More in total than a duplex, and less per unit — because a fourplex shares one foundation, one roof, one set of service connections and one mobilisation across four dwellings. That per-unit efficiency is the real reason the economics improve as you add doors, and it continues right up to the point where financing changes at five units.
- Per-unit cost falls with scale — shared foundation, roof, servicing and site mobilisation
- The fourth unit unlocks the federal GST rental rebate; a triplex cannot reach it
- Five units moves financing from residential to multi-unit — the biggest step change in the range
- Servicing capacity is the largest swing item and the one discovered too late
- In Ontario, the 2nd and 3rd units are DC-exempt; the 4th generally isn’t
This guide is about build cost. Our guide on what a fourplex costs and returns covers the pro forma — total development cost against stabilised income. Read that one if you’re deciding whether the project works; read this one if you’re trying to understand where the money goes.
What a fourplex costs
We could not source a defensible Canadian range, so we’re not publishing one. A new-build fourplex in Metro Vancouver and a conversion in an Alberta satellite are not comparable projects, and the ranges circulating online are single-market, unsourced, or American.
What’s reliable and transferable is the shape of the cost — and the most useful thing to understand about a fourplex is that it is cheaper per unit than anything smaller. That’s the section below.
Why per-unit cost falls with scale
This is the genuine insight of the post, and it’s why “just add another door” is sometimes the right answer.
Several major costs are fixed per building, not per unit:
One foundation. Excavating and pouring for a four-unit building costs more than for a two-unit building — but nowhere near twice as much. The setup, the equipment mobilisation and the engineering are largely the same job at a larger scale.
One roof. Same logic. A larger roof area, but one roof structure, one set of flashings, one mobilisation for the roofer.
One set of service connections. This is the big one. Whether you’re connecting two units or four, you’re doing one connection to the water main, one to the sanitary sewer, and one electrical service — sized differently, but a single job each. Splitting that cost four ways instead of two materially improves the per-unit figure.
One site mobilisation. A builder setting up on site, arranging inspections and managing trades incurs largely the same fixed cost regardless of unit count. Spread over four units, it’s half the per-unit burden of a duplex.
One permit process. More drawings and more review, but one application rather than four.
What genuinely scales per unit: kitchens, bathrooms, the plumbing and ventilation serving them, fire separations between units, and the finishes inside each dwelling. These are real and they’re the bulk of the incremental cost — but they’re a smaller share of the total than people expect.
The practical consequence: if your lot can carry four units and your financing supports it, the per-door economics are better than a triplex, which are better than a duplex. This is the arithmetic behind the “missing middle” argument, and it’s why the fourplex keeps coming up.
Where it stops: at five units, financing moves from residential to multi-unit underwriting. The construction efficiency keeps improving; the financing gets more complicated. That trade-off is covered below.
New build vs converting
Converting an existing house reuses the structure, foundation and existing service connections — but inherits their constraints. You’re working within someone else’s floor plan, ceiling heights and structural grid. Common cost items: fire separation between all four units, sound separation, egress from every unit, ceiling height where a unit is below grade, additional stairs, and a substantial electrical upgrade.
Building new costs more in total but produces an efficient layout — four sensible units rather than four compromised ones — and lets you size servicing correctly from the start.
Which is cheaper depends heavily on the existing house. A large older house with good ceiling heights and a sensible structural grid converts well. A low-basement bungalow does not. Our fourplex conversion guide covers the conversion route.
One thing that pushes toward new build at this scale: servicing. If the existing connection has to be upgraded anyway to carry four units, one of conversion’s main advantages disappears.
Soft costs and fees
At four units, soft costs step up meaningfully from a single-suite project.
- Drawings and design — more involved, and usually requiring a designer or architect rather than a drafter
- Structural engineering — routine at this scale
- Survey, grading and drainage plans
- Studies where required — arborist, geotechnical
- Permits and trade permits
- Contingency — non-optional at four units
And two threshold effects worth naming, because they’re decided at the drawing stage:
The GST rental rebate gate — at four. The enhanced federal rebate relieves the GST (and in Ontario, the provincial HST portion) on qualifying new rental construction. Per PwC Canada, 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. Construction must begin after 13 September 2023 and before 1 January 2031, and complete by 31 December 2035.
A triplex cannot reach this. A fourplex is the smallest building that can. On a qualifying project this removes the federal GST from the build — a substantial figure, not a rounding item.
Watch the 90% test if you intend to live there. One owner-occupied unit in four is 25%, which doesn’t leave 90% held for rental. Get advice before finalising the plan.
The Ontario development charge line — also at four. Bill 23 exempts the second and third units from development charges and parkland dedication. The fourth generally doesn’t get it automatically.
So the fourth unit simultaneously gains you the GST rebate and may cost you a development charge. Which dominates depends on your municipality’s DC rate and your build cost — model it both ways rather than assuming bigger is better.
Provincial cost base: BC charges 7% PST on many construction inputs; Alberta has neither a provincial sales tax nor Ontario-style development charges.
What it earns
Four units of rent, less vacancy, maintenance, insurance, property tax and management if you use it.
Get your market’s rents from CMHC’s Rental Market Report by unit type, and use a realistic vacancy figure — rates across major Canadian centres rose through 2025 and into 2026, easing what had been a very tight market. Assumptions from three years ago overstate revenue.
The most common analytical error at this scale is optimistic rent, and lenders look for it first in a small multi-unit application. Use comparable local rents you can evidence, not the top of the range.
Financing
At four units you’re still residential. Canadian lending treats one to four units as residential — underwritten substantially against your personal income and credit, with a portion of projected rental income counted toward qualifying you.
CMHC’s refinance product supports properties with up to four units at up to 90% of as-improved value, capped below $2 million, subject to owner or close-relative occupancy, funds going to construction, and CMHC approval before construction starts.
At five units and above, you move to multi-unit underwriting and MLI Select becomes available — more powerful terms, earned with long-term affordability, energy or accessibility commitments. Note its terms tightened in July 2025 with premium increases and an amortisation surcharge, so model from current figures.
One 2026 development: the federal Spring Economic Update of 28 April 2026 signalled that private insurers would be permitted to offer multi-unit insurance on five- to eight-unit properties. If your project sits there, ask a mortgage professional what’s actually available now.
Our financing page and multiplex development overview cover the routes.
Get your fourplex costed properly
The two things that decide a fourplex budget are servicing capacity and whether you’re converting or building new — both establishable before design.
HouseLyft’s free property assessment covers what your lot can carry. If your project is at this scale, get qualified is the right starting point for financing.
This guide explains construction cost drivers in general terms and is not a cost estimate or financial, tax or investment advice. No figure here is a quotation — obtain quotes from qualified builders and confirm tax treatment with a qualified accountant.
Checked by Lee Yousaf, Founder