Are you short of space at home, but the idea of moving gives you cold sweats? You're not alone. With home prices soaring 9.1% in the Montreal region last year, more and more Longueuil homeowners are opting to expand rather than search for a new property.

Backyard expansion makes sense for many families. It lets you stay in your neighborhood, keep your kids in the same school, and avoid moving costs that can easily exceed 15,000$.

Cartago Construction has been working with homeowners in Longueuil, Saint-Hubert and Vieux-Longueuil for over 10 years. We know the particular challenges of our region: clay soils that require special foundations, changing municipal bylaws, and the expectations of families who want a project done right, on time and without unpleasant surprises.

In this guide, I'll explain everything you need to know to make your expansion project a success. Permits, actual costs, steps to follow and, above all, mistakes to avoid.

Why Longueuil homeowners choose to expand into the backyard

It's not just about square footage. It's about quality of life.

The families we meet often tell us about the same needs: an office for the telecommuter who has become permanent, an extra bedroom for the new baby, or simply a bigger kitchen for entertaining the family on Sundays.

Pressure from the real estate market and lack of space

According to Nesto data for 2025, the average price of a single-family home in the Montérégie region has risen by 9.1% over the previous year. For many homeowners, this means that buying bigger just isn't a realistic option.

Moving is expensive. Really expensive. Between notary fees (1,500$ to 2,500$), welcome tax, brokerage fees, the move itself, and all the adjustments you have to make in a new home, we're easily talking 25,000$ to 40,000$.

And that's not counting the stress, lost time and adjustment to a new neighborhood.

Unique advantages of a rear extension

Expanding into the backyard offers a number of practical advantages that we've seen with our customers.

First, it adds value to your property. According to the Association des professionnels de la construction et de l'habitation du Québec (APCHQ), a well-designed addition can increase the value of your home by 5% to 10%. On a 450,000$ property, this represents between 22,500$ and 45,000$.

Secondly, you retain control over the design. Unlike buying an existing home, where you have to deal with the choices made by the previous owners, here you create exactly the space you need. A kitchen opening onto a bright living room? Office overlooking the garden? Family room with integrated sound system? The choice is yours.

And finally, you stay in your neighborhood. Your kids have friends over, you already know the local merchants, and you know that Parc Michel-Chartrand is a 10-minute bike ride away.

Want to know if your lot is suitable for expansion? Request a free evaluation of your project.

Longueuil's municipal rules (2025) to know before enlarging

Here, we get to the heart of the matter. Building regulations in Longueuil are not something you can ignore.

The city updated its zoning by-law in 2024 (CO-2024-1286), and many homeowners we meet don't know all the details. Yet an error at this stage can delay your project by weeks, or worse, force you to modify your plans after the fact.

Permits required and lead times (20 to 30 days)

Any extension over 10 square meters (about 108 square feet) requires a building permit. You can't negotiate.

The good news is that Longueuil has set up an online portal that simplifies the process. Processing times are generally 20 to 30 working days once your file is complete.

Permit costs vary according to the size of your extension. Expect to pay between 0.50$ and 1$ per square foot, plus an administrative fee of approximately 500$ to 2,000$, depending on the complexity of the project.

At Cartago, we prepare and submit your permit applications. We know exactly what documents you need and how to present your project, so you don't have to go back and forth with the city.

Zoning, setbacks and backyard encroachment

Here's what often surprises homeowners: you can't build just anywhere on your property.

Longueuil by-law CO-2024-1286 generally requires a 2 to 3-metre (approx. 6 to 10 feet) rear setback between your extension and the lot line. This distance may vary according to your specific residential zone.

The total footprint of your home (including the addition) can generally not exceed 40% of your lot area. If your home already occupies a large portion of your lot, this may limit your options.

There are exceptions in some cases, especially for smaller lots or in heritage areas such as Old Longueuil. That's why a preliminary analysis of your lot is essential.

Energy standards 2025 (-40% consumption)

The Quebec Construction Code has introduced new energy standards that will gradually come into force until October 2025. The objective? Reduce the energy consumption of buildings by 40%.

For your enlargement, that means:

Reinforced insulation (higher minimum R-value for walls and roof). Energy-efficient windows (triple glazing in some cases). Improved airtightness to prevent heat loss.

These requirements slightly increase construction costs, but save you money on heating in the long run. In Longueuil's climate, where winters can be harsh, good insulation really makes a difference on your Hydro-Québec bill.

Cartago can verify your project's compliance with municipal by-laws before you submit your permit application.

How much does a backyard extension in Longueuil cost?

It's THE question we're always asked when we first meet.

The honest answer? That depends. But I'll give you some concrete figures based on the projects we've completed in the region over the past two years.

