Rooftop Mezzanines in Montreal: What You Need to Know
Adding space without leaving your neighborhood is a challenge many Montreal homeowners face—especially those living on the top floor of a duplex or triplex. One option that often comes up is building a rooftop addition, commonly referred to as a “mezzanine.”
In this context, a mezzanine is a structure built on the roof that, under certain conditions, does not count toward the building’s official height. This makes it an attractive solution for expanding living space while staying within zoning limits. But while the idea sounds simple, the reality is far more complex. Between costs, regulations, and structural challenges, these projects require careful planning.
Another key consideration is that a mezzanine occupying less than 40% of the building’s floor area is not classified as an additional storey. This distinction allows a certified professional architectural technologist to prepare the plans and provide services for this type of project.
However, technologists are limited to buildings of up to two full storeys. If the existing building already has three storeys and you plan to add a mezzanine, the project must legally be handled by an architect in accordance with the Architects Act.
Why Consider a Rooftop Addition?
A rooftop mezzanine can:
Increase living space without relocating
Add natural light and outdoor access
Support gentle urban densification
Help families stay in central neighborhoods instead of moving outward
Structural Challenges, and why Anthem is the right fit.
Mezzanine projects are structurally demanding and require close coordination between architectural design and structural engineering from the very early stages. Decisions around layout, loads, and constructability are tightly interconnected, meaning that delays or misalignment between disciplines can quickly impact both timeline and budget.
At Groupe Anthem, these projects are a natural fit for our integrated approach. Because we handle both the architectural and structural scopes in-house, we’re able to streamline communication, anticipate constraints earlier, and develop more efficient solutions. This not only reduces coordination time but also helps control costs and minimizes revisions throughout the process.
From a structural standpoint, rooftop additions are rarely straightforward.
Depending on the building, we as engineers may consider:
Reinforcing the existing structure
Possible in newer buildings
Limited additional capacity
Load transfer to existing walls
Requires careful analysis
Not always sufficient
New structural system (most common)
Steel columns added through all floors
New footings installed in the basement
This last option is often the safest—but also the most disruptive.
It can involve:
Opening ceilings and walls on multiple floors
Work inside neighboring units
Significant coordination with occupants
Zoning and City Constraints
The first step is always verifying what is allowed on your property.
You’ll need to confirm:
Maximum building height
Number of permitted storeys
Specific borough regulations
Even if zoning technically allows an additional floor, approval is not guaranteed. If your building becomes significantly taller than neighboring properties, the city may refuse the project for urban integration reasons.
If a full additional storey is not permitted, a mezzanine (roof structure) may be possible—but only if it meets strict criteria.
In some boroughs, special approvals or minor variances may be required, which can extend timelines and increase costs.
What Does It Cost?
Rooftop additions are major construction projects—not minor renovations. As a general guideline, projects typically start around $275,000 to $350,000, even for modest additions.
For example:
A 35 to 55 m² (375 to 600 ft²) addition can quickly reach or exceed this range
Costs per square foot are often higher than ground-level additions due to complexity
Key Design Rules for Mezzanines
To qualify as a rooftop structure rather than a full storey, typical requirements include:
Maximum area: usually limited to about 40% of the floor below
Front setback: often around 2× the structure’s height
Rear setback: typically around 1× the height
These rules are intended to:
Reduce visibility from the street
Limit shadow impact on neighboring properties
Final Thoughts
Rooftop additions sit at the intersection of design, engineering, and regulation. While they offer unique opportunities, they also come with a high level of complexity.
Before moving forward, the best approach is to:
Evaluate zoning feasibility
Estimate realistic buildable area
Consult with a structural professional early
Align expectations with budget and constraints
With the right preparation, a mezzanine can transform a home. Without it, it can quickly become an expensive and frustrating process.
