Determining occupancy is the first and most fundamental step in any building code evaluation. 

Without knowing your occupancy, you cannot determine what part of the Building Code to apply (Part 9 or 3) and nearly every aspect of design governed by building codes is affected by occupancy.  This post focuses on the British Columbia Building Code (BCBC), however, Canada’s National Building Code (NBC) is the code after which most provincial and territorial building codes are modeled and all have similar requirements around occupancy. 

Please keep in mind reading that this is an area of the Code that is interpreted very differently across provinces, and even jurisdictions in the same province. This post summarizes my personal experience when dealing with occupancy.

The Building Code defines occupancy as “the use or intended use of a building or part thereof for the shelter or support of persons, animals or property.”  Occupancy is often informally referred to as “use” to imply what a building will be used for.  The type of use intended for a building has great bearing on how that building must be constructed and our building codes are the rule books we must follow when constructing buildings. 

When broken down into their most basic form, all building codes are simply documents with a set of rules that describe what you can and cannot do when constructing or renovating a building.  This set of rules is guided mainly by two things:

1. Occupant well being (fire safety, public health, etc.), and
2. The preservation/durability of the building (resistance to damage  as a result of seismic, fire, weather, and other forces). 

To a large extent, the rules are guided by levels of risk and each occupancy has a unique set of risks.  These different levels of risk are mitigated by imposing stricter constraints on buildings perceived to have higher levels of risk.  A facility with biohazard testing laboratories and a single-family residence could be considered two opposite ends of the spectrum.  The activities present in one are considered extremely hazardous and a risk to public and occupant safety whereas the other is considered low risk due to the nature of activities typical in a home and the occupants’ familiarity with the building.  Naturally, one would expect that occupancies with higher levels of risk be constructed to a different standard than those with lower risk. 

Consequently, occupancy plays a huge role in how every building is constructed and what set of rules it needs to follow.

Major, Minor, Subsidiary. What’s the Difference?

The Building Code distinguishes between major occupancy and subsidiary occupancy, sometimes referred to as a minor, ancillary, or supporting occupancy.  Major occupancy is defined as, “the principal occupancy for which a building or part thereof is used or intended to be used, and shall be deemed to include the subsidiary occupancies that are an integral part of the principal occupancy.” 

The major occupancy classifications are:

• A1 – Assembly occupancies intended for the production and viewing of the performing arts 
• A2 – Assembly occupancies not elsewhere classified in Group A 
• A3 – Assembly occupancies of the arena type 
• A4 – Assembly occupancies in which the occupants are gathered in the open air 
• B1 – Detention occupancies in which persons are under restraint or are incapable of self-preservation because of security measures not under their control 
• B2 – Treatment occupancies 
• B3 – Care occupancies 
• C – Residential occupancies 
• D – Business and personal services occupancies 
• E – Mercantile occupancies 
• F1 – High-hazard industrial occupancies 
• F2 – Medium-hazard industrial occupancies 
• F3 – Low-hazard industrial occupancies

In today’s ever-growing world of complexity, buildings are no exception, and neither is the way is which we use them.  Mixed-use buildings are quite common and there is no limit to peoples’ imagination when it comes to proposing combinations of different uses under one roof. 

The Building Code can limit what occupancies can coexist together so it’s important that when a client proposes a use for a building, we understand what occupancy classification it falls under.  The basic nuts and bolts of how Part 3 buildings are constructed are dictated by the construction articles in Subsection 3.2.2.  The details for everything else are covered under all other parts of the Code. 

Construction articles consider major occupancy(s) only.  Subsidiary occupancies, although there may be many in one building or suite and they may contribute greatly to the function of the building, are not considered and do not factor into what set of construction “rules” the building must follow.  It should be clear by now that interpreting between major and subsidiary occupancy is an important distinction.

So how do I know if I am dealing with a major or subsidiary occupancy?  For a lot of buildings, this can be obvious.  Some buildings, however, are a little more complicated and the nuances between major and subsidiary occupancy may not be so clear. 

Going back to the definition of major occupancy, the Code states that major occupancy is the “principal occupancy” and that this includes subsidiary occupancies that are “an integral part of the principal occupancy.”  Subsidiary occupancy is not a defined term, nor is there any additional information provided in the Notes (formerly the ‘Appendix’) on the distinction between major and subsidiary. This lack of supporting information is most likely the reason between the variation of interpretations.

One way to think about it is to ask yourself the question, “Would Use B exist if Use A did not exist?  For example, would the set of three office rooms exist if the car manufacturing plant did not exist?  If the point of those offices is to serve the manufacturing plant, the answer is clearly no.  It is rare that one would ever encounter a large manufacturing plant (F2 occupancy) without the inclusion of at least some office spaces (D occupancy).  A factory like this may also have a cafeteria (A2), or even a gift shop with memorabilia (E).  If the building is owned and operated by one tenant, all subsidiary occupancies (A2, D, & E) typically have no affect on how the building is classified and constructed under 3.2.2. It should be noted that there are differing interpretations of this across jurisdictions, so it is always best to verify with the local authority.

Although subsidiary occupancies do not have an impact on the 3.2.2. classification, keep in mind they are still subject to any occupancy specific requirements, mainly those outlined in Subsections 3.3.2. to 3.3.6.

Change in Occupancy

Another instance where occupancy can trigger building code compliance is when there is a change to the occupancy in a building.  This is a very common occurrence in old buildings whose use is no longer culturally or practically relevant.  Some examples include converting silos into apartment buildings, grain elevators into museums with a restaurant and gift shop, or churches into homes or office buildings. 

In fact, the very first article in Division A of the Code [specifically 1.1.1.1.(1)(c)] makes it clear that the Code, and all its provisions, apply to a change in occupancy of any building.  This means if one were to propose a new use in a building, even if there was not a single change in the layout or any physical characteristic of the fixed and permanent aspects of the building, for all intents and purposes, the building is considered to have a new associated level of risk and must follow a new set of rules that prescribe how the building must be constructed. 

A building owner may want to keep everything as is and simply apply a fresh coat of paint and add new furniture but the AHJ will still require a building code review to determine if the existing building is safe to use for the proposed new use.  

Accessibility and Occupancy

Since 1941 when the National Building Code was first introduced in Canada and later in 1987 for the BC Building Code, building codes throughout our country have moved towards more inclusive design for persons with disabilities.  Today it is referred to as barrier free or accessible design and is defined in the Code as, “an area and its facilities, or both, as required by this Code, which is easy to approach, enter, exit, operate, participate in, pass to and from, and use safely and independently by persons with disabilities.” 

Accessible design is generally considered to be required in all buildings today but there are many exceptions, some of them having to do with occupancy.  Article 3.8.2.1. describes these exceptions.  This is one instance where the Building Code makes no distinction between major and subsidiary occupancy.  In most cases, all types of occupancies, whether major or subsidiary, must be accessible.  A subsidiary use, although perhaps of no significance to the combustible or noncombustible construction of the building, is still considered integral to the overall function of the building and must be designed to allow all occupants, whether disabled or not, to use them. 

In some cases, accessibility is not considered practical such as in some industrial buildings that have large quantities of dangerous materials or where a certain level of mobility is required to ensure the safety of its occupants.  In these cases, accessibility may not be required for some spaces, however, it should be provided for subsidiary occupancies such as offices, lunchrooms, showrooms, and other spaces where it would be reasonable to provide.

I hope that this explanation sheds some light on the topic of occupancy and has helped you gain a better understanding of its crucial role in building construction and safety.

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