Do you have a semi-finished or completely unfinished basement and are thinking of converting it into living space? It's a great way to gain floor space without adding to the size of your house. But in Longueuil, making a basement habitable isn't as simple as installing gyproc and painting.

Montérégie's clay soil, humidity, Quebec Building Code standards and city requirements: all must be taken into account before starting work. Otherwise, you run the risk of spending thousands of dollars on a space that will go mouldy in two years, or that will never be compliant for resale.

At Cartago Construction, we fit out basements in Longueuil and on the South Shore for over 10 years. We know exactly what works in our climate and what causes long-term problems. This guide covers everything you need to know to avoid costly mistakes and create a comfortable, healthy and compliant space.

Why making a basement habitable is more complex in Longueuil than elsewhere

Building a basement in Longueuil isn't the same as in Vancouver or even downtown Montreal. Montreal. Our region has geological and climatic particularities that make certain subsoils difficult to finish properly.

If you follow generic advice found online or copy what a friend has done in another city, you're likely to have problems with damp, mold or non-conformity.

Clay soils, freeze-thaw and water infiltration

Longueuil is built largely on clay soils. Clay has a peculiarity: it swells when it absorbs water and contracts when it dries. This constant movement creates pressure on the foundations and encourages cracks in the concrete.

With our intense freeze-thaw cycles (we can go from -20°C to +5°C in just a few days in spring), water seeps into micro-cracks, freezes and widens them. Year after year, this creates pathways for groundwater.

As a result, many homes in Longueuil, especially bungalows built between 1950 and 1980, have water infiltration problems in the basement. It's not systematic, but it's common.

According to the APCHQ (Association des professionnels de la construction et de l'habitation du Québec), excessive humidity is the number-one cause of basement renovation failures in Quebec. We're talking mold, bad odors, buckling floors and respiratory health problems.

If you finish your basement without fixing the moisture problems first, you're going to have to tear it all out in 2-3 years. We've seen it too many times.

Stricter building and planning codes

The Quebec Building Code (2025 edition) imposes precise standards for a basement to be considered habitable. Minimum ceiling height, emergency windows, insulation, ventilation, heating: everything is regulated.

The Ville de Longueuil strictly enforces these standards. If you're doing major work such as adding rooms, creating a rental unit or changing the use of a space, you need a permit from the City.

A lot of people think, «It's just a small room, you don't need a permit.» Not true. As soon as you turn an unlivable space into a livable one (especially if you're adding a bedroom), a permit is required.

Without a permit, you risk fines, a hold-up on the resale of the house and insurance problems if there's ever an incident (water damage, fire). The buyer or building inspector will notice the undeclared work, and this will lower the value of your property or cancel the sale.

Minimum conditions for a basement to be considered habitable

Before planning anything, you need to know whether your basement meets the basic conditions of the Building Code. If any of these conditions are not met, you'll have to carry out major work or forego certain uses.

Minimum ceiling height (RBQ 2025)

According to the Quebec Building Code 2025, a habitable basement must have a minimum headroom of 2.1 meters (about 6 feet 11 inches) over at least 50 % of the surface. The remainder can be as low as 1.85 meters (about 6 feet).

If your basement is less than 2.1 metres everywhere, you can still use it as a family room or storage space, but you won't be able to create a bedroom or conforming rental unit.

To gain height, some owners lower the floor by excavating. This can be done, but it's complex and costly. You need to reinforce the foundation, manage drainage and often add a new concrete slab. Count on 20,000 to 40,000 $ for this type of work in Longueuil.

Measure your headroom before committing to the project. If you're on the borderline, consult a RBQ-certified contractor to evaluate your options.

Egress windows mandatory for bedrooms

If you want to create a bedroom in the basement, the Code requires an egress window. It's a question of safety: in the event of fire, occupants must be able to get out quickly.

The window must have a clear opening of at least 0.35 square meters (about 543 square inches). It must also be accessible from the inside without tools, and open onto a skylight or directly to outside.

If your basement doesn't have large enough windows, you're going to have to add some. This means cutting the concrete foundation, installing a new window and digging a skylight on the outside. Expect to pay between 2,500 and 5,000 $ per window installed.

