How to Legally Convert a House Into a Duplex

Converting a house into two legal units used to be a fight with the planning department. Since 2022 it mostly isn’t — three provinces moved at once, and two units now sits comfortably inside what’s permitted almost everywhere urban.

What that means practically is that the project has shifted from a permission problem to a construction problem. This guide covers the construction problem. Our guide on what a duplex is covers the definitions.

Is it allowed where you live?

Almost certainly, if you’re in an urban area of Ontario, BC or Alberta.

Ontario. The More Homes Built Faster Act, 2022 (Bill 23) requires most residential lots zoned for a single home to permit up to three residential units as-of-right — on urban residential land served by municipal water and sewer. Two units is well inside that. The second and third units are also exempt from development charges and parkland dedication.

British Columbia. The Province’s small-scale multi-unit housing rules require most municipalities to permit a minimum of three units on parcels of 280 m² or smaller and four on larger parcels, rising to six near frequent bus service. A further compliance round under Bill 25 is due by 30 June 2026.

Alberta. Municipal. Edmonton’s Small Scale Residential zone permits up to eight units on a sufficiently large mid-block lot. Calgary is mid-change — the blanket rezoning repeal takes effect 4 August 2026 while suites separately move toward permitted-use status — so check your specific address.

As-of-right means no rezoning and no council decision. It does not mean no permit. You still need a building permit, and the conversion must meet the objective zoning standards and the building code.

What still varies: lot geometry after setbacks, parking requirements where they apply, and servicing capacity. Those are the checks worth doing before design.

The code work that defines the budget

This is where the money goes, and it’s a finite list.

Fire separation between the units. Required, and absent in a house built as a single dwelling. Ratings differ by province:

• Ontario: 45 minutes for new construction; houses more than five years old may use a 30 or 45 minute separation depending on circumstances — a genuine concession worth establishing early
• BC: 45 minutes, reduced to 30 minutes where additional photo-electric smoke alarms are installed in each unit and interconnected
• Alberta: a smoke-tight separation between the units and in common spaces and the furnace room, with doors in those locations solid core wood at least 45 mm thick with self-closing devices

In an existing house, achieving this generally means opening up floor and ceiling assemblies rather than adding a layer over the top.

Sound transmission between units. A distinct requirement from fire separation, and the one that most determines whether the finished units are comfortably liveable and rentable. Skimping here is a false economy — you’ll live with it or your tenants will complain about it.

Egress for both units. Under Section 9.9.10, each bedroom needs an escape window or exterior door — unless the suite is sprinklered — giving an unobstructed opening of at least 0.35 m² with no dimension under 380 mm, openable from inside without tools. Window wells need at least 760 mm of clearance in front.

Ceiling height, where a unit is below grade. 1.95 m in Alberta and Ontario basements (1.85 m below beams and ducts); 2 m in British Columbia. This is the one that can’t be fixed cheaply — underpinning is structural work — so measure it before anything else.

Separate heating and ventilation, as required, so the units can be controlled independently and the separation isn’t compromised by shared ductwork.

Electrical capacity and metering. Two kitchens and two sets of appliances frequently require a service upgrade. Separate metering isn’t always mandatory but is usually worth doing — it removes a whole category of landlord-tenant friction.

Interconnected smoke alarms across both units, plus carbon monoxide alarms.

The permit process

1. Measure ceiling height and check egress openings. This tells you which category of project you’re in before you spend anything.
2. Confirm zoning permits two units, and check parking and any amenity space requirements.
3. Establish servicing capacity — water, sewer and electrical. The earlier the better.
4. Get drawings prepared showing the unit layouts, separation assemblies, egress and mechanical arrangement.
5. Apply for the building permit. In Alberta, expect a development permit first as well.
6. Apply for trade permits — electrical and plumbing.
7. Build, with inspections at stages.
8. Register where your municipality requires it.

The step people skip: starting work before permits. Retroactive permitting is available in most municipalities but costs more and may require opening finished work for inspection.

What it costs

We don’t publish a figure — the range between a house that already has the height and services and one needing underpinning and a service upgrade is too wide for one number to be useful.

The drivers, in order of impact:

• Ceiling height, if you’re short. Underpinning changes the budget category entirely.
• Egress retrofits — cutting concrete and building window wells.
• Fire and sound separation across the full shared assembly.
• Electrical service upgrade and metering.
• A second kitchen and bathroom — plumbing and ventilation, repeated.
• Provincial cost base. BC charges 7% PST on many construction inputs; Alberta has no provincial sales tax and no Ontario-style development charges.

The offset in Ontario: development charges are waived on the second and third units under Bill 23. GTA development charges per unit are among the highest in Canada, so on an Ontario conversion this exemption frequently exceeds every other saving available. Look up your municipality’s rate schedule to see the scale.

What it earns, and how it’s financed

On the income side, three honest caveats: a below-grade unit rents for meaningfully less than an equivalent above-grade one; unit quality matters more below grade than above; and any projection ignoring vacancy, maintenance and tax on rental income isn’t a projection.

On financing, the good news is that a duplex stays firmly in residential territory. Canadian mortgage lending treats one to four units as residential — underwritten substantially against your personal income and credit — with five or more moving to multi-unit and commercial-style underwriting. That five-unit line is the biggest step change in the whole range, and a duplex is nowhere near it.

Two routes to fund the conversion:

• CMHC’s refinance product, available since 15 January 2025 — up to 90% of the as-improved value on properties with up to four units, as-improved value capped below $2 million. Conditions include owner or close-relative occupancy, funds going to construction rather than equity take-out, no short-term rental of the new unit, and CMHC approval before construction starts.
• Conventional refinance or HELOC — lower leverage, typically 80%, but fewer conditions.

Lenders will typically count a portion of projected rental income toward qualifying you — commonly up to half — but generally only where the unit is legal. That’s a concrete financial reason to permit the conversion properly rather than a moral one. Our financing page covers the routes.

One tax point to raise before you build: creating a self-contained second unit 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.

Mistakes that force expensive rework

Covering work before inspection. Framing, separation assemblies and egress openings are checked before they’re concealed. Drywalling early is how a modest job becomes an expensive one.

Designing before measuring ceiling height. If you’re under the minimum, everything downstream changes. Measure first.

Treating sound separation as optional. It’s a code requirement and a liveability one. Retrofitting it after the fact means reopening assemblies.

Undersizing the electrical service. Discovering mid-project that the panel won’t carry two units means a service upgrade at the worst possible time.

Assuming a shared stairwell won’t work. It often will — Calgary accepts a shared stairwell provided each unit has its own private entrance off it. People budget for cutting a new exterior entrance they may not need.

Ignoring the fourth-unit line. If there’s any chance you’ll later go to four units, plan the servicing and layout for it now. Retrofitting capacity is far more expensive than installing it once. Our fourplex conversion guide covers what changes at that threshold.

Find out whether your house converts economically

The difference between a straightforward conversion and an expensive one comes down to ceiling height, egress and servicing capacity — all establishable before you commission a single drawing.

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