What Is a Fourplex? Four Units on One Lot, Explained

If you have followed the “missing middle” conversation in Canada, you will have noticed how often it lands on the fourplex. That is not a design preference. Four units is where several separate Canadian rules happen to converge — financing, tax and zoning — and it produces a building that is meaningfully easier to fund than a five-unit one.

This guide explains what a fourplex is, how it compares to smaller buildings, why the four-unit line matters so much in Canada, whether you can build one on an ordinary residential lot, and how the financing works.

What is a fourplex?

A fourplex is one building containing four separate, self-contained dwelling units. Each has its own entrance, kitchen, bathroom and living space, and each can be occupied independently. All four sit on a single lot, typically under one owner and one title.

The physical arrangements vary. A fourplex can be two units up and two down, four units stacked in a two-and-two arrangement across a shared corridor, or four side-by-side in a row. What makes it a fourplex is the count of self-contained units, not the layout.

The term overlaps with “multiplex,” which municipalities increasingly use as a catch-all for three-to-six-unit buildings in low-rise neighbourhoods. If your city’s bylaw talks about multiplexes, a fourplex is one.

Fourplex vs duplex vs triplex

The vocabulary is a simple count — two, three, four units in one building. What changes as you go up is which rulebook applies:

That table is the whole argument for the fourplex. It is the largest building that stays inside residential mortgage treatment while being the smallest that can reach the federal rental rebate. Nothing else in the range sits in both.

Why four units is the number that matters in Canada

Three separate rules converge here, and none of them were designed together.

The financing cliff at five units

CMHC’s mortgage loan insurance for multi-unit residential properties applies to buildings with five or more self-contained units. Below that — one to four units — a property is generally handled under homeowner mortgage rules instead.

This is a bigger deal than it sounds. Crossing from four units to five typically means moving from residential underwriting, which leans on your personal income and credit, to commercial underwriting, which leans on the building’s net operating income, and usually means different rates, different amortisations, different documentation and a different class of lender. A great many small Canadian projects stop at four units for exactly this reason.

CMHC’s refinance product for secondary suites, introduced January 15, 2025, is built around the same boundary: it supports properties with up to four units, to a maximum of 90% of the as-improved value, with that value capped below $2 million.

The GST rental rebate gate at four units

The enhanced federal purpose-built rental housing rebate relieves the GST — or the federal portion of HST — on qualifying new rental construction. As PwC Canada sets out, the property must be a multiple unit residential complex containing “four or more stand-alone apartments, each with a private kitchen, bath and living area, or 10 or more residential units,” with all or substantially all — 90% or more — of the units held for long-term rental as a tenant’s primary residence.

Construction must also begin after September 13, 2023 and before January 1, 2031, and be completed by December 31, 2035. A condominium complex is excluded.

Two things worth flagging. First, a duplex or triplex simply cannot qualify — the four-unit floor is absolute. Second, that 90% test deserves attention if you intend to live in the building yourself: in a four-unit property, one owner-occupied unit is 25% of the total, which does not leave 90% held for rental. Whether and how that affects a specific project is a question for an accountant before you start, not after.

Zoning reform landed near here

Canadian zoning reform since 2022 has pushed small multi-unit buildings into as-of-right territory, though not uniformly at four:

• Ontario — Bill 23 requires most residential lots to permit three units without a rezoning. Three, not four. Some municipalities permit four, but that is a local decision rather than the provincial floor.
• British Columbia — the Province of British Columbia’s SSMUH rules require a minimum of three units on parcels of 280 m² or smaller and four on larger parcels, rising to six near frequent bus service.
• Alberta — Edmonton’s Small Scale Residential zone under Zoning Bylaw 20001 permits up to eight units on a mid-block lot large enough to carry them.

So a fourplex is as-of-right on a reasonably sized B.C. lot and comfortably within Edmonton’s limits, but in Ontario the fourth unit usually requires checking your municipality rather than relying on the province.

Can you build a fourplex on a normal residential lot?

Sometimes — and the constraint is more often physical than legal now.

Even where four units are permitted, the lot has to carry them. The rules that decide it are the unglamorous ones: site coverage, height, setbacks, parking where still required, amenity space, and separation from neighbouring buildings. Edmonton’s approach illustrates the arithmetic — its guidance works out to roughly 75 m² of site area per unit, meaning a lot of about 600 m² to reach the full eight units the zone allows.

Then there is servicing, which is the constraint nobody checks first and the one that most often kills a project quietly. Four units need water, sanitary sewer and electrical capacity that a single-family lot was never sized for. Where the existing connection is undersized, upgrading it can involve work in the street, and that is both expensive and slow.

Converting an existing house into four units brings its own list: egress from every unit, fire separation between all of them, sound separation, ceiling heights, and often a second stair. Our fourplex conversion guide covers that route specifically.

What does a fourplex cost to build?

We don’t publish a figure. A four-unit new build in Metro Vancouver and a four-unit conversion in an Alberta satellite are not comparable projects, and any single national number is invented rather than researched.

What actually moves it:

• New build vs conversion. A conversion reuses the structure and foundation but inherits its constraints; a new build costs more and gives you an efficient layout.
• Servicing capacity. The single largest swing item at this scale. Confirm it early.
• Number of kitchens and bathrooms. Four of each is the expensive part of a fourplex — plumbing and ventilation, repeated.
• Fire and sound separation between four units, plus the assemblies and possibly sprinklers your code requires at this scale.
• Parking and site works, where your municipality still requires them.
• Provincial cost base. B.C. charges 7% PST on many construction inputs; Alberta has neither a provincial sales tax nor Ontario-style development charges.

The one genuinely large documented saving at this scale is the GST rental rebate above — on a qualifying project it removes the federal GST from the build, which is not a rounding item. It is worth structuring the project deliberately to qualify rather than discovering afterwards that it doesn’t.

How is a fourplex financed?

At four units, you are on the residential side of the line, which is the good side for most homeowners.

That generally means the mortgage is underwritten against your personal income and credit rather than purely against the building’s operating income, with residential rates and amortisations. Lenders will typically consider a portion of the projected rental income in qualifying you, though how much varies by lender and product.

The routes people actually use:

• Refinancing the existing property to fund construction — CMHC’s secondary suite refinance supports up to four units at up to 90% of as-improved value, subject to owner or close-relative occupancy, funds going to construction rather than equity take-out, and no short-term rental use.
• Construction financing, drawn in stages against progress, then converted to a standard mortgage on completion.
• Conventional refinancing once the units are built, tenanted and the value is established.

At five units and above, this changes completely — you move into CMHC’s multi-unit programmes and commercial underwriting. That is a legitimate path and often the right one for a larger project, but it is a different process with different requirements. Our financing page covers how these projects get funded, and multiplex development covers what happens above four.

Confirm current terms with CMHC or a mortgage professional — products and rules change, and the details above are a starting point rather than an offer.

Find out whether four units fit your lot

Whether a fourplex works comes down to four questions that can all be answered before you spend real money: what your zoning permits, whether the lot’s dimensions physically carry four units, whether servicing capacity is there, and how the project would be financed given the thresholds above.

HouseLyft’s free property assessment works through those for your specific address. If your project is likely to sit at four units or above, our get qualified page is the faster route in. When you want the answer for your own lot, start with a free report.