be rezoned, allowing for different occupancies/mixed-use program, and constructing taller buildings. Designing a building is a network of information and knowing how the zoning code interfaces with the building code and vice versa is the strongest start to the process.