Code Compliance For Enlarging Existing Buildings – Beware the Exceptions

By Austin Regan

We have touched on Administrative Code section AC 28-101.4.5 “Work that increases existing floor surface area of a prior Code building by more than 110 percent.” This section, introduced in the 2014 Code, clarifies that when enlarging a building built under a previous Code, if the enlargement increases the size of the building more than a certain amount, then the application will be treated as a new building and the entire building will be required to follow the 2014 building Code.

As the title suggests, once you more than double the size of the building, it will be treated like a new building. Titles can be misleading. The trigger for 2014 new building compliance can occur in a much smaller enlargement depending on the proposed work and how it is performed.

The measurement to determine when one exceeds the 110% threshold is the amount of “floor surface area.” This is a defined term. “Floor surface area is the gross square foot area of all horizontal floor and roof surfaces, including roofs of bulkheads and superstructures, including, cellar, attic and roof.” That sounds pretty generous since cellars and attics are included. The Department must have thought so too because they added the following exceptions which clarified what counts as existing floor surface area.

Exception: “… the following shall be excluded from the measured square footage of floor surface area:

1.       The square footage of floors removed during the course of work when such floors are removed together with the supporting beams, joists, decking slabs on grade.”

Take the hypothetical case of a two story building with a cellar. Per the definition of floor surface area, this building has four levels of existing floor surface area. Assuming all levels are of equal size, one could enlarge this building to six stories (the new roof counts as a level). If the new floor plates were smaller in size then the existing, the building could theoretically add more stories since the ultimate determinant is total square footage not number of levels.

Now, when analyzing the same two story with a cellar, if it was determined that the roof structure needed to be replaced because it was not adequate to serve as a floor and the cellar needed to be lowered to provide proper headroom. Now, the only existing floor surface area that would count would be the first and second floor. The 100% enlargement threshold is reached once the new cellar floor slab is poured and the roof structure replaced. In this case no additional levels can be added without the building needing to comply completely with the 2014 Code ( Note that the threshold is 110% not 100% but we are using the latter to make the math simpler).

There is a second exception.  This one disallows any new floor surface area that was installed prior to the application for enlargement if the application related to that work is signed off less than 12 months prior to the submittal of the new application.

So architects and engineers must be very careful when specifying the removal of any floor structures, roof structures or slabs on grade. In the case of enlarging our example building, changing from a two story building to a 5 or 6 story may trigger Code requirements under previous Codes for an elevator or a second egress stair. The partial floor removals to accommodate these items would also reduce the total amount of existing floor surface area used to calculate compliance with AC 28-101.4.5.

So if approached incorrectly, straight removal and  replacement of floor and roof structures could lead to an existing building having to fully comply with the 2014 Code. Value Engineering exercises that determined that it was easier to fully replace a floor structure as opposed to reinforcing it can have severe implications on the ability to enlarge or even modify a building under a previous Code.