Extension to a floor or width: comparison of costs, floor constraints and choices to make
- coraliefortin076
- Oct 1
- 11 min read
When the time comes to expand their home—whether it's a house, a cottage, or a garage—many homeowners hesitate between two major options: expanding vertically (adding a story) or expanding horizontally (in width). This choice isn't purely aesthetic: it involves significant differences in terms of costs, technical constraints, foundation and soil requirements, building permits, and architectural integration. At Plan Maison Québec, we frequently encounter these questions among our clients, and this article aims to shed light on them so you can make the best choice for your situation.
We'll first examine the typical costs for these two types of expansion, then the soil and structural constraints to anticipate, finally the determining criteria for choosing between the two approaches, and some practical recommendations. In conclusion, I invite you to contact Plan Maison Québec to bring your project to fruition with rigor and expertise.
Cost comparison: floor vs. width
One of the first things a homeowner thinks about is, "What will be the cost difference between expanding upstairs or down?" The answer depends on many factors—finishing, foundation, existing structure—but some range estimates can be given to guide the thinking.
Cost ranges according to the type of expansion
According to RénoAssistance data, a "raw" floor addition costs around $195 to $235/sq ft, while a "turnkey" floor addition costs between $225 and $270/sq ft. For a turnkey extension on a foundation or slab, the cost ranges from $265 to $310/sq ft.
Other specialized sources in Quebec provide similar or complementary ranges. For example, the Soumission Rénovation blog mentions that a side or rear extension can cost between $200 and $500/sq ft, depending on its complexity. For a full floor addition, the same source cites costs between $175 and $400/sq ft.
Overall, the cost of a residential addition in Quebec often ranges between $150,000 and $300,000, depending on the scope of the project.
These figures help to understand the scale of the investment - but they remain estimates, and the particularities of the land, the existing structure, access, finishes, etc. must be taken into account.
Why does adding a floor often cost less per square foot?
A key point to understand: per square meter (or square foot), adding a story can sometimes cost less than a horizontal (width) expansion for an equivalent surface area. Why?
Less new foundation: To expand in width, it is often necessary to build new foundations or load-bearing walls on the ground, which involves excavation, formwork, concrete pouring, drainage, etc. In contrast, adding a story takes advantage of the existing foundation (with reinforcements often required, but less open ground work).
Less excavation and backfilling: Horizontal expansion often requires earthworks, adjacent excavation, and backfilling around the perimeter, which generates machinery and site development costs.
Optimization of the vertical structure: Elevation allows for the design of floors and walls that integrate into the existing structure, which sometimes reduces the need for additional materials (fewer foundations, fewer exposed walls).
Economies of scale: For a vertically larger project, certain fixed costs (architect, engineering, permits) can be amortized over a larger area.
That said, adding a floor imposes technical constraints (reinforcement of load-bearing walls, structural compliance, modifications to existing staircases, etc.), which can bring the total cost to a comparable level, or even higher in some cases.
Estimated differential cost according to typical cases
Let's take a few illustrative examples (these figures are indicative—your project may vary):
A 1,000-square-foot extension on a turnkey foundation could cost $265,000 to $310,000 (at $265–310/square-foot).
The same extension spread over several floors (i.e., without enlarging the footprint) could, with reinforcements and a high-end finish, cost $225,000 to $270,000, depending on structural capabilities.
If the site is difficult (rocks, unstable soil, slope, formwork constraints), horizontal extensions can incur significant surcharges, making the one-story option more competitive.
On the other hand, if the existing house is structurally weak or does not allow for a vertical addition without major alterations, the horizontal option, despite its foundation costs, may be the only viable one.
At Plan Maison Québec, we always recommend obtaining precise estimates based on each project, rather than relying solely on generic figures.