Without a compliant egress window, you can't legally call a room a «bedroom». This is a systematic checkpoint for municipal inspections.

Heating, ventilation and humidity control

A habitable basement needs to be adequately heated to maintain a comfortable temperature all year round. If your current heating system doesn't go down to the basement, you'll need to extend it or install an electric radiant heating system.

Ventilation is even more critical. A poorly ventilated basement accumulates humidity, especially in a well-insulated, airtight house like the ones we build today. The ideal humidity level is between 40 and 60 %. Below that, it's too dry. Above that, you're encouraging mold.

Many people think that heating the basement will solve the problem of humidity. It won't. The heat can actually make the problem worse if warm air comes into contact with cold surfaces (like uninsulated concrete walls). This creates condensation.

The solution is to install a mechanical ventilation system such as an air exchanger (HRV). It circulates fresh air from outside and exhausts stale air, while recovering heat to avoid wasting energy.

A dehumidifier can also be useful, especially in the first few years after finishing the basement, while concrete and other materials dry out completely.

Step 1 - Adjust humidity BEFORE finishing

This is the most important step, and the one many people skip because they can't wait to see the final result. Big mistake.

If you finish a damp basement, you'll trap the moisture behind the walls. This will create mold, odors and health problems. Even if it looks nice at first, you'll have to tear it out in a few years.

Waterproofing and drainage (exterior vs. interior)

The first thing to do is identify where the water is coming from. Have you ever had seepage? Is the floor damp or wet in places? Does it smell musty?

If so, you need to solve the problem at source before thinking about finishing the basement.

There are two main approaches: external sealing and internal sealing.

Exterior waterproofing is the most effective method, but also the most expensive. We excavate around the house, clean the foundation, apply a waterproof membrane and install a French drain (foundation drain) that directs water away from the house. Expect to pay between 10,000 and 25,000 $, depending on the size of the house and the accessibility of the site. Longueuil.

Internal waterproofing (cuvelage) is less expensive but less durable. An interior drain is installed at the foot of the foundation, cracks are sealed and water is directed into a sump with a pump. It generally costs between 5,000 and 12,000 $.

When should you choose one or the other? If your infiltrations are severe or recurrent, exterior waterproofing is the only real solution. If it's just light damp or minor infiltration, casing may suffice.

At Cartago Construction, we always assess the condition of the foundation before proposing a solution. We tell you honestly what will fix the problem for good.

Insulation adapted to Longueuil's climate

Once humidity is under control, it's time to insulate the foundation walls properly. Concrete alone provides almost no insulation (about R-1), which means you lose a lot of heat through the basement.

In Quebec, we recommend achieving at least R-12 to R-15 for basement walls. This represents about 2 to 3 inches of rigid extruded polystyrene insulation board (pink or blue XPS).

Why XPS and not mineral wool or fiberglass? Because XPS is moisture-resistant. In the event of minor infiltration, the panel will dry out without losing its insulating properties. Wool, on the other hand, will absorb water and remain wet, creating mold and mildew.

XPS is installed directly on the concrete with concrete adhesive or mechanical fasteners, then a wooden furring strip (2×3 or 2×4) is added to secure the gyproc. Some contractors install the furring directly on the concrete, but this is a mistake: wood in contact with cold concrete creates a thermal bridge and can rot if moisture accumulates.

According to Transition Énergétique Québec, good basement insulation can reduce your heating costs by 15 to 25 %. This is significant in Longueuil, where winters are long and cold.

Mechanical ventilation and dehumidification

As mentioned above, mechanical ventilation is essential. An air exchanger (HRV) will circulate fresh air and evacuate humidity and pollutants.

Installing an HRV for the basement generally costs between 2,500 and 5,000 $, depending on the model and the complexity of the ventilation system. It's a worthwhile investment for the air quality and durability of your finish.

In addition to HRV, we often recommend installing a dehumidifier for the first 6 to 18 months after finishing. Concrete, wood and gyproc still contain residual moisture that will gradually evaporate. A dehumidifier will speed up the drying process and avoid unpleasant surprises.

A good basement dehumidifier costs between 300 and 600 $. Make sure it has a continuous drain so you don't have to empty the tank every day.

