What Is a Duplex? How Duplexes Work in Canada

“Duplex” is one of those words everyone uses and few people define the same way. A real-estate listing, a zoning bylaw and your neighbour may all be describing different buildings with it. That matters more than it sounds, because the version you actually have determines what you can build, how you finance it, and what taxes and rebates apply.

This guide sets out what a duplex is, how it differs from the buildings people confuse it with, what it takes to convert a house into one in Canada, and how duplexes are financed and taxed.

What is a duplex?

A duplex is one building divided into two separate, self-contained dwelling units. Each unit has its own entrance, kitchen, bathroom and living space, and each can be occupied independently. Both units sit on a single lot, usually under a single owner and a single title.

That last point is the one that does the work. Because a duplex is one property, it is bought, financed, insured and taxed as one property — not as two. You cannot normally sell half of it without a separate legal process to divide the land.

In practice, Canadians use “duplex” for three distinct buildings:

• The purpose-built side-by-side. Two units next to each other under one roof, sharing a vertical wall, built as a duplex from the start.
• The up-down duplex. One unit stacked above the other, sharing a horizontal floor assembly. In several major Canadian cities this is the default meaning of the word.
• The converted house. A single-family home divided into two legal units — most often a main floor and a basement suite, each brought up to code with its own entrance.

The third is what most homeowners actually mean when they ask about “making my house a duplex,” and it behaves quite differently from the first two.

Duplex vs semi-detached vs townhouse

Canadians mix these up constantly, and the difference is not architectural — it is about how the land is divided.

A semi-detached house and a side-by-side duplex can look identical from the street. The difference runs down the property line: a semi is two separate houses, each on its own lot, each independently sellable. A duplex is one property containing two units.

This distinction has been litigated in Canada, and municipalities define the terms in their own bylaws — which means the word your city uses may not match the word your real-estate listing uses. When it matters, read the zoning definition for your municipality rather than assuming.

Duplex vs triplex vs fourplex

The naming is simply a count of self-contained units in one building: two is a duplex, three a triplex, four a fourplex. What changes as you go up is not the vocabulary but the rules.

Three units is roughly where Canadian zoning reform landed — Ontario and B.C. now require most residential lots to permit three or more units without a rezoning. Four units is where several financing and tax thresholds sit. Five units is where residential lending generally gives way to commercial treatment.

So the count is not cosmetic. Each step up changes which rulebook you are in, which is why it is worth deciding early how many units you are actually aiming at.

Can you convert a house into a duplex in Canada?

In most of the country, yes — and the permission side has become dramatically easier since 2022. Three regimes matter:

Ontario. The More Homes Built Faster Act, 2022 (Bill 23, Royal Assent November 28, 2022) requires most residential lots zoned for a single home to permit up to three residential units without a rezoning. Two units — a duplex — sits comfortably inside that. The as-of-right units are also exempt from development charges and parkland dedication fees where they meet the building code and municipal standards.

British Columbia. Under the SSMUH rules in Bill 44, the Province of British Columbia requires most municipalities to permit a minimum of three units on parcels of 280 m² or smaller and four on larger parcels, within urban containment boundaries in municipalities over 5,000 people. Near frequent bus service the minimum rises to six. A second round of changes under Bill 25 is due in local bylaws by June 30, 2026.

Alberta. Edmonton’s Small Scale Residential (RS) zone under Zoning Bylaw 20001, in effect since January 1, 2024, permits up to eight dwelling units on a mid-block lot large enough to carry them — roughly 75 m² of site area per unit. Calgary and other Alberta municipalities set their own rules; there is no province-wide equivalent to Bill 44.

The honest caveat: “permitted” is not the same as “possible on your lot.” Lot width, rear-yard depth, servicing capacity, parking and overlay zones all narrow the answer, and the local bylaw is what a building official actually applies.

What does a duplex conversion cost and earn?

We do not publish a cost figure for this, and you should be sceptical of any page that does without naming a source. The range across provinces, forms and servicing conditions is wide enough that a single number is misleading rather than helpful.

What actually moves the cost:

• Whether the second unit needs a new entrance and stairwell. Cutting a separate exterior entrance, particularly a below-grade one, is often the largest single item in a basement conversion.
• Egress and ceiling height. Existing basements frequently fail on both. Underpinning to gain height is a structural job, not a finishing one.
• Fire separation and sound separation between units. Required, and rarely present in a house built as a single dwelling.
• Servicing and electrical capacity. A second kitchen and separate metering may need a service upgrade.
• Provincial cost base. B.C. charges 7% PST on many construction inputs; Alberta has no provincial sales tax and no Ontario-style development charges.

On the income side, be equally careful. Rent depends on your local market, the unit’s size and whether it is above or below grade — a below-grade suite typically rents for meaningfully less than an equivalent above-grade unit in the same neighbourhood. Any projection that ignores vacancy, maintenance and the tax on rental income is a marketing number, not a plan.

How are duplexes financed and taxed?

Two federal rules shape this, and both have thresholds that a duplex sits on one side of.

CMHC refinancing tops out at 90% loan-to-value. CMHC’s refinance product for secondary suites, introduced January 15, 2025, lets an owner refinance to fund the construction of additional units on a property with up to four units, to a maximum of 90% of the as-improved value, with the as-improved value capped below $2 million. The conditions are real: you must already own the home, you or a close relative must live in it, the funds must go to the construction itself rather than being taken out as equity, and the new unit cannot be operated as a short-term rental. Confirm current terms with CMHC or a mortgage professional before relying on them.

The federal GST rental rebate does not apply to a duplex. The enhanced purpose-built rental housing rebate relieves the federal GST/HST on qualifying new rental construction — but as PwC Canada sets out, the property must contain “four or more stand-alone apartments, each with a private kitchen, bath and living area, or 10 or more residential units,” with at least 90% held for long-term rental. Two units does not clear that gate. Construction must also begin after September 13, 2023 and before January 1, 2031, and be completed by December 31, 2035.

That four-unit line is the single most useful thing to know if you are choosing between a duplex and something larger — it is a real financial difference, not a rounding one. If a fourplex is plausible on your lot, our fourplex conversion guide covers what changes at that threshold.

Rental income is also taxable, and the expenses you can deduct against it depend on how the property is used and structured. This is worth an hour with an accountant before you start, not after.

Find out what your lot supports

Whether a duplex is the right move usually comes down to four things: what your zoning permits, what your lot’s dimensions physically allow, whether servicing is already in place, and how the build would be financed. Those are answerable — but only for a specific address.

HouseLyft’s free property assessment looks at exactly those factors and tells you which configurations are realistic on your property. If you want to read further first, our multiplex development overview covers what happens above two units, and our financing page explains how these projects get funded. When you want a straight answer for your own address, request your free report.