Renovating · Extension · 2026
Four hundred more square feet is a luxury-car budget, a year of construction and a resale value that is never guaranteed. Here are the 2026 costs, permits city by city, and the extend-or-move calculation.
Extending a house in Quebec costs, in 2026, $200 to $450 per square foot depending on the type of work: a ground-level extension on a full foundation runs $250 to $450, an added storey $200 to $400 and more, an attached garage $180 to $240, a four-season sunroom $250 to $325; a 400-square-foot rear extension therefore comes to $100,000 to $160,000, plans, permit and taxes extra. The permit is mandatory everywhere as soon as floor area increases, and its cost depends on the city: in Laval, $92 for the first $20,000 of work plus $2.50 per $1,000, with a posted delay of 50 days; in Brossard, a $51 base plus $3.50 per $1,000 of estimated cost; in Montreal, $9.80 per $1,000 of work with a $167.40 minimum, issued by the borough; in Longueuil, the application is made online 24 hours a day with digital plans. Count six to fourteen months from plans to completion, more if the project needs a minor variance or goes through an architectural integration plan. The question to ask before the plans: the value created at resale is capped by the sector's comparable sales, while selling and buying bigger costs about 8% of the price in fees; a broker prices both within a week.
| Type of extension | Typical 2026 cost (Dessins Drummond estimates, August 2026) | Typical project |
|---|---|---|
| Ground-level extension on a full foundation (basement) | $250 to $450 per square foot | 400 sq ft at the back: $100,000 to $160,000 |
| Ground-level extension on piers or a slab | $200 to $390 per square foot | Cheaper, no basement |
| Second storey added on the existing house | $200 to $400 and more per square foot | 800 sq ft: $160,000 to $320,000 and more, depending on structure and roof |
| Attached garage | $180 to $240 per square foot | 400 sq ft: $72,000 to $96,000 |
| Four-season sunroom | $250 to $325 and more per square foot | 300 sq ft: $75,000 to $97,500 |
These ranges are industry estimates, before taxes, excluding plans ($2,000 to $8,000 depending on the architect or technologist), permits and contingencies (allow 10% to 15%); an extension that touches the roof, plumbing, electrical or load-bearing structure lands at the top of the range. Three detailed quotes from RBQ-licensed contractors, with a schedule and a holdback, remain the only way to get a price.
| City | Residential extension permit fee (2026, municipal websites) | Delay and filing |
|---|---|---|
| Laval | $92 for the first $20,000 of work + $2.50 per $1,000 above; connection fee $450 if applicable | Posted delay of 50 days; online application; PIIA check at step 1 |
| Brossard | $51 base + $3.50 per $1,000 of estimated cost (residential); veranda: $126 review fee; permit valid 12 months | Form, certificate of location and site plan to direction.urbanisme@brossard.ca or at city hall |
| Montreal (by borough) | $9.80 per $1,000 of work, minimum $167.40 for a residential building; renewal at 25%; possible extra fees in heritage sectors | Issued by the borough, online or at the counter; siting rules and PIIA specific to each borough |
| Longueuil | Per the city's fee by-law; amount shown on filing | Online application 24 hours a day with digital plans and a credit card; one file per type of work |
Everywhere: an up-to-date certificate of location, scaled plans (often signed by a technologist or an architect), compliance with setbacks, height and lot coverage under zoning, and a call to Info-Excavation before digging. An extension in a PIIA zone or on a building of heritage interest adds an advisory committee approval and several weeks. Details by city: Brossard, Longueuil, Laval, Montreal.
An extension is rarely recovered in full at resale: in a sector where comparable houses already have the added area, the value created comes close to the cost; in a sector of smaller houses, it caps at what the market pays for the best house on the street, and the excess is lost. The honest comparison: the cost of the extension (plus 6 to 14 months of work and municipal taxes that rise with the assessed value) against the cost of selling and buying bigger (commission, notary, welcome tax, moving: about 8% of the price, $50,000 to $60,000 on a $650,000 to $700,000 house) and the price gap between the two houses. A broker who knows the sector's closed sales can price both options within a week, before you pay for plans.
Paying for plans before checking zoning (setbacks, lot coverage, height, shoreline strip); underestimating structure (a storey often means redoing the foundations and the framing); forgetting that the assessment roll and taxes will follow the added value; starting without a permit (stop-work order, fine, and a title defect at resale that the buyer discovers on the certificate of location); and extending the most expensive house on the street.
Yes, for any increase in floor area, including a storey, an attached garage, a sunroom or a veranda; the permit is obtained before the work and an inspector visits the site.
About $100,000 to $160,000 in 2026 for a ground-level extension on a full foundation, taxes, plans and permit extra, according to industry estimates.
Partly: the value created depends on the sector's comparable sales and caps at what the market pays for the best house on the street; compare with selling and buying bigger.
Six to fourteen months from plans to completion, including several weeks for the permit, longer if the project needs a minor variance or an architectural integration approval (PIIA).
Before extending, compare with the other option: the cost of selling, renovating before selling and the 2026 renovation grants.
The Loaa & Manseur team, led by Lotfi Manseur and Loaa Al Daow, is the number 1 team at RE/MAX Platine in 2024 and 2025, 6th RE/MAX team in Quebec, 15th in Canada and in the global top 50 of large residential teams according to the official RE/MAX ranking, computed on actual transactions. Its Google profile counts 221 reviews with a 4.9 rating. Its offices are at 55 avenue de l'Équinoxe in Brossard, one bridge from the island, and every broker's licence can be checked in the OACIQ public register. Three things to verify yourself before calling us, and to demand of any other broker.
Also available in French: version française de ce guide.
Published September 7, 2026 by the Loaa & Manseur team, the number 1 team at RE/MAX Platine in 2024 and 2025, 6th RE/MAX team in Quebec, 15th in Canada and in the global top 50 according to the official RE/MAX ranking. Tell us about your project.
Équipe no 1 de RE/MAX Platine en 2024 et en 2025, 6e au Québec, 15e au Canada et dans le top 50 mondial au classement officiel RE/MAX. Réponse en moins de 24 heures, sept jours sur sept.