For a quality extension in Longueuil, expect to pay between 225$ and 270$ per square foot. This is slightly higher than in some regions of Quebec, mainly due to two factors: clay soils that require pile foundations, and the cost of skilled labor in the greater Montreal area, which increased by about 7% in 2024.

Cost per type of foundation (piles vs. slab)

Soils in the Montérégie region are not easy. Much of Longueuil, Saint-Hubert and Greenfield Park is marine clay.

This clay contracts and expands according to the season and humidity level. The result? If you build on a simple concrete slab without adequate preparation, you risk cracks and structural problems within a few years.

That's why most extensions in our region require piles. These steel or concrete pillars seek out stable soil deep beneath the problematic clay layer.

How much does it cost? Pile foundations cost around 20% more than a standard slab. On a 100,000$ extension, that's about 20,000$ more. But it's an investment that protects your home in the long term.

We work with soil experts who analyze your land before we start. No guesswork, just recommendations based on geotechnical testing.

Total cost based on surface area (500 to 1,000 sq. ft.)

Here are some realistic ranges for 2025:

• 500-square-foot expansion (about 46 square meters): Between 112 000$ and 135 000$. This gives you a large room such as a kitchen opening onto a living room, or two bedrooms with a bathroom.
• 750-square-foot expansion (about 70 square meters): Between 168 000$ and 202 000$. You can create a real addition with several rooms, for example a complete master suite with walk-in closet and bathroom, plus an office.
• 1,000-square-foot expansion (about 93 square meters): Between 225 000$ and 270 000$. We're talking about a complete floor or a major extension that completely transforms your home.

These prices include everything: foundations, structure, insulation, electricity, plumbing, complete interior finishing, windows and doors. However, they do not include furnishings, appliances, or landscaping to redo your yard after the work.

Hidden expenses (excavation, drainage, electricity)

Here's where many owners get caught.

Excavating in Longueuil clay can be challenging. If your soil retains water or existing drainage is deficient, you may need to install a perimeter drainage system. Plan on 3,000$ to 8,000$, depending on complexity.

Electrical upgrades are sometimes necessary. If your main electrical panel is already close to its maximum capacity, the addition of an extension with kitchen, bathroom and air conditioning may require an increase in service (from 100 to 200 amps). Cost: 2,500$ to 4,000$.

Temporary shoring if your extension requires opening up an existing load-bearing wall. This is an essential safety measure that costs between 2,000$ and 5,000$.

And the unexpected. Even with the best planning, older homes sometimes hold surprises (old heating system to be moved, non-compliant plumbing to be brought up to standard). We always recommend a reserve of 10% to 15% of the total budget.

Myth to debunk: «Doing the work myself will save me 50%». In reality, without the proper experience and tools, homeowners who try to manage their projects themselves often end up paying 20% to 30% more due to mistakes, duplicate materials, or redo work.

Get a detailed, transparent quote for your project, free of charge and with no obligation.

Steps to a successful rear extension in Longueuil (Complete Guide)

A successful expansion isn't just a question of budget. It's about planning and sequencing.

Here's the complete process we follow at Cartago, from the first phone call to the handover of the keys.

Site analysis and choice of foundation (piles recommended in Montérégie)

It all starts with a site visit. We can't give you a fair price without seeing the condition of the soil, the slope of your lot, access for machinery, and the current configuration of your home.

For lots in Longueuil, especially in older sectors near the river, a geotechnical study is almost always recommended. It costs between 1,200$ and 2,000$, but it saves you from problems that could cost 10 times as much to correct later.

This analysis tells us exactly what type of foundation to choose. In most cases in the Montérégie region, it will be screw piles or concrete piles that go down to bedrock or stable ground.

We also evaluate drainage. If your lot is directing water toward your house instead of away from it, we need to correct this before building.

Architectural or technological drawings (2 400$ to 3 600$)

Once we know your terrain's capabilities, we move on to design.

For an extension, you need professional plans prepared by an architect or architectural technologist. These plans show the structure, electricity, plumbing and all the technical details needed to obtain your permit.

The cost varies according to complexity: between 2,400$ and 3,600$ for a standard project. If your extension includes complex structural elements (removal of a load-bearing wall, major opening, cathedral roof), you may need a structural engineer. Add 1,500$ to 2,500$.

These plans must comply with the Quebec Construction Code and Longueuil municipal bylaws. That's why we work with professionals who are familiar with local requirements.

Submission of municipal permit

With plans in hand, we prepare your permit application.

