Legal Basement Apartments & Second Units
Legal Basement Apartment & Second Unit (ADU) Builder in Ontario
A legal basement apartment — also called a second unit, secondary suite, or accessory dwelling unit (ADU) — is a self-contained rental unit built in your basement to Ontario Building Code and Fire Code standards: its own entrance, kitchen, bathroom, code-compliant egress, and fire separation from the main home. GSP Builder is a design-build contractor that designs, permits, and builds legal basement apartments across the GTA and Durham Region — to the Ontario Building Code and Fire Code, with the building permit, ESA-certified electrical, and second-unit registration that make the finished suite legal, insurable, and rent-ready.
What it costs
What a legal basement apartment costs in Ontario
There's no flat rate for a legal basement apartment — the cost depends on the scope of work and the finish level, and both vary widely from one basement to the next. A straightforward build-out with a clean rental finish sits at one end of the range; a high-end suite with premium millwork, hardwood, and home automation sits at the other. Structural work — underpinning to gain ceiling height, a new separate entrance, or egress excavation — adds to any project. Every basement is bespoke, so we don't post a rate: we provide accurate, itemized figures for your specific space only after a free consultation and on-site review.
What moves the number
- Ceiling height — underpinning to lower the floor if the basement doesn't have adequate ceiling height (the minimum is set by code and confirmed at design)
- A separate entrance — excavation, a walk-out or side stair, and a code exit
- Egress windows and window wells in every bedroom
- Fire separation between units, plus interconnected smoke and CO alarms
- Plumbing and drainage — a sewage ejector pump if the suite sits below the sewer line
- Heating and ventilation — usually a second zone or dedicated system for the unit
- Electrical — a compliant panel and ESA-certified wiring
- Finish level — from a builder-grade rental finish to premium work (hardwood, premium millwork, home automation)
The legal reality
What makes a basement apartment “legal” in Ontario
Ontario permits second units — and now up to three units on most residential lots — but a finished basement is not a legal apartment until it meets the Building Code and Fire Code and passes inspection. Here is what that means in plain terms:
- Adequate ceiling height over the required areas — the minimum is set by code and confirmed at design
- A code-compliant means of egress — an exit plus proper egress windows in bedrooms (minimum opening size, maximum sill height, a window well where below grade)
- Fire separation between the two units (typically a 30–45 minute fire-resistance rating) — or interconnected alarms under the Ontario Fire Code retrofit provisions where full separation isn't feasible
- Interconnected smoke alarms and carbon-monoxide alarms in both units
- A separate entrance and, in most cases, a dedicated heating and ventilation provision
- Electrical work inspected and certified by the ESA (Electrical Safety Authority)
- Zoning compliance (unit size, parking) and, in many municipalities, second-unit registration
How we work
Our planning-first process
Most basement projects fail before a single wall goes up — unclear pricing, permit delays, and scope changes mid-build. We plan every step first, so cost, design, and timeline are settled before construction starts.
- 01
Feasibility & Zoning Review
We assess your basement's structure, ceiling height, drainage, and your municipality's second-unit rules before any design — so you know early whether it's viable and what it will take.
- 02
Budget Planning & Free Estimate
Clear numbers up front. You get an honest, itemized estimate — free — so cost is a decision you make at the start, not a surprise you absorb later.
- 03
Design & 3D Renderings
We design the suite and show you 3D renderings so you can see the layout, entrance, and finishes before we build a thing.
- 04
Permits & Approvals
We prepare submission-ready drawings and shepherd the building permit through municipal review — the exact step where most basement projects stall.
- 05
Construction
Licensed trades, one accountable contractor. Framing, egress, fire separation, plumbing, HVAC, electrical, and finishes — all built to code.
- 06
Inspections, Occupancy & Handover
We coordinate every inspection (building, ESA, final occupancy) and hand you a legal, insurable, rent-ready suite.
Questions people ask
Legal basement apartment FAQs
How much does a legal basement apartment cost in Ontario?
The honest answer is that it depends on your space and finish level — there's no single number. The factors that drive the cost are ceiling height (whether underpinning is needed to lower the floor), whether a separate entrance and egress windows have to be excavated, the fire separation and alarms required between the units, plumbing and drainage (a sewage ejector if the suite sits below the sewer line), HVAC and electrical/ESA work, and how far you take the finishes — from a builder-grade rental finish to premium millwork, hardwood, and home automation. We give you real, itemized numbers after a free on-site consultation, not a guessed figure up front.
Do I need a permit to build a second unit or basement apartment in Ontario?
Yes. A building permit is required. Ontario now allows up to three residential units on most residential lots as-of-right, so you generally won't need a zoning amendment — but the unit must be designed to the Ontario Building Code and Fire Code, permitted, and inspected. We handle the drawings, permit application, and inspections for you.
What makes a basement apartment legal in Ontario?
A legal basement apartment needs adequate ceiling height (the minimum is set by code and confirmed at design), a compliant means of egress (an exit and proper egress windows), fire separation between the two units (or interconnected alarms under the Fire Code retrofit provisions), interconnected smoke and carbon-monoxide alarms, ESA-certified electrical, and compliance with local zoning and second-unit registration. Miss any one of these and the unit isn't legal — or insurable.
How long does it take to build a legal basement apartment?
Typically about 3 to 5 months end to end. Construction of a straightforward suite runs roughly 8 to 14 weeks; the bigger variable is the municipal permit timeline, which ranges from a few weeks to a couple of months. Underpinning, if it's needed, adds time. We map the full schedule during planning so you're not left guessing.
Do you provide a free estimate and design before I commit?
Yes. Every project starts with a free, itemized estimate and 3D design renderings so you can see the layout and the real numbers before any work begins. Planning first is how we prevent the mid-build cost jumps and delays that sink most basement projects.
Proof
Recent & current work
Recent and current projects include a legal basement second unit in Burlington (in permitting and build), a fire-damaged commercial building restored as offices in Barrie, a CSA Group commercial fit-out in Etobicoke, and a major renovation in Woodstock. Fully insured and WSIB-covered. (GSP is also an HCRA-licensed new-home builder, licence #B64606.)
Where we build
Service area
We build legal basement apartments across the Greater Toronto Area and Durham, York, Peel, and Halton Regions — and out to Barrie and Woodstock. Whitby is home base.
Get a free estimate & 3D design
See your suite in 3D and get real, itemized numbers before you commit. Fully insured, WSIB-covered, and built to the Ontario Building & Fire Code.
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