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ADU Construction
ADU Construction designed, permitted and built across the 30 municipalities of the Greater Toronto Area - zoning review, permit drawings and construction handled by one team.
ADU Construction - Overview
Key Takeaways
- Key features: Build from your existing approved drawings, Excavation, foundation and framing, Trades coordination and site supervision
The construction phase itself - excavation, foundation, framing, services and finishes - for homeowners who already hold approved ADU drawings and a permit.
Features & Specifications
- Build from your existing approved drawings
- Excavation, foundation and framing
- Trades coordination and site supervision
- Municipal inspection scheduling
- Fixed construction schedule with milestones
Key takeaways
- ADU construction is the build phase: it starts when drawings are stamped and the permit issues, and ends at municipal occupancy sign-off - run as one general contract, one schedule, one accountable team.
- Foundation is chosen from soils and water table, not preference: a frost-protected slab, a helical pile array, or a full basement.
- Restricted rear-yard access is the single biggest cost multiplier, because material handling and crane positioning drive labour hours - we price it as a line, not a mid-build surprise.
- Construction on a detached 500-600 sq ft unit runs four to eight months at $300 to $600 per square foot, on top of four to eight weeks of permit review.
ADU construction is the build phase itself: the work that starts once drawings are stamped and the permit is issued, and ends when the municipality signs off on occupancy. We run it as a general contract - one team, one schedule, one point of accountability - rather than coordinating a designer, a framer and a trades list the owner has to manage personally.
The sequence on a detached unit is consistent. Site protection and excavation. Foundation, whether a frost-protected slab, a helical pile array or a full basement, chosen from the soils and the water table rather than from preference. Framing and sheathing. Roof and building envelope. Rough-in for electrical, plumbing and HVAC, each with its own inspection. Insulation and air sealing to the SB-12 energy target. Interior finishing, kitchen and bathroom. Servicing connections for hydro, water and drainage. Then the inspection sequence to final occupancy sign-off.
What this includes
- Single general contract from excavation to occupancy sign-off - one schedule, one accountable team
- Foundation selected from soils and water table: frost-protected slab, helical piles or full basement
- Staged inspection sequence - footing, framing, electrical, plumbing, HVAC, insulation, final occupancy
- Air sealing and insulation built to the SB-12 energy target filed in the permit package
- Third-party drawings reviewed against the issuing municipality's current requirements before pricing
- Restricted rear-yard access planned as a costed line - material staging and crane positioning, not a mid-build surprise
- Four to eight months construction on a detached 500-600 sq ft unit, quoted against the $300-$600/sq ft GTA band
- WSIB clearance certificate and $2M general liability supplied on request, re-verified every 90 days
Owners hire us at this stage for one of two reasons: they have permit-ready drawings from another designer and need a builder, or they want design and construction under one contract so nobody can attribute a delay to the other party. Both are common. Where drawings arrive from outside, we review them against the issuing municipality's current requirements before pricing, because a set drawn eighteen months ago may predate O. Reg. 462/24 or a local by-law amendment.
Construction on a detached 500 to 600 sq ft unit runs four to eight months in Toronto, on top of four to eight weeks of permit review. End to end, including design and approvals, eight to fourteen months is the honest range. Cost is $300 to $600 per square foot depending on site access and finish, or roughly $200,000 to $350,000 for a typical detached suite in the Greater Toronto and Hamilton Area. Restricted rear-yard access is the biggest single multiplier, because material handling and crane positioning drive labour hours.
Everything is built to the Ontario Building Code with SB-10/SB-12 energy compliance filed in the permit package, electrical filed with the ESA, and a fixed-scope contract issued before work starts. We carry WSIB registration and $2M general liability and supply a clearance certificate on request - under the Construction Act and the WSIA an owner is liable for an unregistered contractor's unpaid premiums, and the certificate needs re-verification every 90 days.
If you do not yet have drawings, our ADU plans service is the earlier step; if you have drawings but no permit, the ADU building permit service handles the submission.