Required documents include:

• Complete architectural plans, signed and sealed
• Up-to-date certificate of location (if your last transaction was more than 10 years ago)
• Completed permit application form
• Photos of existing property
• Description of exterior materials

Drop everything off at the Service de l'urbanisme de Longueuil, either online via their portal, or in person at 4250 chemin de la Savane.

Analysis time is generally 20 to 30 working days. During this time, the inspector may ask for clarifications or minor adjustments to the plans. This is normal, and we take care of all these communications.

Construction - structure, insulation, energy 2025

Once the permit has been obtained, work can begin.

Excavation and foundations come first. For piles, allow 2 to 3 days. For a slab on prepared ground, 5 to 7 days including concrete drying time.

Next, the structure is erected: exterior walls, roof structure, opening in the existing wall if necessary. This phase generally takes 2 to 3 weeks.

Then comes the envelope: insulation to the new 2025 standards, vapour barrier, exterior cladding, windows and doors. Air-tightness is of the utmost importance. A blower door test may be required for certain projects.

Plumbing, electricity and heating are installed before the interior is finished. Now is the time to think about your future needs: integrated USB sockets, wiring for sound systems, intelligent thermostats.

Interior finishing (drywall, paint, floors, cabinets) comes last. This is the phase when your extension really comes to life.

A 500-square-foot extension typically takes 12 to 16 weeks from start to finish. Larger projects can take 20 to 24 weeks.

Interior finish + home automation integration

What are the trends for 2025? Homeowners want smart technology and sustainable materials.

On the home automation side, we often install: zone-programmable thermostats, smart lighting controlled by phone, electrical outlets with integrated USB-C ports, and wiring for security cameras.

On the design side, we're seeing a lot of demand for: hardwood or engineered wood floors (more stable than solid wood with variations in humidity), large windows to maximize natural light, open spaces that connect the extension to the rest of the house, and eco-friendly materials like VOC-free gypsum or recycled quartz countertops.

The aim is for your extension to blend in perfectly with the rest of your home. No one should be able to tell where the old ends and the new begins.

Cartago can manage your project from A to Z: plans, permits, construction and finishing. One team, no stress.

Common mistakes to avoid (myths vs. reality)

After 10 years in the business, we've seen a lot of mistakes. Some are costly, others delay projects by months.

Here are the most common, and more importantly, how to avoid them.

Myth: «You don't need a permit if it's in the back».»

It's not true. Very wrong.

In Longueuil, any extension of more than 10 square meters requires a building permit, no matter where it's located on your lot. Back, side, front: it doesn't matter.

Building without a permit can cost you dearly. The city can:

• Fine you up to 10,000$
• Require you to stop work immediately
• Demanding that you demolish what has been done
• Refuse to issue a certificate of location upon resale

What about your home insurance? It could refuse to cover damage caused by non-conforming work.

In short, a license costs between 1,000$ and 3,000$. Taking shortcuts can cost you 10 times that amount.

Myth: «DIY saves 50%».»

We understand the appeal. You watch YouTube videos, you have tools in your garage, and you think, «I can do this myself.»

The reality? Extensions are complex structural projects. A mistake in the foundation, framing or insulation can cause major problems:

• Structural collapse
• Mold due to poor sealing
• Wall cracks
• Heating problems and high energy bills

Homeowners who try DIY often end up calling in professionals to fix their mistakes. And repairing usually costs 20% to 30% more than doing things right the first time.

Not to mention the value of your time. A project that might take an experienced team 3 months can easily stretch to a year if you do it evenings and weekends.

Myth: «All extensions increase value».»

Not automatically.

A well-designed, well-built extension, well integrated with the rest of the house, can increase its value from 5% to 10%. But a poorly done or poorly planned addition can actually hurt resale value.

This reduces the value:

• An addition that doesn't match the architectural style of the house
• An extension that creates an awkward flow of traffic (forcing you to cross one room to access another, for example).
• Finishes inferior to the rest of the house
• An extension that completely eliminates your backyard and reduces outdoor space

This increases the value:

• An addition that creates a modern open-plan layout
• An extension that adds a bedroom and bathroom (particularly useful for families).
• Materials and finishes that match the standing of your neighborhood
• A design that maximizes natural light

Before you take the plunge, think long-term. Even if you don't plan to sell tomorrow, you'll want to recoup your investment someday.

Why choose Cartago Construction for your extension in Longueuil?

We're not going to tell you that we're the only contractors capable of carrying out your project. That would be dishonest.

But here's what really sets us apart.

Local expertise Montérégie + clay soil management

Yassine, our founder, has been working in residential construction in Quebec for over 10 years. He grew up in the region and knows the particular challenges of Longueuil, Saint-Hubert and Greenfield Park.

We know that the clay soils of the Montérégie require a different approach. We've been working with the same geotechnical experts for years, and we know exactly what type of foundation is appropriate for each area.