Step 2 - Planning the layout according to actual use

Once the humidity has been dealt with and you know that your basement meets basic standards, you can plan the layout.

What do you want to do with this space? A family room for watching movies? A home office? Rooms for children or family? A rental unit to generate income?

Each use has its own requirements and implications in terms of permits, costs and design.

Family room, office or bedrooms

A family room or office is the simplest use. You need basic finishes (walls, ceiling, floor), adequate lighting and good thermal comfort. You don't need an egress window if you're not adding a bedroom.

If you want to create one or two bedrooms in the basement, you're in another category. Each bedroom must have its own emergency window, a compliant closet and access to a bathroom complete (if the basement is separate from the rest of the house).

Basement rooms must also offer good acoustic comfort. No one wants to hear every footstep on the floor above when they're trying to sleep. We recommend installing a floating subfloor with acoustic insulation on the floor above, or adding insulation to the basement ceiling.

Rental or intergenerational housing

Creating an accessory dwelling unit in the basement (a small, self-contained 3½ or 4½-unit apartment) is an excellent way to generate rental income, or to accommodate an aging parent while offering privacy.

But beware: a basement rental unit in Longueuil is subject to specific regulations. You need to check the zoning in your area. Some residential zones allow secondary suites, others do not.

You'll also need to obtain a conversion permit and, in some cases, amend the certificate of location. The city will inspect to ensure that the dwelling meets all standards: height, windows, independent exit, soundproofing, etc.

The return on investment for a basement rental unit can be excellent. In Longueuil, a well-located 3½-unit apartment easily rents for between 900 and 1,200 $ per month. Over 10-15 years, this can more than repay the initial investment and significantly increase the resale value of your property.

Thinking of creating a rental property? Cartago Construction can guide you through the entire process, from zoning verification to handing over the keys to the tenant.

Step 3 - Actual costs to make a basement habitable in Longueuil

Let's talk money. That's the question we're asked at every first meeting: how much will it cost?

The answer depends on the current state of your basement, the extent of the work required and the level of finish you want. But we can give you realistic ranges based on our experience in Longueuil.

Average costs 2025-2026

For a standard 600 to 800 sq. ft. basement with medium-quality finish (not luxury, but not cheap either), count between 50 and 100 $ per sq. ft.

This gives a total budget of 35,000 to 60,000 $ for a complete project including :

• Humidity correction (if necessary)
• Wall and ceiling insulation
• Electrical installation and lighting
• Plumbing (if adding a bathroom)
• Wall and ceiling finishing (gyproc, paint)
• Flooring (vinyl, laminate or floating)
• Doors and mouldings
• Mechanical ventilation

If you opt for premium finishes (ceramic instead of vinyl, custom cabinetry, recessed lighting throughout, high-end bathroom), the cost can rise to 100 to 120 $ per square foot, or up to 80,000 $ or more for a large basement.

Conversely, if your basement is already dry, you don't have any major problems and you're just doing a basic finish without a bedroom or bathroom, you can get away with 30,000 to 40,000 $.

These figures include labor, materials and compliance with standards. They do not include major exterior waterproofing work (if required), which can add 10,000 to 25,000 $ to the budget.

Budget-busting factors

There are three main factors that can push costs well beyond expectations:

Untreated damp. If you discover mid-project that you've got water ingress, and you have to rip out everything that's already been done to waterproof it, you'll easily double the budget. That's why we insist: fix the moisture BEFORE you do.

Insufficient height. If your basement has less than 2 meters of headroom and you have to excavate to gain height, you instantly add 20,000 to 40,000 $ to the project. This is major work that affects the foundations and structure.

Unsuitable materials. Using materials unsuited to the climate or humidity (such as ordinary wood instead of treated wood, or fiberglass insulation instead of XPS) will cause problems in the medium term. You'll end up paying twice: once for the initial finish, and a second time to redo everything.

Get a realistic, detailed estimate for your basement project in Longueuil with Cartago Construction. We'll come to you free of charge to evaluate your basement and give you an accurate figure.

