Home / Code & Permits / OBC key definitions
Plain-language guide · Beta
Key Ontario Building Code definitions
The words that decide which rules apply to your project: building area, storey, major occupancy, limiting distance and more. Each term is explained in plain language, with the Code reference so you can check the official wording.
How to use this page
Plain words first, official wording second
In the Building Code, a defined word has one exact meaning. It is shown in italics in the Code text, and the meaning can be different from everyday use. For example, building area is not the total floor area of a house.
The explanations below are our summaries, not the official definitions. Most terms are defined in Division A, Article 1.4.1.2. of the 2024 Building Code (O. Reg. 163/24), which took effect on January 1, 2025. You can read the official text free in Ontario’s 2024 Building Code Compendium.
Size and height Occupancy and use Fire separations Spatial separation Exits Construction type Existing buildings Compliance
Topic 1
Size and height
These terms decide whether a building is “small” (Part 9) or “large” (Part 3), and how many fire and structural rules apply.
Grade
The average level of the finished ground around the building, taken at all of its exterior walls. Many other measurements, such as the first storey and building height, start from grade.
Div. A 1.4.1.2.
Building area
The largest horizontal area of the building above grade, measured to the outside face of the exterior walls (or to the centre line of a firewall). Think of it as the biggest footprint above ground, not the total of all floors.
Building area is what the Code’s size limits use, for example the 600 m² limit for Part 9.
Div. A 1.4.1.2.
Gross area
The total area of all floors above grade, measured to the outside face of the exterior walls (or to the centre line of firewalls). The Architects Act and the Code use gross area to decide when an architect is needed.
Div. A 1.4.1.2.
Floor area
The usable space on one storey between the exterior walls and any required firewalls. Exits, shafts and similar vertical service spaces are left out. Occupant load and many fire rules work per floor area.
Div. A 1.4.1.2.
Storey and first storey
A storey is the part of a building between one floor and the floor (or roof) above it.
The first storey is the storey whose floor is closest to grade and whose ceiling is more than 1.8 m above grade. Everything below it is a basement.
Div. A 1.4.1.2.
Basement
Any storey below the first storey. Basements are not counted in building height, but they still count for fire safety, exits and many other rules.
Div. A 1.4.1.2.
Building height
Measured in storeys: the number of storeys between the roof and the floor of the first storey. Part 9 is limited to buildings of not more than 3 storeys. Zoning by-laws measure height in metres, often in their own way.
Div. A 1.4.1.2.
Mezzanine
A partial floor between the floor and ceiling of a room or storey, including an interior balcony. A mezzanine may or may not count as a storey, depending on its size and how open it is.
Div. A 1.4.1.2. · Div. B 3.2.1.1. / 9.10.4.1.
Topic 2
Occupancy and use
How a building is used matters more than how it looks. The major occupancy decides which Part applies and which rules follow.
Occupancy
The use, or intended use, of a building or part of it, for sheltering or supporting people, animals or property.
Div. A 1.4.1.2.
Major occupancy
The main use of a building or part of it. Smaller uses that support the main use are treated as part of it. The Code sorts major occupancies into groups:
- A Assembly (divisions 1 to 4)
- B Care, treatment or detention
- C Residential
- D Business and personal services
- E Mercantile
- F Industrial: F1 high, F2 medium, F3 low hazard
- G Agricultural
Div. A 1.4.1.2. · Div. B 3.1.2.
Occupant load
The number of people a building, or part of it, is designed for. It is usually worked out from the floor area and the use, and it drives exit widths, washroom numbers and fire alarm needs.
Div. A 1.4.1.2. · Div. B 3.1.17.
Suite and dwelling unit
A suite is a room or group of rooms used by one tenancy, such as an apartment, office or store unit.
A dwelling unit is a suite used as a home, normally with its own cooking, eating, living, sleeping and sanitary facilities. A house with a secondary suite has two dwelling units.
Div. A 1.4.1.2.
Post-disaster building
A building needed to provide essential services to the public after a disaster. Examples include hospitals, emergency response facilities, fire, rescue and police stations, and communications facilities such as radio and television stations. These buildings must be designed under Part 3 and Part 4, whatever their size.
Div. A 1.4.1.2.
Topic 3
Fire separations
These terms describe how a building stops fire and smoke from moving between rooms, floors and suites.
Fire separation
A wall, floor or other assembly that acts as a barrier against the spread of fire. A fire separation may or may not need a fire-resistance rating; the Code says when it does.
Div. A 1.4.1.2.
Fire-resistance rating (FRR)
How long, in minutes or hours, a material or assembly can hold back flame and heat in a standard fire test. Wall and floor assemblies are rated, for example 45 min or 1 h.
Div. A 1.4.1.2. · SB-2
Fire-protection rating
The rating for a closure, such as a fire door, fire shutter or wired glass. It is how long the closure can hold back flame in a standard test. It is not the same as a fire-resistance rating.
Div. A 1.4.1.2.
Closure
A device or assembly, such as a door, shutter or window, that closes an opening in a fire separation or an exterior wall.
Div. A 1.4.1.2.
Firewall
A special fire separation built of noncombustible materials that divides a building, or separates two buildings, and stays standing if the building on one side collapses in a fire. Areas on each side of a firewall can be treated as separate buildings for many rules.
Div. A 1.4.1.2. · Div. B 3.1.10. / 9.10.11.
Fire compartment
An enclosed space separated from the rest of the building by enclosing construction that gives a fire separation with any required rating.
Div. A 1.4.1.2.
