Legal basement suites (ARUs): what qualifies, what it costs, and what trips people up

An Additional Residential Unit — an ARU, what most people still call a legal basement apartment — is one of the most sensible things you can add to a house in Ontario right now. Provincial rules changed to let most homes have up to three units, and the math is compelling: rental income in the range of $1,600 to $2,400 a month across much of the GTA and surrounding towns, plus real value added to the property. But the gap between a finished basement and a legal suite is wider than people expect, and it's usually the boring details — ceiling height, a window a few inches too small, where the furnace lives — that decide whether the project pencils out.

So before you hire anyone, it helps to understand what you're actually being asked to build. A legal ARU is a self-contained dwelling: its own kitchen, its own bathroom, its own sleeping and living space, and — critically — its own safe way in and out. It has to meet the Ontario Building Code and Fire Code, and it has to satisfy your municipality's zoning. Those are two separate approvals, and a suite can clear one while failing the other.

Start with ceiling height, because it's the one thing you can't negotiate. The Building Code wants a minimum clear height — generally around 1.95 metres (roughly 6'5") over the required area, measured to the underside of beams and ducts, not just the joists. Older homes with 7-foot foundations often lose the fight to a low duct run or a dropped bulkhead. If your basement is genuinely short, you're either underpinning (expensive) or you don't have a project.

Then there's egress. Every bedroom needs a window big enough to climb out of — the code specifies a minimum openable area, and it can't be blocked. That usually means enlarging a window and building a window well below grade. A second exit from the suite, fire separation between the units (typically a 45-minute fire-rated ceiling assembly), interconnected smoke and carbon-monoxide alarms, and sound separation between floors round out the safety picture.

Parking and zoning are where local rules bite. Many municipalities have relaxed or removed the extra-parking requirement for a second unit, but not all, and setbacks for a separate entrance matter. If your only viable exit lands in a side yard that's too narrow, that's a zoning problem before it's ever a construction one. The mechanical side is just as real: a single furnace can sometimes serve two units, but you'll need proper ductwork, and separate hydro metering — while not always required — makes life far easier when a tenant moves in.

For a straightforward suite in a house with decent ceiling height and a workable exit, a realistic all-in budget lands somewhere between $65,000 and $120,000 in today's market. The spread is wide for good reasons. A new below-grade entrance with excavation, a stairwell, and a window well can add $15,000 to $30,000 on its own. Underpinning to gain height is a different order of magnitude — often $50,000 and up — and it's the single biggest reason a project's numbers stop working.

On timing, expect the design and permit phase to run six to twelve weeks depending on your municipality's queue, and construction to take twelve to twenty weeks. Committee of adjustment — needed if you're seeking a minor variance — can add two to four months. None of that is the fun part, but it's the part that protects you: a permitted, inspected suite is insurable, financeable, and legal to rent. An unpermitted one is a liability that surfaces at the worst possible moment.

That's the honest throughline. The projects that go sideways almost never fail because of finishes or fixtures. They fail because someone framed and drywalled before confirming the egress window would pass, or assumed the existing furnace could carry two units, or discovered mid-build that the side-yard entrance needed a variance. Each of those is fixable on paper and painful in the field.

A few things quietly recur. The first is measuring ceiling height to the joists instead of to the ducts and beams — the code cares about clear height, and the bulkheads are what eat it. The second is treating the separate entrance as an afterthought; it's often the most expensive and most zoning-sensitive element, so it should be settled early. The third is underestimating fire and sound separation between units, which is not the same as the insulation you'd put in a finished rec room. And the fourth is skipping the permit to save time — which almost always costs more later, in re-work, in insurance gaps, or when a future buyer's lawyer asks for the paperwork.

None of this is meant to discourage. Across Milton, Oakville, Mississauga, and the wider region we work in, ARUs are one of the best-returning things a homeowner can do — they add income, flexibility, and lasting value, and the province has made the path more open than it's ever been. The point is simply that a legal suite is a small building inside your house, and it rewards the same discipline any building does: confirm the constraints first, design to them honestly, and permit the work. Do that, and the numbers usually take care of themselves. Skip it, and the basement will find a way to remind you.

If you're at the research stage, the most useful thing you can do before spending money is a feasibility check: verify your ceiling height at the low points, find where a legal exit could go, and confirm what your municipality's zoning actually allows on your lot. Those three answers tell you whether you have a $70,000 project or a $150,000 one — and whether you have a project at all.