Garden Suites, Bunkies and Secondary Dwellings: Septic Rules in Ontario

Bill 23 Says You Can Build a Garden Suite. Your Septic System May Disagree.

Ontario’s More Homes Built Faster Act opened the door to garden suites, bunkies with plumbing, and secondary dwellings on most residential properties. On municipal sewer and water, the conversation is mostly about zoning and construction. On a private well and septic lot, the conversation is different — and the septic system is often the deciding factor.

This guide is specifically for rural Ontario property owners on private septic systems. If your property is connected to municipal water and sewer, the septic question does not apply to you. If you are on a well and septic — which describes most of cottage country, Simcoe County, Haliburton, the Kawarthas, and rural Ontario generally — read this before you hire an architect or buy a prefab bunkie.

What Bill 23 Actually Changed — and What It Did Not

The More Homes Built Faster Act, 2022 (Bill 23) made several significant changes that affect secondary dwellings in Ontario. Ontario Regulation 299/19, as amended most recently in November 2024, now establishes that most residential lots — detached houses, semi-detached houses, and rowhouses — can accommodate up to three total residential units: the primary dwelling plus up to two additional units.

Bill 23 also eliminated development charges that previously added $10,000 to $30,000 to the cost of building a detached garden suite on many Ontario properties. That charge exemption applies to qualifying additional residential units built to the provincial framework.

What Bill 23 did not change: the Ontario Building Code. Specifically, it did not change Part 8 — the part that governs on-site sewage systems. A garden suite, bunkie with plumbing, or any secondary dwelling on a rural property still requires the existing septic system to be capable of handling the additional load, or the system must be upgraded to handle it.

The Core Septic Rule for Secondary Dwellings

Under Part 8 of the Ontario Building Code, an on-site sewage system and all the buildings it serves must be located on the same parcel of land. You cannot share a septic system across property lines. One lot, one system (or more than one system if the lot is large enough and permits allow).

More importantly for garden suite planning: the daily design flow calculation for the septic system must account for all bedrooms and all fixture units across every building the system serves — the main house and any additional dwellings. Adding a garden suite with one bedroom, a bathroom, and a kitchen sink adds both bedrooms and fixture units to the total flow calculation. That combined total is what the system must be sized to handle.

In plain terms: if your current septic system was permitted and sized for a 3-bedroom house, it may not legally support a 3-bedroom house plus a 1-bedroom garden suite. The system was not sized for that combined load. Whether it needs to be upgraded — and how significantly — depends on the original system design, the soil conditions on your lot, and what class of system you have.

How the Daily Design Flow Calculation Works for Secondary Dwellings

Ontario sizes septic systems using a daily design flow calculation based on bedrooms and fixture units across all buildings served. The base flow rate under Part 8 is approximately:

• 1 bedroom: 800 litres per day
• 2 bedrooms: 1,200 litres per day
• 3 bedrooms: 1,500 litres per day
• 4 bedrooms: 2,000 litres per day
• Each additional bedroom beyond 4: add 450 litres per day

Additional flow is added for fixture units above the included threshold and for living area above 200 square metres. The point is that adding a garden suite with one bedroom and standard fixtures to a 3-bedroom house does not simply add the garden suite’s flow to the calculation — it changes the total bedroom count for the whole property, which changes the base rate, which changes the required leaching bed area.

This is why the septic implications of a secondary dwelling need to be calculated by a qualified designer before you commit to the project — not after you have already poured the footings.

The Four Possible Outcomes on a Rural Lot

When you have a designer assess your existing septic system’s capacity for a secondary dwelling, the outcome falls into one of four categories:

Outcome	What It Means	Typical Next Step
System has adequate capacity	The original design flow exceeds the combined load of main house plus secondary dwelling. System can serve both without modification.	Proceed. May still need a permit amendment — confirm with health unit.
System is close but marginal	The combined load slightly exceeds the design flow. May be addressable with conservative assumptions or minor modifications.	Get a professional assessment. Designer will advise on options.
Leaching bed needs expansion	The tank is adequate but the leaching bed area is undersized for the combined load. New bed area is needed — either enlarging the existing bed or adding a second bed.	Permit a leaching bed expansion. Cost: $8,000 to $20,000+ depending on soil and system type.
Full system replacement or upgrade required	The existing system is already at or near capacity, aging, or the lot conditions require a higher system class to handle the additional load.	Budget for a full system assessment and likely replacement. Plan the replacement and the garden suite together — one excavation covers both.

What About Bunkies?

This is where a lot of Ontario cottage country homeowners run into surprises. The word “bunkie” covers a range of structures from a simple sleeping cabin with no plumbing to a fully serviced guest cottage with a bathroom, kitchen, and multiple bedrooms. The septic implications are completely different depending on which one you are building.

