Yes, you can likely add an additional residential unit (ADU) in Mississauga today. The confusion homeowners run into isn’t about whether it’s legal. It’s that the rules governing it come from two different governments stacked on top of each other, and most articles online blend them into one list without saying which layer each rule actually comes from.
The two-layer rule: what Ontario sets, and what Mississauga decides
Ontario’s Bill 23, the More Homes Built Faster Act, amended the Planning Act and updated O. Reg. 299/19 effective November 28, 2022, to allow up to three units per lot as-of-right province-wide on serviced residential land, either three units in the principal building, or two in the principal building plus one in an ancillary building. That change is documented in Environmental Registry of Ontario notice 019-6197. The same notice bars municipalities from requiring more than one parking space per additional unit, setting a minimum unit size, charging development charges, or requiring parkland dedication for a second or third unit in this category.
What the province does not do is tell Mississauga how big those units can be, how tall, or how far from the property line. That’s the second layer, and it’s the one that actually shapes what you can build.
| Layer | Who sets it | What it covers |
|---|---|---|
| Provincial | Government of Ontario, via Bill 23 and O. Reg. 299/19 | Whether ADUs are allowed at all, and the unit-count ceiling |
| Municipal | City of Mississauga, via its zoning by-law and building rules | Size, height, setbacks, permits, fees, and process |
“ADUs are legal” is a provincial statement. “This is what fits on your lot” is a municipal one, and it’s the one that requires a phone call before you design anything.
So how many units can you actually add?
Mississauga’s own guidance confirms up to two additional residential units per eligible detached, semi-detached, or townhouse property, on top of the existing home. Only one garden suite is allowed per lot, from the same source, which means the second unit is typically an interior or attached one, like a basement apartment, rather than two detached suites.
Put together, that’s one principal dwelling plus two additional units, three total. Call it the three-unit ceiling. It mirrors the province’s as-of-right cap exactly, which isn’t a coincidence, since Mississauga isn’t allowed to set the ceiling any lower for eligible properties. What it can and does set is everything below that ceiling, which is where the next section matters more than the number itself.
Why the by-law keeps moving
Mississauga regulates additional residential units under By-law 0174-2023 and fourplexes under a separate By-law 0196-2023. Both are recent, and both sit inside a zoning framework that changed again in 2025: the City’s Neighbourhood Zoning Review consolidated most low-rise residential zones into two new zones, Residential Large Lot and Residential Small Lot, in force since April 2, 2025, with the RL and RS regulations revised again on April 30, 2025. Details are in the City’s zoning by-law materials.
That timeline matters for one reason: the structure underneath every number in this post has been rewritten within the past year. Treat any figure you read here, or anywhere else online, as a starting point rather than a final answer, and confirm it against the current by-law before you commit to a design.
The one number we won’t give you
Most ADU articles quote a maximum floor area, height, and setback for a detached unit as if it’s one fixed figure. We looked for that number directly on the City’s own pages and couldn’t confirm one. Mississauga’s published guidance describes the envelope in general terms, staying within maximum building heights and sizes, and maintaining setback distance from property lines, without publishing a single city-wide figure, because it varies by zone.
Third-party sites disagree with each other on the specifics, which is itself a signal. If the number were truly universal, those sites would match. They don’t, because the real figure depends on which zone your property falls into under the by-law, and that has to be checked directly with the City’s zoning or building division for your specific address. We’d rather tell you that plainly than repeat a number we can’t stand behind.
A shortcut around some of the guesswork
One place the City has removed uncertainty on purpose: pre-approved garden suite plans, launched in 2024. A studio option runs roughly 40 m² (430 sq ft) and a one-bedroom option roughly 55 m² (600 sq ft), sized and pre-cleared by the City itself, according to the garden suite plans announcement. Using one of these plans sidesteps a chunk of the custom-design uncertainty that the previous section describes, though it doesn’t remove the need to confirm siting and setbacks for your specific lot.
Garden suites also carry a specific access requirement: the fire-access pathway to the unit must be at least 1.2 m wide and 2.1 m high, and no more than 45 m long measured from the edge of the street, per the City’s additional residential units page. If you’re adding a new civic address for a garden suite, the current fee, as of 2025, is $81.90 plus HST. Fees like this change, so confirm the amount before you budget.
Registering a unit isn’t the same as permitting one
These two processes get conflated constantly, and they’re not interchangeable. Building a new unit requires a building permit through the City’s ePlans portal, compliance with the applicable ARU or Fourplex by-law, and sign-off from City inspectors before anyone moves in.
Registering an existing unit is a different process entirely, and it’s for units that are already built, not ones you’re about to build. It’s free. You submit proof of ownership, permit documentation (or apply for a permit retroactively if none exists), and a registration form to secondunits@mississauga.ca. The unit has to meet the Ontario Building Code, the Ontario Fire Code, and the zoning by-law before it goes on the City’s Second Units Registry, which updates monthly, according to the City’s registering a second unit page. If your basement apartment was never permitted in the first place, registration doesn’t skip that step. It comes after it.
Three assumptions worth checking before you go further
“ADUs are legal now, so mine will be approved.” As-of-right unit count is not the same as automatic approval. Size, height, setback, servicing, and site-specific factors can still disqualify a design that would otherwise be allowed on paper.
“Registering my basement apartment is the same as getting a permit.” Registration is a compliance record for a unit that already meets code. An unpermitted unit still needs a permit and inspection first, and registration comes after that, not instead of it.
“One rule applies everywhere in the city.” The province sets the floor, up to three units. What actually fits on your lot depends on which zone your property sits in under Mississauga’s by-law, and that’s not the same answer for every property in the city.
What this post can’t tell you
Everything above is the rule structure: what’s allowed, how many units, which by-laws apply, and how the registration and permit paths differ. None of it tells you whether an ADU actually works on your specific lot. That’s a separate question involving net rear-yard space after existing structures and setbacks, servicing capacity, zoning overlays, and lot coverage, and it’s the one our site assessment post walks through. Read the two together: this post is the rulebook, that one is the reality check for a specific address.
Talk to the City, then talk to us
Confirm the current by-law, zone, and fees with the City of Mississauga’s building and zoning division for your specific address before you commit to a design. Once you know what the rules allow, our ADU work in Mississauga picks up from there with a feasibility review that tells you what your lot can actually hold. If you’re earlier in the process and still comparing builders, what to ask before hiring one is a reasonable next read.
