Can You Build a Backyard Home on Septic in Ontario?
Short answer: it depends on your municipality. Some allow it with conditions, some do not allow a detached unit on septic at all. Here is how it breaks down.
London: municipal services required
In London, a detached additional residential unit must connect to municipal water and sanitary sewer. A lot on a private septic system does not qualify for a detached ARU as-of-right. We have asked City planning staff whether any variance or alternative approval path exists and will update this post when they reply.
Rural areas: possible, with conditions
Outside fully serviced areas, the rules are set lot by lot. In Hamilton's agricultural zones, for example, a detached accessory dwelling needs a minimum lot size of 1.5 hectares — most rural residential lots do not clear that bar, and Greenbelt lands add another layer of restriction.
Where a detached unit is allowed on private servicing, the Ontario Building Code (Part 8) requires the septic system to be sized for the combined sewage flow of both dwellings. In practice that usually means:
- A new or substantially enlarged septic system, designed by a qualified professional
- A sewage system permit from the municipality or health unit
- A well adequacy assessment if the lot is on a private well
Some municipalities also require nitrate or hydrogeological studies before they will sign off, and some apply holding provisions that require proof the system can handle the additional load before a building permit issues.
Detached vs. internal units
Where a detached unit is not permitted on septic, an internal second unit — inside the existing house — is sometimes still possible. The zoning treatment is different, but the septic system still has to be proven adequate for the combined occupancy. Do not assume the existing system qualifies; most were sized for a single dwelling.
Why servicing is the gating item
On urban lots, people focus on setbacks and height. On private servicing, the septic system is usually what decides feasibility — before setbacks even matter. A system replacement can add tens of thousands of dollars, and a lot that cannot support an enlarged bed may not be feasible at any price.
Check servicing early. It is the cheapest question to answer and the most expensive one to get wrong.
The bottom line
- London: detached ARU requires municipal sewer and water — septic lots do not qualify as-of-right.
- Rural municipalities: sometimes allowed subject to lot size minimums and a septic system sized for both dwellings.
- Everywhere: the existing septic system must be assessed; assume it needs upgrading until proven otherwise.
If your property is on well and septic, start with a free property review. We check the zoning, the servicing rules for your municipality, and whether the lot can realistically support a second dwelling — before you spend money on design.
Last updated: September 30, 2026. Servicing rules vary by municipality and change; confirm with your local planning and building departments.