House extension on a slab: what to know before starting?
- coraliefortin076
- Jul 1
- 5 min read
Updated: Aug 15
In Quebec, many homeowners choose to expand their homes rather than move. Whether it's to create an additional room, a garage, or even an intergenerational home, an addition is an excellent way to increase the value and functionality of a property. Among the crucial decisions to make in such a project is the choice of foundation type.
One of the most popular options, especially for affordable construction or renovation projects, is the floating concrete slab. Plan Maison Québec, a specialist in custom residential plan design in Canada, can help you better understand this technical choice and its implications. Here is how to do a house extension on slab in Quebec.
What is a floating concrete slab?
A floating concrete slab is a monolithic foundation, meaning it is poured in a single step, without joints or gaps. Unlike a footing foundation or a basement, it does not rest on deep foundation walls. It is laid directly on a well-compacted gravel base, sometimes covered with a waterproofing membrane and an insulating layer.
This "floating" structure is so called because it is not anchored deep in the ground, but rather rests on the surface. This allows it a certain flexibility in the face of ground movements, which is particularly relevant in Quebec, where freezing and thawing can cause the ground to heave (the famous winter frost).
Why choose a slab for your house extension?
When it comes to adding to a home, the choice of foundation depends on several factors: budget, soil type, land topography, the use of the new structure, and, of course, municipal bylaws. Plan Maison Québec assists its clients in this strategic assessment to ensure that each project is sustainable, compliant, and adapted to their needs.
A concrete slab is an effective solution for several types of additions, including:
A single-story extension without a basement
An attached or detached garage
An intergenerational dwelling
A workshop or home office space
Stages of construction of a concrete slab
Building a concrete slab requires meticulous attention to detail. Here are the main steps our experts at Plan Maison Québec consider when planning and designing:
Soil assessment
Excavation and grading
Installation of insulation and vapor barrier
Installation of ducts
Pouring of concrete
Curing and curing
Advantages of a concrete slab for an extension
Fast and economical construction
One of the biggest advantages of a concrete slab is its speed of execution. By eliminating the complex steps of deep excavation and foundation wall construction, construction times are shorter and costs are reduced. This makes the slab ideal for projects where budget is a determining factor.
Energy performance
With good peripheral and sub-slab insulation, this foundation can offer excellent energy performance. Concrete acts as a thermal mass, storing heat and gradually redistributing it, which is very useful in a northern climate.
At Plan Maison Québec, we recommend adding a radiant floor system, an unparalleled thermal comfort solution.
Moisture resistance
Unlike basements, which can be prone to water infiltration, a well-constructed slab with a vapor barrier and good drainage is less vulnerable to moisture. This makes it a sound foundation for habitable extensions.
Less risk of radon
Radon, a naturally occurring radioactive gas found in some soils, can more easily seep into basements. A well-sealed slab with a continuous membrane and sealed joints effectively limits this exposure.
Construction Costs: What to Expect for a Concrete Slab
The cost of a concrete slab extension varies depending on several factors: the surface area, the complexity of the project, the desired level of finish, the accessibility of the land, and the materials used. On average, in Quebec, the cost can be estimated between $200 and $350 per square foot for a turnkey construction on a slab. This price generally includes site preparation, the insulated slab, the structure, and basic interior finishes.
Note that elements such as underfloor heating, high-end glazing, or a complex roof can increase this estimate. One of the major advantages of a slab is its positive impact on the overall budget: it eliminates the costs associated with digging a basement and simplifies several stages of the project. At Plan Maison Québec, we help you maximize every dollar invested thanks to well-thought-out plans tailored to your financial situation.
Disadvantages of a concrete slab
No basement
The main disadvantage of a slab extension is that it doesn't allow for the creation of a basement. This limits storage space and prevents the possibility of creating additional rooms below the ground floor. For some projects, this is a major constraint.
Accessibility to services
Once the slab is poured, the integrated plumbing or heating pipes become difficult to modify. Proper planning is therefore crucial in advance. Plan Maison Québec ensures that all service locations are planned to avoid unforeseen future costs.
Ground movements
In clay or very wet soil, the slab may be more susceptible to seasonal soil movement, especially in the absence of adequate drainage. A good soil analysis and effective peripheral drainage are therefore essential.
Concrete slab vs. other types of foundations
There are other types of foundations, such as crawl spaces or screw piles. Each has its own advantages depending on the type of project. However, the floating slab is known for its simplicity, speed, and thermal performance, especially when used for a single-story addition.
At Plan Maison Québec, we guide you through comparing these options and recommend the solution best suited to your lot, budget, and needs.
Standards, subsidies and local considerations
In Quebec, some municipalities impose specific standards for slab foundations. Requirements may include concrete thickness, minimum insulation, or frost protection. Plan Maison Québec designs all of its plans with these regulations in mind to avoid unpleasant surprises when applying for a permit.
It's also worth noting that some energy efficiency programs, such as those offered by Hydro-Québec or Écohabitation, may offer grants if the slab insulation meets certain standards. Our team can also incorporate these criteria into your plans.
Conclusion : house extension on slab in Quebec
A concrete slab extension is an affordable, efficient, and durable option when well planned. This solution is particularly attractive for single-story projects, garages, or additional dwellings. However, it requires rigorous design and careful coordination between professionals.
At Plan Maison Québec, we have the expertise to design custom plans tailored to your needs, your lot, and your budget. We support you every step of the way, from the initial sketch to the final design, with an approach focused on quality, comfort, and compliance with Quebec standards.
Contact Plan Maison Québec today to get a free quote and personalized support for your project!
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