Canada Builds

Height, Area, and Use

In order for a building to be classified under Part 9 of the Ontario Building Code there are three conditions that must be satisfied:


  1. The building must be no more than 600 m2 in building area
  2. The building must be no more than 3 Storeys in building height
  3. The building must be of C, D, E, F2 or F3 occupancy


Items 1 and 2 are easy to determine if one has a clear understanding of the terms ‘Building Area’ and ‘Building Height’. Item 3, occupancy, is determined by identifying the use of the building (or part of the building) and assigning a letter designation according to the code. A description of occupancies and their corresponding designations can be found in Part 3 on OBC Table

A more detailed description of occupancies and their letter designations can be found on the Major Occupancy Classifications Page. If any of the uses are classified as Assembly ‘A’, Industrial ‘F1’ or Care and Detention ‘B’, then the building must be designed according to Part 3 of the Ontario Building Code.

The description of major occupancies permitted under Part 9 can be found on OBC Table

Building Area

Building Area does not refer to Gross Building Area which is the addition of all floor areas above grade. The calculation of Gross Building Area is made for other reasons and does not apply to the determination between Part 9 and Part 3 buildings.

Gross Building Area is used for purposes such as:

·        Determining the need for an Architect or Engineer. Generally, any building exceeding 600 m2 (6460 ft2) in gross building area or any building exceeding 3 storeys in building height requires the services of an Architect or Engineer. Refer to Table

·        Determining the ratio of floor space to lot area. This is a zoning requirement and is used to determine what is referred to as GFA (Gross Floor Area). In some jurisdictions it is also referred to as FSI (Floor Space Index). Whatever the term, the calculation is the same – a ratio of the Lot Area to the Gross Building Area. Note here how words and syntax are used in interchanging ways. In the building Code, the term is ‘Gross Building Area’, for zoning purposes it is ‘Gross Floor Area (GFA)’. When the building code refers to 'Floor Area', the definition is different.


Figure 1                showing a building with 600 m2 of Gross Building Area (200m2 per floor)

The building Area in the example above is 200 m2 and the Gross Building Area is 600 m2. The best way to think of building area is to imagine a bird’s eye view of the building. For instance, when the second floor is overhanging the first floor, it needs to be counted as part of the overall building area. Consider the following example:

Figure 2                Building Area Calculation

In the diagram above, the building area is 240 m2 and the Gross building area is 600 m2. Note that this building is the same size as the building shown in Figure 1 but the ‘Building Area’ is larger.



Consider the same building in Figure 2 above with a wall thickness. For the purpose of making the calculations easier, say the wall thickness is 500 mm. This would create interior dimensions of 19 m x 9 m for a total floor area of 171 m2. This value of Floor Area would be used for various calculations such as occupant load, and allowable mezzanine area. The building in Figure 2 then has three very different numbers associated with area descriptions:


Gross Building Area              600 m2

Building Area                        240 m2

Floor Area                            171 m2    (for one floor with wall thickness of 500 mm)


Note these calculations and how they are derived because they will need to be used for any number of different situations and for different purposes. When reading a question or trying to solve a problem, check the definitions in the building code for ‘keywords’ that form the question. In this case, an important distinction is made between floor area, building area, and gross building area.

Floor area in the examples above is essentially the area of the floor to the inside face of exterior walls.

Building area is essentially the largest floor area (including all walls)

Gross building area is essentially all floors combined.


Consider the following building descriptions below and indicate which are classified as Part 9 and which are classified as Part 3


A       An unsprinklered 3 storey Warehouse (F3) with a building area of 500 m2


B       A restaurant with a building area of 400 m2 and seating capacity of 46


C       A 3 storey Concrete Apartment building with an area of 400 m2 per floor


D       An unsprinklered wood frame Restaurant with seating for 25 people


Building Height

Building Height means the number of storeys contained between the roof and the floor of the first storey.

First Storey means the storey with its floor closest to grade and having its ceiling more than 1.8 m above grade.

Grade means the average level of proposed or finished ground adjoining a building at all exterior walls.


These three terms – building height, first storey, and grade are defined terms in the building code (OBC Determining building height requires that the first storey and grade are properly identified. Consider the following example:


Figure 3                Building Height in relation to Grade

Both of these buildings seem identical in size. The 3 storey building on the right however is lowered slightly further into the ground than the 4 storey building. This minor adjustment has a significant impact on how the building needs to be considered and built. The 4 storey building would need to be considered under Part 3 and the 3 storey building could be considered under Part 9. The implications in terms of cost are significant. This is why an accurate determination of grade and height is important to identify early on in the design process.