We don't guess. We test, we analyze, and we build according to the real data in your field.

Turnkey approach (plans, permits, construction)

You don't have time to run back and forth between the architect, the engineer, the city and the subcontractors. We get it.

That's why we offer a complete service:

We coordinate the preparation of your plans with our partner architects and technologists. We prepare and submit your permit application. We manage all communications with the city. We carry out construction with our team and trusted subcontractors. Follow-up until final inspection.

You have a single point of contact. You receive regular updates. And you always know where your project stands.

Full insurance + RBQ 2025 compliance

Cartago Construction is fully insured and holds a valid RBQ license. It's not just a piece of paper: it's your protection.

Our RBQ number guarantees that:

• Our work complies with the Québec Construction Code
• You are protected by the new residential buildings warranty plan
• In the event of a problem, you have legal recourse
• Your investment is secure

And we keep up to date with all the new standards, including the 2025 energy changes.

Examples of projects (Longueuil case study)

Last year, we completed a 650-square-foot extension for a family in Saint-Hubert. They were running out of space with the arrival of their third child.

We created a large kitchen opening onto a living room with a wall of windows overlooking their garden, plus two bedrooms and a full bathroom.

Total budget: 156,000$. Duration of work: 14 weeks.

The result? They had their house appraised six months later to refinance their mortgage. The appraiser added 42,000$ to the property's value, for a return of 27% on their investment in less than a year.

This is the kind of project we love: well planned, well executed, and truly transforming people's lives.

Discuss your project with Yassine. Free site visit available this week.

Frequently asked questions about extensions in Longueuil

How much will a rear extension in Longueuil cost in 2025?

For a quality extension, expect to pay between 225$ and 270$ per square foot, all inclusive. A 500-square-foot project therefore costs between 112,000$ and 135,000$. This price includes foundations adapted to clay soils, structure, insulation to 2025 standards, electricity, plumbing and complete finishing.

Do I need a permit to expand into the backyard?

Yes, absolutely. In Longueuil, any extension over 10 square metres (about 108 square feet) requires a building permit. It generally takes 20 to 30 working days to obtain one. Costs range from 0.50$ to 1$ per square foot, plus administrative fees.

Piles or slab: what's best for my lot in Longueuil?

In the majority of cases in Longueuil, piles are recommended because of the clay soils present in the region. A geotechnical study (1 200$ to 2 000$) will tell you exactly what your land needs. Pile foundations cost around 20% more than a standard slab, but they protect your long-term investment against ground movement.

Is it cost-effective to expand instead of relocate?

In today's market, often yes. Moving easily costs 25,000$ to 40,000$ between selling, buying, notary, and moving costs. A well-done extension increases the value of your home by 5% to 10%, lets you stay in your neighborhood, and gives you exactly the space you need.

What is the total lead time for a residential extension?

From design to completion, allow 5 to 7 months for a standard project:

• Plans and permits: 6 to 10 weeks
• Construction: 12 to 16 weeks for a 500 ft² extension
• Finishing and inspections: 2 to 3 weeks

More complex or larger projects can take up to 9 or 10 months.

Can I live in the house while the work is being carried out?

Yes, in most cases. The addition is made to the outside of your existing house. You'll have noise during the day, and there will be a period when the opening to your new space won't be completely closed (usually 2 to 5 days). We plan this step carefully to minimize inconvenience.

What RBQ standards will apply in 2025?

The Quebec Construction Code has introduced new energy standards aimed at reducing energy consumption by 40%. This means reinforced insulation, higher-performance windows and better airtightness. These requirements slightly increase initial costs, but reduce your heating bills in the long term.

What subsidies are available for energy-efficient extensions?

Rénoclimat offers financial assistance of up to 5,000$ for energy-efficient renovations. Hydro-Québec's Chauffez vert program may also apply if you take advantage of your extension to upgrade your heating system. Criteria change regularly, so check available programs when planning your project.

Ready to extend your home?

Now you know a lot more about what's involved in a backyard extension in Longueuil. Regulations, costs, steps and pitfalls to avoid.

But reading an article is one thing. Having a concrete plan adapted to YOUR house and YOUR plot of land is quite another.

At Cartago Construction, we offer a free, no-obligation initial consultation. We come and look at your property, assess the feasibility of your project, and give you a realistic idea of costs and deadlines.

No pressure, no hard sell. Just honest advice based on 10 years of experience in residential construction in Quebec.

Contact Yassine today to discuss your project: 438-932-4326 or [email protected]

Cartago Construction 2580 Rue Cartier, Longueuil, QC RBQ certified | Fully insured | Full service from conception to completion