Permits and compliance in Longueuil - what the city requires

We can't stress this enough: if your project requires a permit, get it before you start work. It may sound bureaucratic and slow down the start-up, but it's infinitely less complicated than dealing with a non-compliance issue later on.

When a license is required

In Longueuil, you must obtain a conversion permit if you :

• Change the use of a space (e.g. transform a storage basement into living space)
• Add rooms
• Create a rental or accessory home
• Major plumbing or electrical modifications
• Make structural changes (remove or add load-bearing walls, enlarge openings)

Even if you're just making a family room without a bedroom, if you change the official use of the space, a permit is usually required. It's best to contact the City of Longueuil's urban planning department to clarify your specific situation before getting started.

Process with Ville de Longueuil

The Ville de Longueuil has set up an online portal to facilitate permit applications. You can submit your request with the required plans and documents directly on the site.

Typical documents requested :

• Basement plan with dimensions and room locations
• Electrical plan showing sockets, lights and panel
• Plumbing plan if you are adding or modifying installations
• Proof of compliance with minimum heights and egress windows

Processing times vary according to the time of year, but generally take 3 to 6 weeks. In peak season (April to June), it may take a little longer.

Permit fees depend on the value of the work, but for a standard basement project, allow between 200 and 500 $.

The risk of building without a permit? Fines that can run into the thousands of dollars, a hold-up on resale if the buyer or his inspector notices the undeclared work, and complications with your insurance in the event of a claim. It's really not worth the risk.

At Cartago Construction, we take care of the entire permit process for you. We know exactly what the city requires, and we avoid the back-and-forth that can delay your project.

Subsidies and return on investment in Longueuil

Building a basement is a major investment. But there are subsidies that can reduce the cost, and the long-term return on investment is often excellent.

Rénoclimat - Foundation insulation

The Quebec government's Rénoclimat program offers grants to improve the energy efficiency of your home, including foundation insulation.

If you insulate your basement walls to the recommended standards (R-12 minimum), you can receive financial assistance of up to 1,500 to 1,625 $, depending on your region and the type of work.

To qualify, you must have a certified Rénoclimat advisor conduct a pre- and post-retrofit energy evaluation. The process adds a few weeks to the schedule, but the savings are well worth it.

You can also combine Rénoclimat with other grants if you make several energy improvements at the same time (changing windows, improving heating, etc.).

ROI over 10-15 years in Montérégie

Beyond the subsidies, a basement adds value to your property. In Longueuil, especially for bungalows built between 1950 and 1980, a well-finished basement can increase resale value by 15,000 to 40,000 $, depending on the quality and scope of the work.

If you create a rental property, the return on investment is even faster. With a rent of 1,000 $ per month, you generate 12,000 $ per year. Over 10 years, that's 120,000 $ in gross income (minus maintenance costs and taxes, of course).

Even without renting, simply having more living space improves your quality of life. A large family room in the basement is ideal for kids, watching movies or setting up a home gym.

On the energy front, a well-insulated basement permanently reduces your heating costs. Over 15-20 years, energy savings can easily amount to 5,000 to 10,000 $.

Common myths about basement design (deconstructed)

We often hear the same misconceptions about basements. Here's the reality behind four common myths.

Myth 1: «If I heat the basement properly, the humidity will disappear».»

False. Heat can even aggravate the problem if you haven't dealt with the source of the humidity. Warm air contains more moisture than cold air. If this warm air comes into contact with cold surfaces (such as uninsulated concrete walls), water vapour will condense and create liquid water. The result: mold.

Heat alone never solves humidity. You need waterproofing, insulation and mechanical ventilation.

Myth 2: «Carpet is comfortable in the basement».»

Carpeting in the basement is a very bad idea in Longueuil. If water infiltrates (and it often does, even in newer homes), the carpet will absorb the water and stay damp for days. It's the perfect environment for mold and mildew.

Plus, you won't see the problem until it's advanced. With vinyl or laminate flooring, you can see right away if there's water and react quickly.

If you want comfort underfoot, opt for a floating floor with insulating underlay. It's warm, comfortable and much safer.

Myth 3: «My basement is new, so it's dry».»

Even a basement in a new house can have moisture problems. Concrete takes months, sometimes more than a year, to dry completely. During this time, it releases a great deal of moisture.