Comparison table: extension in floors and width
Criteria | Two-story extension | Enlargement in width |
Average cost per square foot | $195 to $270/sq ft (depending on finish) | $265 to $310/sq ft (with foundation) |
Necessary foundations | No (structural reinforcements possible) | Yes (formwork, excavation, drainage) |
Groundworks | Low (work concentrated on the structure) | Important (excavation, backfilling) |
Impact on the yard/field | No impact | Reduction of outdoor space |
Soil constraints | Less sensitive | Very sensitive (soil type, slope) |
Zoning and Regulations | Maximum height, easements | Setback margins, footprint |
Aesthetics / integration | Changing the building's silhouette | Must harmonize laterally with the existing |
Disruption during works | Possible if interior modification | No more external impact, construction site access |
Access to new parts | Via a staircase (addition or modification) | Direct access to the ground floor |
Recommended use | Bedrooms, offices, bathroom | Kitchen, living room, living room extension |
Energy efficiency | Less efficient (vertical exposure) | Better thermal inertia |
Resale / Added value | Good, if well integrated | Very good, especially with usable basement |
Soil constraints and ground conditions
The soil your property sits on plays a key role in deciding whether to expand horizontally or vertically. Stable soil can make it easier to expand in width, while difficult soil can make it easier to add more floors. Here are the main constraints to consider.
Soil types and impacts on foundations
Rocky ground or outcropping rock
If your land is rocky or has rocky outcrops, horizontal excavation for foundations becomes very expensive. Drilling, blasting, or clearing the rock increases the costs of foundations, formwork, and connections. In this context, a multi-story extension is often preferable to limit open ground work.
Expansive clay soil or unstable soil
Expansive clay soils (which expand or contract depending on humidity) or undersized soils require special foundations (wide footings, piles, deep foundations). If the width expansion requires new foundations, these reinforcements can increase costs. Adding a story also imposes additional loads, but these are distributed over the existing foundation—as long as it can be reinforced.
Silty, sandy or micaceous soil
These soils may require appropriately sized footings or screw piles to ensure stability. Horizontal expansions often require excavation and drainage, which adds cost. Vertical expansions, on the other hand, require less new foundation work, but load-bearing capacity analysis is crucial.
Slope or gradient of the land
Sloping land poses a greater challenge for a horizontal extension, as excavation, soil retention, and landings can complicate the work. Here, adding a floor can be a simpler alternative to gaining space without affecting the slope of the site.
Existing foundations and structural compatibility
Before considering a story addition, it is important to ensure that the existing foundations (footings, foundation wall, heat sinks, drainage) can support the additional load. This often requires structural studies and an in-depth inspection (cracks, settlement, moisture). If the current foundation is weak, reinforcements, micropiles, or enlarged footings will be required, which may reduce the advantage of the vertical option.
In the case of a horizontal expansion, it is also important to ensure that the new foundations are compatible (same type, same level, same frost resistance, same drainage) to avoid cracking between the old and new buildings.
Access conditions, underground infrastructure and legal constraints
When expanding horizontally, the work often involves dealing with underground networks (sewers, pipes, cables, existing drains). This infrastructure must be located and possibly relocated, which increases costs. The vertical option often avoids this type of intervention, since it remains within the existing footprint.
Furthermore, some municipal or zoning regulations impose setbacks, land use coefficients, or maximum heights. If your land is subject to such restrictions, horizontal expansion may be limited or impossible, leading to a vertical option.
Another constraint that should not be overlooked is the geotechnical impact: differential settlement between the existing structure and the new extension can cause cracks or deformation if the soils are not properly reconciled.
Choice between floor or width: determining criteria
After looking at the costs and floor constraints, let's examine the key criteria that should guide your choice between extending upstairs or in width.
1. Land availability and size
If your lot has sufficient lateral room, a horizontal extension may be feasible. However, if the backyard, setbacks, or easements limit the space, a vertical extension may be the only viable option for adding living space.
2. Purpose of the expansion (intended use)
If the extension is for a large room like a kitchen, living room, or den, the cost per square meter increases. Adding a floor often allows you to reserve the ground floor for heavy-duty uses and add lighter rooms upstairs. Furthermore, if maximum space savings are required with minimal disruption to the landscape, the vertical approach is attractive.
