Section 9.31 of the Ontario Building Code is short — two pages of Division B carry the entire residential plumbing-fixture conversation — because the underlying rule is simple. Every self-contained dwelling unit gets the same fixed set of fixtures: a place to cook, wash, bathe, use the toilet, and do laundry. The code does not scale that list with bedroom count or square footage. What scales is the number of dwelling units: add a secondary suite and you add a full second set of fixtures, with one carve-out for shared laundry.
9.31.4.1.(1) spells the core list in one sentence. Every dwelling unit served by a water distribution system shall have a kitchen sink, a lavatory, a bathtub or shower, and a water closet. 9.31.4.1A.(1) adds the fifth — laundry facilities, or at minimum a space for laundry facilities. 9.31.3.2 assigns the water connections: hot and cold to the kitchen sink, lavatory, bathtub, shower, slop sink, and laundry area; cold to every water closet. The list does not change with household size. What changes is whether the water closet, lavatory, and tub or shower are in one bathroom or spread across two — driven by 9.5.3F.1, which requires an enclosed space of sufficient size to accommodate the fixtures, plus design conventions for layouts with bedrooms on a separate storey. Every fixture needs its own drain and vent per Part 7, and every bathroom needs a door per 9.5.3F.2.
A secondary suite — legal basement apartment, coach house over a garage, in-law suite with its own entrance — is by definition a second dwelling unit. 9.31.4.1 does not read "per house," it reads "per dwelling unit," so the suite inherits the full core list independently of the primary: one additional kitchen sink, water closet, lavatory, and tub or shower, each with its own supply lines and drain and vent. 9.31.4.1A is the single relaxation the OBC offers: laundry may be grouped in a location conveniently accessible to every dwelling unit. A shared laundry in the basement, reachable from both the primary stair and the suite's private entrance without going outside, is legal and common. Bathrooms and kitchens have no equivalent carve-out — the suite kitchen and bathroom must be inside the suite. Beyond the OBC, most Ontario municipal zoning bylaws define a secondary suite as self-contained, meaning a private bathroom and kitchen are a zoning condition even where the building code might theoretically permit sharing. The practical answer: design the suite with its own full set of fixtures plus optional shared laundry, and never count on sharing a bathroom.
Strictly speaking, 9.31.4.1 does not set a bedroom-count threshold. A legal Ontario house could have fifteen bedrooms and one bathroom and still satisfy the letter of 9.31.4. In practice that house does not get built — 9.5.3F.1 requires an enclosed space of sufficient size to accommodate the fixtures, and a single bathroom for five or more bedrooms does not pass the sufficient-size test in front of any building official in the province. The universal design rule, backed by local AHJ custom rather than an explicit clause, is that five or more bedrooms trigger a second bathroom, ideally on a different storey from the first. The logic is queuing and egress — a family of seven cannot share a single basement bathroom. The same principle produces the more common two-storey pattern: a main-floor powder room plus an upstairs bathroom, even in a three-bedroom build, because the code is a minimum and the design standard is what people actually live in.
The OBC is explicit about clearances only in Section 3.8 for public and barrier-free facilities. For private dwelling-unit fixtures, the numbers come from Part 7 plumbing-code cross-references, industry convention, and typical AHJ practice. The residential minimums every designer and inspector uses: 500 mm of clear floor space in front of the water closet, measured from the front of the bowl to the opposite wall or fixture; 380 mm from the centreline of the water closet to each side wall, making 760 mm the minimum wall-to-wall enclosure; 500 mm in front of the lavatory; 600 mm of clear access in front of a tub or shower; and 610 mm minimum for the bathroom door per Table 9.5.5.1. Inspectors measure the water closet front clearance because it is the dimension that gets eaten by last-minute layout changes — a tight layout ends up with the tub curb 400 mm in front of the toilet bowl, and a person of average build cannot sit down. Plan to 500 mm and the fixture inspection is uneventful.
A bathroom rough-in is the cheapest piece of future-proofing available on a new build or a gut renovation. At framing stage, the plumber runs the drain stack to the future water closet location, the vent up through the ceiling, and the hot and cold supply lines to the future lavatory and tub. The pipes are capped, pressure-tested, and left in the slab or the wall behind drywall. Cost is typically $600 to $1200 on top of the existing first-fix plumbing bill because the plumber is already on site with the tools, the materials, and the opening. The retrofit scenario ten years later looks entirely different. A finished basement has drywall, flooring, lighting, HVAC ducts, and furniture. Adding a bathroom means cutting an eight-foot by three-foot section of concrete slab for the new drain, coring through the framed ceiling to tie the vent into the main stack, re-routing ducts, patching the floor and ceiling, and redoing all the finish work from scratch. Realistic numbers are $6000 to $15000 for the same physical outcome — one toilet, one sink, one shower. If there is any chance the basement will be finished within ten years, rough it in now. The math is one-sided.
9.31.4.1A.(1) is one of the most useful carve-outs in the residential OBC. Laundry facilities, or a space for laundry facilities, shall be provided in every dwelling unit OR grouped elsewhere in the building in a location conveniently accessible to occupants of every dwelling unit. The OR is load-bearing — a duplex, a house with a secondary suite, or a small multi-unit building can install one shared laundry room in the basement and meet the code for every unit simultaneously, as long as the shared space is reachable from every unit without going outside. Reachable-without-going-outside is the test, and it is where shared laundry in a detached secondary unit (a laneway suite or garden suite) fails — if the occupant has to step outside to do laundry, the shared space is not conveniently accessible and each unit gets its own. Beyond the OBC, municipal zoning adds its own layer. Most Ontario cities permit shared laundry for secondary suites but require written access rights in the tenant's lease. Smaller municipalities vary — check the zoning bylaw and the building division's interpretation before finalizing a suite design, because the zoning test can be stricter than 9.31.4.1A.