What's more, builders don't always provide perfect drainage or optimal waterproofing. We've seen 5-year-old houses with water infiltration because the French drain was poorly installed or clogged.

Before finishing a basement, even in a new house, always test the humidity level with a hygrometer. If it's above 60 %, wait or install a dehumidifier before starting.

Myth 4: «I don't need a permit if I do the work myself».»

Le permis n’a rien à voir avec qui fait les travaux. Ce qui compte, c’est la nature des travaux. Si vous changez l’usage de l’espace, vous avez besoin d’un permis, que vous le fassiez vous-même ou avec un entrepreneur.

En fait, si vous faites les travaux vous-même sans permis et que vous ne respectez pas les normes, vous prenez un double risque : la non-conformité municipale ET le non-respect du Code du bâtiment. C’est encore pire lors de la revente.

Pourquoi confier votre projet à un entrepreneur RBQ local à Longueuil

Aménager un sous-sol correctement, c’est un projet qui touche plusieurs corps de métier : excavation parfois, étanchéité, isolation, électricité, plomberie, ventilation, finition. Coordonner tout ça demande de l’expérience et une connaissance approfondie des normes.

Chez Cartago Construction, on applique une méthodologie éprouvée pour garantir un résultat durable et conforme.

Recommended methodology

On commence toujours par une inspection complète de l’état actuel du sous-sol. On vérifie les signes d’humidité, on mesure la hauteur libre, on évalue l’état des fondations et du drainage.

Si on détecte des problèmes d’humidité, on identifie la source et on propose une solution adaptée. Parfois c’est juste une question d’améliorer le drainage autour de la maison. D’autres fois, il faut étanchéiser les fondations ou installer un système de drainage intérieur.

Une fois que l’humidité est sous contrôle, on passe à l’isolation et à la ventilation. On utilise des matériaux adaptés au climat québécois et on installe un système de ventilation mécanique si nécessaire.

Ensuite vient la finition : électricité, plomberie, gyproc, peinture, plancher, portes et moulures. On coordonne tous les sous-traitants pour que le projet avance de façon fluide et respecte le calendrier.

À chaque étape, on s’assure que tout respecte le Code du bâtiment et les exigences municipales. On prépare les documents pour le permis et on accompagne le processus d’inspection.

Avantages d’un expert local

Un entrepreneur local comme Cartago Construction connaît les particularitésdu territoire. Les sols argileux de Longueuil, les problèmes de drainage typiques du secteur, les exigences spécifiques de la Ville : tout ça fait partie de notre travail quotidien.

On sait aussi où trouver les bons matériaux et les bons sous-traitants. On a des relations établies avec des fournisseurs locaux, ce qui nous permet d’obtenir des prix compétitifs et des délais de livraison plus courts.

En plus, on est certifiés RBQ et assurés responsabilité. Ça vous protège et ça garantit que nos travaux respectent les normes professionnelles.

Si jamais il y a un problème après la fin du projet, on est là. On répond aux appels, on vient vérifier et on corrige ce qui doit l’être. Vous avez pas à vous battre avec un entrepreneur qui a disparu ou qui refuse de revenir.

Faites vérifier si votre sous-sol est réellement habitable avant d’investir des milliers de dollars. Contactez Cartago Construction pour une évaluation gratuite à Longueuil et sur la Rive-Sud.

FAQ  Sous-sol habitable à Longueuil

Puis-je aménager un sous-sol avec un plafond à 1,95 m?

Techniquement, vous pouvez créer un espace de rangement, une salle de jeux ou même une salle familiale, mais vous ne pourrez pas y mettre de chambre ni créer un logement locatif conforme. Le Code du bâtiment exige 2,1 m de hauteur libre sur au moins 50 % de la surface pour un espace habitable. Si vous voulez vraiment des chambres, vous devrez excaver pour gagner de la hauteur.

Est-ce rentable d’aménager un sous-sol à Longueuil?