Sprinklered
Equipped with an automatic sprinkler system that meets the Code. Being sprinklered often allows larger buildings, more openings and lower ratings.
Div. A 1.4.1.2. · Div. B 3.2.5.
Topic 4
Spatial separation
Spatial separation limits how fire can spread from one building to a neighbouring building. It is one of the most common reasons drawings get revised.
Exposing building face
The part of an exterior wall that faces one direction, from the ground up to the ceiling of the top storey (or the part of that wall belonging to one fire compartment). It is the wall area used in spatial separation tables.
Div. A 1.4.1.2.
Limiting distance
The distance from an exposing building face to the property line, to the centre line of a street, lane or public thoroughfare, or to an imaginary line between two buildings on the same property. It is measured at right angles to the wall.
The closer the wall is to the line, the fewer windows it may have and the more fire-resistant it must be.
Div. A 1.4.1.2. · Div. B 3.2.3. / 9.10.14. / 9.10.15.
Unprotected opening
A door, window or other opening in an exposing building face that does not have a rated closure. Wall areas that do not have the required fire-resistance rating can also count as unprotected openings.
The Code limits the total area of unprotected openings based on limiting distance and wall area.
Div. A 1.4.1.2.
Want the table worked out? The OBC code matrix builder has fields to record limiting distance, wall area and openings for each exposing building face.
Topic 5
Exits
The Code separates the route people take inside a floor area from the protected exit itself.
Means of egress
The continuous path a person uses to escape from any point in a building to a safe place outside. It includes exits and the routes that lead to them.
Div. A 1.4.1.2.
Exit
The part of a means of egress, including doorways, that leads from the floor area it serves to a separate building, an open public thoroughfare, or an exterior open space protected from fire. Exit stairs and exterior exit doors are typical exits.
Div. A 1.4.1.2.
Access to exit
The part of a means of egress inside a floor area that leads to an exit, such as a corridor or aisle. Travel distance rules apply to it.
Div. A 1.4.1.2.
Topic 6
Construction type
Part 3 tables allow combustible or require noncombustible construction depending on occupancy, height, area and sprinklers.
Combustible construction
Construction that does not meet the rules for noncombustible construction, such as wood framing. Most houses and many Part 9 buildings are combustible construction.
Div. A 1.4.1.2.
Noncombustible construction
Construction where fire safety comes from using noncombustible materials, such as concrete, masonry and steel, for the structure and other parts. Limited combustible elements are allowed.
Div. A 1.4.1.2. · Div. B 3.1.5.
Encapsulated mass timber construction
Construction using large timber elements, such as cross-laminated timber, that are protected (encapsulated) with fire-resistant materials like gypsum board. It allows taller wood buildings under Part 3.
Div. A 1.4.1.2. · Div. B 3.1.6.
Topic 7
Existing buildings
Work on buildings that already exist has its own vocabulary and its own Parts of the Code.
Construct
Under the Building Code Act, to construct means to do anything in the erection, installation, extension or material alteration or repair of a building. That is why many renovations need a permit.
Building Code Act, 1992, s. 1(1)
Renovation (Part 11)
Work on an existing building, such as material alteration or repair. Part 11 lets an existing building be renovated without bringing every part of it up to today’s Code, as long as performance is not reduced. It usually applies to buildings that have existed for at least 5 years.
Div. B Part 11 · 11.1.
Basic and extensive renovation
In general terms, a basic renovation keeps the existing building elements and replaces or alters parts of them, while an extensive renovation is so substantial that the work area is essentially rebuilt. Which one applies changes how much you must upgrade. Check the official definitions and Section 11.3. before you decide.
Div. B 11.3.
Change of use
A change in how a building, or part of it, is used. If the change increases the hazard, for example a house becoming a daycare or an office becoming apartments, a permit is needed even if no construction is planned. Part 10 sets the rules.
Building Code Act, 1992, s. 10 · Div. B Part 10 · Div. C 1.3.1.4.
Not sure if your project is a renovation, an addition or a change of use? Start the OBC code matrix builder. Its first step asks these questions and suggests which Part applies.
Topic 8
Compliance
How you show the building department that the design meets the Code, and who is responsible for it.
Acceptable solution
A requirement stated in Parts 2 to 12 of Division B. Following an acceptable solution is the usual way to show your design complies.
Div. A 1.4.1.2.
Alternative solution
A different way of meeting the Code that gives at least the same level of performance as the acceptable solution it replaces. It must be proposed with supporting documents and accepted by the chief building official.
Div. A 1.4.1.2. · Div. C Part 2
Designer
The person responsible for the design. Depending on the building, this must be an architect, an engineer, or a qualified designer registered with a BCIN, unless an exemption applies.
Div. A 1.4.1.2. · Div. C 3.2.
Who may design what, and when you need a permit, is explained on our Building Code Act essentials page.
Put the definitions to work
Use the free matrix builder to record your project data step by step. It shows these definitions next to each field and links to your municipality’s zoning by-law, map and building department.
Please read
Disclaimer
This page is general information written in plain language by LA Design Inc. It is not legal or professional advice and it is not the official text of the law. The definitions here are summaries and may leave out conditions that matter for your project. Laws and the Building Code change. Always read the current official version and confirm with your municipality’s building department, the Ontario Association of Architects, Professional Engineers Ontario, or a lawyer before you rely on it. LA Design Inc. accepts no liability for decisions made using this page. Not affiliated with or endorsed by the Province of Ontario, the OAA or PEO.
Beta page · Last checked November 2026. Spotted an error? Please tell us through our contact page.