A bunkie with no plumbing

A structure with no toilet, sink, or any plumbing fixture does not add to the sewage flow and does not trigger a septic system assessment under Part 8. It is not considered a dwelling unit under the Building Code in the same way. You still need a building permit for the structure itself, and the municipality will have setback and lot coverage requirements — but the septic system is not affected.

A bunkie with a bathroom or kitchen

The moment a structure has plumbing — any toilet, any sink, any drain that connects to the sewage system — it becomes subject to the same Part 8 requirements as the main house. The fixture units and any sleeping areas in the structure must be added to the design flow calculation for the septic system. This is where many cottage owners are caught off guard. They build what they consider a modest guest cabin, add a bathroom “for convenience,” and discover they have just triggered a septic assessment requirement.

The Ontario Building Code also counts any room that could reasonably be used as a bedroom — including lofts, dens, and office spaces in outbuildings — toward the bedroom count for septic sizing purposes. The local health unit or Conservation Authority will interpret this conservatively. A loft with a bed in a bunkie counts as a bedroom.

The Lot Feasibility Question: Does Your Property Have Room?

Even if your existing system has capacity, adding a secondary dwelling may require a new or expanded leaching bed — which requires space on the lot that meets all required setbacks from the new dwelling, the main house, property lines, wells, and water bodies. On a typical rural Ontario lot with generous space, this is usually manageable. On a small cottage lot near a lake, it may not be.

The questions a designer will need to answer:

• Is there adequate usable area on the lot to accommodate an expanded or new leaching bed, meeting all setbacks from both dwellings, the property lines, any wells, and any regulated water bodies?
• Is there suitable soil in the proposed expansion area — or is the best soil already occupied by the existing system?
• Where will the replacement bed area be located when the current bed eventually fails — and does adding the garden suite now use up that future replacement area?
• Does the lot’s topography and drainage allow for a compliant system in the available area?

That last point is particularly important and often overlooked. Every lot should have a designated future replacement area reserved for when the current leaching bed eventually fails. If adding a garden suite uses up that replacement area — either because the new leaching bed occupies it or because the garden suite building sits on it — you may be compromising the property’s long-term septic capacity for short-term density.

What Municipalities Are Saying

Municipal responses to rural garden suites on private septic have been consistent: confirm your septic capacity first, before spending money on plans.

Tiny Township’s guidance to property owners considering garden suites states explicitly that homeowners should determine whether the sewage system is large enough before proceeding. Essa Township’s guide notes that private well and septic connections may be permitted if municipal services are not accessible, but only subject to municipal and servicing conditions. The North Bay-Mattawa Conservation Authority, which handles septic permits in Nipissing and Parry Sound Districts, requires a septic clearance review for any accessory building application — they are checking whether the proposed structure is within required setback distances from the existing septic system components.

The pattern is consistent across Ontario: local authorities want the septic question answered before the building permit is issued, not after. Attempting to get a building permit for a garden suite without addressing the septic capacity question first will result in a stop at the permitting stage.

Step-by-Step: How to Assess Your Property for a Secondary Dwelling

• Step 1: Locate your existing septic permit. Contact your local health unit or Conservation Authority and request the permit records for your property. You need to know what class of system is installed, what flow rate it was permitted for, and where the components are located. If you do not have these records, our guide on finding septic records in Ontario explains how to request them.
• Step 2: Have a designer calculate the combined design flow. Provide the designer with the bedroom count and fixture units for both the main house and the proposed secondary dwelling. The designer will calculate whether the combined flow exceeds the permitted system capacity.
• Step 3: If the system needs upgrading, plan it into the project. Get the septic upgrade permitted and priced as part of the garden suite project. If the system is aging anyway, plan the replacement to the appropriate size for the combined dwelling from the start.
• Step 4: Check lot area for the expanded or new leaching bed. Have the designer confirm there is adequate space on the lot for any required bed expansion, meeting all setbacks from both structures, property lines, and water features.
• Step 5: Submit the septic permit amendment or new permit before or alongside the building permit. The two permit applications can often proceed in parallel, but the health unit septic permit must be in order before installation begins.

The Bedroom Addition Connection

This entire issue is closely related to the bedroom addition trigger that most Ontario homeowners already know about. Under the Ontario Building Code, adding a bedroom to an existing home triggers a mandatory septic assessment. A garden suite or secondary dwelling is essentially the same trigger — you are adding bedrooms and fixture units that the existing system was not designed for.

The Building Code does not care whether the new bedrooms are in the main house or in a separate structure on the same lot. It cares about the total daily design flow the system is required to handle. That total determines the required system capacity.

Bill 23 changed the zoning and development charge landscape for secondary dwellings in Ontario. It did not change the laws of physics or the Ontario Building Code. On a private septic lot, the septic system still has to handle whatever you connect to it. The question is whether your current system can handle the load, and if not, what the upgrade costs and whether it changes the project’s economics. Those are questions worth answering before the concrete truck arrives.