3. The aesthetic and architectural imprint
The extension must harmonize with the existing architectural style. If you are extending in width, you must carefully coordinate the materials, roofs, and volumes so that the extension does not appear grafted on. Adding a floor may require changes to the roof, slopes, or even adjustments to the ground floor, but often allows for a more visually integrated approach.
4. Disruptions and duration of works
Horizontal extensions often require major earthworks, foundations, and excavation work, which can cause major disruptions (roadway, access, excavation, backfilling). Adding a floor, although more technical, can be built in a certain isolation from the site, limiting the footprint of the groundwork. This can accelerate certain phases and limit the impact on the immediate environment.
5. Energy efficiency and insulation
A horizontal extension on a foundation allows for more uniform insulation, better management of thermal bridges, and better floor control. An extension on piles (for example, an elevation) can be more heat-hungry (less inertia, more exposure), which requires high-performance insulation materials, heated floors, or other techniques to prevent heat loss.
6. Resale potential and added value
A well-designed extension, whether horizontal or vertical, can increase the value of your property. However, an extension on a foundation offering a basement or greater structural homogeneity can sometimes have a better return upon resale. Similarly, a well-integrated upper floor, which maintains balanced proportions, can add a certain charm. However, it's always worth checking how extensions are valued in your area.
7. Overall budget and room for maneuver
If you're on a tight budget, the easiest option to budget for is often a modest horizontal expansion (bump-out, slight lateral extension). If your budget is higher and you want to maximize the surface area, the vertical option can offer a better return per square foot, provided the reinforcements are manageable. At Plan Maison Québec, we always recommend setting aside a budgetary cushion (contingency) of 10 to 15% for unforeseen events (floor problems, structural discoveries, additional material costs, etc.).
Essential steps for a successful expansion
Here are the essential steps to follow to make your extension project a success, whether it is upstairs or in width:
Feasibility analysis
First and foremost, have a soil study and structural inspection conducted. These provide a solid foundation for your decisions. Diagnose the condition of the existing foundation, measure the load-bearing capacity, identify the nature of the soil, access constraints, and zoning regulations. Plan Maison Québec recommends this crucial step to minimize surprises.
Architectural design and structural choices
Then, design the plans taking into account aesthetic integration, structural constraints, circulation, openings, ventilation, natural lighting, etc. The architect or technologist must propose solutions (beams, reinforcements, foundations, septic standards, drainage) adapted to your project.
Permit application and municipal compliance
Obtaining a building permit is a prerequisite. You will need to submit plans, geotechnical studies, structural designs, landscaping plans, drainage details, and compliance with setbacks, maximum heights, etc. Adding a story is often more closely scrutinized by urban planning departments because it affects the building's volume.
Earthworks, foundations and structure
For a horizontal extension, this involves the excavation, formwork, foundation, drainage, and backfilling phases. For a story addition, this involves reinforcing the existing foundations, installing temporary supports, and constructing the new structure (beams, floors, walls). Coordination between the old and the new is essential to avoid differential settlement.
Building envelope and closure
Once the structures are in place, we proceed to the roof, exterior walls, insulation, windows, doors, etc. Airtightness, management of thermal bridges and the compatibility of existing versus new materials are essential to limit infiltration and ensure energy efficiency.
Interior and finishes
The final stage involves plumbing, electrical, heating systems, ventilation, partitions, plasterboard, painting, floor coverings, etc. This is also the stage where unforeseen events (such as unanticipated work on load-bearing walls) can increase the bill, which is why a contingency fund is recommended.
Inspections, warranties and acceptance
Before taking possession of the extension, ensure that all required inspections (structural, electrical, plumbing, building code compliance) have been completed. Obtain contractor warranties, a warranty plan such as a latent defect warranty, and complete documentation. This protects you in the event of future defects.
