Adding a garden suite or basement unit in Northumberland County

What Ontario allows

Since the province's 2022 housing changes, Ontario municipalities must allow up to three residential units on most lots with full municipal services: the main house plus two additional residential units (ARUs), in the house, in a detached building, or one of each. Rural lots on wells and septic are often allowed fewer, depending on the township and septic capacity.

Cramahe Township (Colborne)

Northumberland County approved Cramahe's Official Plan Amendment No. 16 to allow additional residential units. A 2024 draft amendment to Zoning By-law 08-18 proposes up to three units per lot in urban areas, and two units on non-urban lots (one in the house and one in an accessory building). Confirm the current status with the Township before you plan. The process starts with a preliminary zoning review, using the ARU schedule on the Township's form, before the building permit. Septic permits go through Northumberland County, and properties without an approved entrance need an entrance permit and 911 number from Public Works.

Cobourg and Port Hope

Both towns allow additional units under provincial rules, with their own zoning standards for size, setbacks and parking. Exterior work in a heritage conservation district also needs a heritage permit. Ask the planning department, or ask us to check your property.

Rural lots: septic decides

On a rural lot, the septic system often decides whether a second unit is possible. A new unit adds daily flow, and the system has to be sized for it. Plan on a septic assessment, and possibly a new or expanded system, before design.

Step by step

1. Zoning review with the municipality
2. Septic assessment on rural lots
3. Design to the Ontario Building Code: fire separation, egress, alarms, ceiling heights, heating
4. Building permit, plus septic and entrance permits where needed
5. Construction and inspections

Costs

A legal basement unit typically costs $120 to $200 per square foot in Ontario in 2026. A detached garden suite often costs $350 to $450 per square foot or more, plus servicing, which on rural lots may include septic upgrades.

Sources

• Cramahe Township: Draft ARU revisions (2024)
• Cramahe Township: Preliminary zoning review application
• Cramahe Township: Permit package for new homes and additions
• Northumberland 89.7: County approves rural housing measures
• MoneySense: Are laneway homes worth it?

Last updated October 2026 by NEUE. Market figures change; your project gets its own fixed price after a site visit.