How Much Does It Cost to Build a Basement Apartment?

This guide is written for Ontario and the GTA, where the term is “basement apartment.” (In Alberta and BC it’s a “basement suite” and the numbers differ — see that guide instead.)

What it costs

We could not source a defensible Canadian range, so we’re not publishing one. The figures circulating come from individual contractors describing their own market, from US data, or from articles citing other articles — and the variance between an already-finished basement and a bare unfinished one is wider than any published range admits.

What is reliable:

A basement apartment is normally the cheapest ADU you can build. The foundation exists. The roof exists. Water, sewer and electrical already reach the building. You’re doing fit-out plus code compliance rather than constructing a dwelling from nothing. Compare that with a garden suite, where every one of those has to be created.

But the range within that is enormous, and it’s driven by one measurement.

The cost drivers that decide it

1. Ceiling height — the cost cliff.

The Ontario Building Code permits a minimum finished ceiling height of 1.95 m in basements over the required floor area, with beams, ducts and bulkheads permitted to drop to 1.85 m.

If you clear it, everything else on this list is ordinary construction.

If you don’t, the fix is underpinning the foundation or lowering the slab — excavating beneath the existing footings in sections, or removing and re-pouring the floor. That’s structural work requiring engineering, and it changes the budget category entirely. It’s also the single biggest reason a basement conversion goes from “the cheap option” to “not worth doing.”

Measure this before anything else. It is the highest-value ten minutes in the whole project.

2. Egress windows.

Every bedroom needs an escape window giving an unobstructed opening of at least 0.35 m² with no dimension less than 380 mm, and the sill no more than 1.5 m above the floor.

Below grade this means cutting the concrete foundation, installing a lintel, fitting the window, and excavating a window well with at least 760 mm of clearance in front and proper drainage. A real job, but a defined one — and considerably cheaper than underpinning.

3. Fire separation — and an Ontario concession worth finding.

New construction requires a 45-minute fire-resistance rating between units. But existing homes more than five years old may use a 30-minute or 45-minute separation depending on circumstances and the alarm arrangement.

That five-year provision can materially reduce the separation cost. Establish which route your house qualifies for before pricing the work.

4. A separate entrance — and a possible saving.

Cutting a new below-grade entrance means excavation, concrete, a stairwell, drainage and often a retaining structure. It’s a substantial line item.

But you may not need one. The requirement is independent access — the occupant reaching their unit without passing through your living space. A shared stairwell can satisfy that where each unit has its own private entrance off a common landing. Check your municipality’s position before budgeting for excavation.

5. Plumbing runs and a second kitchen.

Adding a kitchen and bathroom means new supply and drainage. How far they are from existing stacks matters — a suite laid out near the existing plumbing costs meaningfully less than one at the opposite corner of the basement.

6. HVAC and electrical.

Separate heating and ventilation as required, so the units are independently controlled and the fire separation isn’t compromised by shared ductwork. Plus an ESA electrical permit, and often a service capacity review — a second kitchen on an older panel frequently triggers an upgrade.

7. Sound separation.

A code requirement, and the thing that most determines whether the finished unit rents well and whether you can live above it comfortably. This is the one place we’d tell you not to economise.

Legalising an existing suite vs building new

Two genuinely different projects, and people conflate them.

Building new means creating a unit where there wasn’t one — full fit-out, all the items above.

Legalising an existing suite means bringing something that already functions up to code. The work is whatever’s missing, which ranges from a weekend of interconnected alarms and a self-closing door to structural underpinning.

Legalising is usually far cheaper — unless ceiling height is the failure, in which case it can cost more than building elsewhere on the property.

Our guide on legalising a basement apartment in Ontario covers the process step by step, and there’s a dedicated guide on what legalisation costs.

Permits and fees

The permit itself is a small share. Ontario municipalities publish their fee schedules; Toronto calculates building permit fees as a rate per square metre of proposed work. Look up your own municipality’s current schedule rather than relying on a figure in an article.

The big one is what you don’t pay. Under Bill 23, the creation of additional residential units in existing houses is exempt from development charges and parkland dedication — specifically the second and third units. GTA development charges per unit are among the highest in Canada, so this exemption frequently exceeds every other saving available to you.

Look up your municipality’s residential DC rate on its own schedule to see the scale of what you’re avoiding. Our development charge guide covers it in detail.

In Toronto specifically: rates are frozen at 2024 levels for 2025–26, with reductions of 40–60% planned between 2026 and 2029.

Also budget for: an ESA electrical permit, a plumbing permit, and drawings — which are usually the largest approval-stage cost.

Rent and payback

On rent: get your market’s figure from CMHC’s Rental Market Report, then adjust downward — a below-grade unit rents for meaningfully less than an equivalent above-grade one. Then subtract vacancy, maintenance, incremental insurance and tax on the rental income.

On payback: net rent against the increase in your carrying cost, over the hold period you actually expect. A basement apartment is the fastest-paying ADU because it’s the cheapest — but it’s still a multi-year proposition, and under about five years the numbers rarely justify it.

One tax point to raise before you build: creating a self-contained suite is generally a structural change, which can affect how the principal residence exemption applies to the rented portion when you sell. Worth an accountant’s hour up front.

Where people underestimate

Six recurring ones:

Ceiling height, checked last. People design the layout, then measure. Reverse it.

Egress treated as “a bigger window.” It’s cutting a concrete foundation and excavating a well. Price it as structural work.

Sound separation treated as optional. It’s a code requirement and a liveability one, and retrofitting it means reopening assemblies.

Electrical capacity. Discovering mid-project that the panel won’t carry a second kitchen means a service upgrade at the worst moment.

Plumbing distance. Laying the suite out far from existing stacks adds cost invisibly.

No contingency. Basements produce surprises — moisture, unexpected structure, undersized existing work. A budget without a contingency line converts an ordinary surprise into a problem.

Our Toronto building guide covers the local process.

Find out which category your basement is in

Ceiling height sorts your project into “ordinary construction” or “structural work,” and it takes ten minutes to check.

HouseLyft’s free property assessment establishes what your property supports and what compliance would involve. Request your free report.