ADU Construction - Permits and Approvals by Municipality
What approval typically costs, and what the lots look like, in the GTA municipalities we build in most.
| Municipality | Typical approval fees | Lot typology |
|---|---|---|
| Toronto | $8,000-$14,000 in permit, development and plan-review fees | Narrow downtown lots with rear public laneways - Toronto has over 300 km of them, more laneway frontage than any other Canadian city. |
| Brampton | $5,500-$10,000 including Brampton registration and Peel charges | Deep newer-subdivision lots, many with an existing unregistered basement apartment that must be brought to Ontario Building Code fire-separation standard. |
| Mississauga | $6,500-$11,000 in City of Mississauga and Region of Peel charges | Large 1970s-90s subdivision lots with attached garages; garage conversions and rear coach houses are both common outcomes. |
| York | $8,000-$14,000 under the City of Toronto fee schedule | Older narrow semi-detached lots on a laneway grid, with frequent shared-driveway conditions that must be resolved before permit. |
| Etobicoke | $8,000-$14,000 under the City of Toronto fee schedule | Split personality - laneway-served lakeshore grid in Mimico and New Toronto, versus large treed lots in The Kingsway where garden suites dominate. |
| Ajax | $7,000-$12,000 including Durham Region charges | Compact 1990s-2000s subdivision lots where the required 1.2 m side-yard construction access sets the maximum module width we can crane in. |
| King | $9,000-$16,500 including York Region and septic charges | Estate and agricultural parcels, many over an acre, on private well and septic with moraine core-area restrictions. |
| North York | $8,000-$14,000 under the City of Toronto fee schedule | Deep post-war bungalow lots, commonly 50 ft × 120 ft, with rear yards large enough for a detached garden suite plus the required 1.5 m separation. |
Fee bands are indicative, drawn from project experience - confirm current tariffs with the municipality before budgeting.
Related Unit Types
Frequently Asked Questions
Everything you need to know about our services.
The construction phase itself - excavation, foundation, framing, services and finishes - for homeowners who already hold approved ADU drawings and a permit. Concretely that means: build from your existing approved drawings; excavation, foundation and framing; trades coordination and site supervision; municipal inspection scheduling. One contract covers design, permit and construction, so there is no handoff between a designer and a builder halfway through.
Related service: Accessory Dwelling Units
On a 700 sq ft reference unit our estimate band runs $268,000-$359,500 in Brock up to $281,500-$377,000 in East York - roughly $383 to $402 per square foot. The spread is not a markup: this work involves a new foundation, new servicing and a full building envelope, and the per-municipality difference comes from approval fees, frost depth and snow load. Final pricing follows a site visit.
Related service: Accessory Dwelling Units
Yes - a building permit is required in every GTA municipality. Bill 23 (More Homes Built Faster Act, 2022) and O. Reg. 299/19 permit up to three residential units as-of-right on most serviced lots, so a rezoning application is usually not needed.
Municipal approval costs range from $5,500-$10,000 including Brampton registration and Peel charges in Brampton to $9,000-$16,000 including Peel Region and septic permit charges in Caledon, quoted separately from construction. Plan review typically takes four to eight weeks.
Related service: ADU Building Permit
All 30 municipalities of the Greater Toronto Area, from Toronto out to Clarington. Each one has its own by-law, fee schedule and review stream, and we file into the local one - the municipal page for your address carries that city's approval band, frost depth and building season.
Related service: Accessory Dwelling Units
Eight to fourteen months is the honest end-to-end range: design and drawings, four to eight weeks of plan review, then construction. Because this work is a new foundation, new servicing and a full building envelope, the build phase is the part that moves most between projects. Excavation is scheduled inside the local building season, and interior work carries on through winter.
Related service: Accessory Dwelling Units
Ready to Get Started?
Ready to start your adu construction project? Send us your address and we will tell you what your lot allows, what it costs and how long the permits take.