Oui, dans la plupart des cas. Un sous-sol bien aménagé peut augmenter la valeur de votre propriété de 15 000 à 40 000 $ et rendre votre maison plus attrayante pour les acheteurs. Si vous créez un logement locatif, le retour sur investissement peut être excellent, avec des revenus de 900 à 1 200 $ par mois. Même sans location, vous gagnez beaucoup d’espace habitable pour un coût au pied carré beaucoup plus bas qu’un agrandissement.

Combien de temps prend le processus de permis à Longueuil?

Comptez généralement 3 à 6 semaines pour obtenir votre permis de transformation une fois que vous avez soumis tous les documents requis. En haute saison (printemps et début d’été), ça peut prendre un peu plus de temps. Préparez vos plans et vos documents à l’avance pour éviter les délais.

Peut-on isoler un sous-sol par l’intérieur seulement?

Oui, c’est même la méthode standard pour les sous-sols existants. On installe des panneaux de polystyrène extrudé (XPS) directement sur les murs de béton, puis on construit une ossature de bois pour fixer le gyproc. L’isolation extérieure des fondations existe, mais elle est beaucoup plus coûteuse et se fait généralement seulement lors d’une construction neuve ou d’une excavation majeure.

Que se passe-t-il si je finis mon sous-sol sans permis?

Vous risquez des amendes de la Ville si les travaux sont découverts lors d’une inspection ou d’une plainte. Plus grave encore, lors de la revente de votre maison, l’inspecteur en bâtiment ou l’évaluateur va remarquer les travaux non déclarés. Ça peut faire baisser la valeur de votre propriété, bloquer la vente ou forcer l’acheteur à exiger une réduction de prix importante. Vos assurances peuvent aussi refuser de couvrir un sinistre s’il affecte un espace non conforme.

Le programme Rénoclimat est-il encore disponible en 2026?

Oui, le programme Rénoclimat continue d’offrir des subventions pour l’amélioration de l’efficacité énergétique, incluant l’isolation des fondations. Les montants et les critères peuvent changer d’une année à l’autre, alors vérifiez toujours les conditions actuelles sur le site de Transition Énergétique Québec avant de planifier vos travaux. Vous devez faire réaliser une évaluation énergétique avant et après les travaux pour être admissible.

Combien de temps faut-il pour aménager un sous-sol complet?

Pour un sous-sol standard de 600 à 800 pieds carrés avec finition complète, comptez entre 6 et 12 semaines de travaux, selon la complexité du projet. Si vous devez d’abord régler des problèmes d’humidité ou d’étanchéité, ajoutez 2 à 4 semaines supplémentaires. Les projets avec logement locatif ou avec plusieurs salles de bain prennent généralement plus de temps.

Quelle est la meilleure période de l’année pour aménager un sous-sol à Longueuil?

Contrairement aux travaux extérieurs, l’aménagement d’un sous-sol peut se faire toute l’année. Par contre, si vous devez faire des travaux d’excavation ou d’étanchéité extérieure, le printemps, l’été et l’automne sont préférables. L’hiver, le sol gelé rend l’excavation beaucoup plus difficile et coûteuse. Pour la finition intérieure seule, n’importe quelle saison convient.

Rendre un sous-sol habitable à Longueuil, c’est un projet qui peut transformer votre maison et votre qualité de vie. Que vous vouliez créer un espace pour la famille, ajouter des chambres ou générer un revenu locatif, les possibilités sont grandes.

Mais pour que ça fonctionne à long terme, vous devez absolument respecter trois règles : régler l’humidité avant toute finition, respecter les normes du Code du bâtiment et obtenir les permis requis. Sauter une de ces étapes, c’est s’exposer à des problèmes coûteux et stressants.

Chez Cartago Construction, on accompagne les propriétaires de Longueuil et de la South Shore dans leurs projets de sous-sol depuis plus de 10 ans. On connaît les défis du territoire, les exigences de la Ville et les meilleures pratiques pour créer des espaces sains, confortables et durables.

Prêt à démarrer votre projet? Appelez-nous au 438-932-4326 ou écrivez à [email protected] pour une consultation gratuite. On évalue votre sous-sol, on répond à toutes vos questions et on vous donne un plan d’action clair avec un budget réaliste.

📍 Cartago Construction 2580 Rue Cartier, Longueuil, QC Certifié RBQ | Entièrement assuré | Turnkey service