Advantages and disadvantages compared
To summarize, here is an overview of the advantages and disadvantages of the two options (to read and compare for your project):
Horizontal enlargement (in width)
Advantages:
Simplicity of structure: few structural reinforcements if the soil permits
Better thermal insulation (fewer exposed surfaces)
Ability to keep the existing floor intact
Less disruption to the vertical circulation
Disadvantages:
High cost of foundations and excavation
Ground disturbance, backfilling, drainage
Risk of differential settlement between the old and new
Sometimes limited by zoning or setbacks
Vertical expansion (addition of floor)
Advantages:
Less need for new foundations (as long as existing structures can be reinforced)
No massive lateral earthworks
Potentially better yield per square meter
Allows for increased living space without encroaching on the yard
Disadvantages:
Requires structural reinforcement (load-bearing walls, beams, foundations)
Increased cost for modifications to the envelope, stairs, roof
Small spaces or galleries are sometimes compromised
Complexity of architectural integration (roof pitch, maximum height, visual proximity)
Exemplary cases and concrete scenarios
To help illustrate these differences, let's consider some typical scenarios that we have observed or that are frequently found in Plan Maison Québec consultations:
House on sloping land with little lateral clearance: adding a story is often the most rational solution, as a horizontal extension would require costly retaining walls and complex earthworks.
House with solid foundations and load-bearing walls capable of supporting an increased load: In this case, adding a story can be achieved at a reasonable cost (compared to a horizontal extension). Space can be maximized without affecting the land.
Large, flat lot, agricultural property, with few zoning constraints: In this case, a horizontal extension is often preferred because it expands the ground floor spaces, simplifies access, and eliminates the need for stairs for certain rooms.
Project to convert a garage into a dwelling or add a story above the garage: Here, the vertical option is relevant because it takes advantage of the existing footprint.ante. On évite de nouveaux fondations pour l’extension horizontale.
Heritage-style or complex-roofed homes: The expansion must be carefully integrated to maintain consistency. In some cases, a more discreet horizontal extension blends better with the original architecture; in others, a well-designed elevation blends in better.
In each of these scenarios, Plan Maison Québec recommends having a 3D model and cost simulation performed to compare the two options before making a decision.
Practical advice for controlling costs and risks
Here are some recommendations based on experience and best practices to help you manage risks and optimize your expansion project:
Have a detailed geotechnical study conducted from the outset: it's better to pay for a good study than to face costly surprises during construction.
Build in a contingency margin (10 to 15%) for unforeseen events, especially for underground or structural work.
Request several detailed quotes: compare items, materials, deadlines, guarantees, and hidden costs (drainage, connections, formwork).
Choose a team skilled in residential expansion: this type of project requires careful coordination between engineers, architects, contractors, and suppliers.
Plan the construction schedule wisely: carry out exterior work (excavation, drainage) during favorable periods, avoid freezing seasons, and anticipate municipal deadlines.
Integrate energy efficiency from the start: efficient insulation, reduced thermal bridges, choice of sustainable materials, balanced ventilation, heat recovery. Clearly communicate the junctions between old and new: plan expansion joints and ensure material compatibility to avoid bond cracks.
Monitor compliance with building codes and local standards: Denied permits or delayed inspections can significantly increase costs.
Plan aesthetic integration: choose siding, roofing, windows, and finishes that match the existing structure as closely as possible for a seamless visual continuity.
Document every step: plans, modifications, additions, and decisions—this protects your rights and facilitates risk management.
By applying these tips, you minimize the gap between the initial estimate and the actual final cost.
Conclusion: floor or width extension
Choosing between a two-story or a full-width extension is a strategic decision that combines financial, technical, aesthetic, and regulatory constraints. For a homeowner, the challenge is to accurately assess soil conditions, the load-bearing capacity of the existing foundation, the land footprint, local regulations, and the overall budget.
At Plan Maison Québec, we support our clients from the planning phase, conducting geotechnical studies, designing optimized plans for each scenario, comparing projected costs, and overseeing the project's implementation to avoid unpleasant surprises.
Don't let doubts hold back your space dream—let's talk about it together.
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