The short answer
Most Durham municipalities now allow at least one additional unit in a detached, semi-detached or townhouse home, and some allow more. Every legal suite needs a building permit, and most towns also require you to register it. A full legal suite typically costs more than a standard finished basement because of fire separation, safe exits and separate systems.
What makes a basement apartment legal
A legal unit is one that was built with a building permit, passed its inspections and meets zoning. In practice that means:
- Fire separation between the units, with fire-rated ceilings and walls, and interconnected smoke and CO alarms
- A safe way out: a code-compliant exit, plus egress windows in bedrooms
- An entrance that’s separate or properly protected
- Minimum ceiling heights and window area for living spaces
- Safe heating, ventilation and electrical, with a panel that can carry both units
An unpermitted apartment can void your insurance, block refinancing and cost you at resale. A legal one is an asset lenders, insurers and buyers recognize.
Basement apartment rules by town
| Municipality | What the town says |
|---|---|
| Oshawa | Up to three residential units on a lot in many zones, one in an accessory building at most. One extra parking space per added unit. Permit required; units are registered as two-unit houses, with the certificate within 60 days of final inspection. |
| Whitby | ADUs allowed in certain detached and semi-detached homes. Permit through the E-Portal. Registration with Enforcement Services ($268). Housing Accelerator Fund fee reimbursement for eligible ADUs, with a complete permit application due by Dec 31, 2026. |
| Pickering | Up to two ADUs on most properties with a detached, semi-detached or townhouse home. Permit required even for pre-approved designs. Registration under By-law 8040/23. |
| Ajax | Second units are a moderate-construction permit, with separate Building Code and Fire Code guides. Apply through the Online Application Portal. |
| Clarington | Expanded permissions for additional dwelling units took effect in January 2022. Check your zoning for what your lot allows. |
| Uxbridge | Secondary suites in existing houses: confirm with Development Services, get a permit, then register the two-unit house. |
From each municipality’s website, October 2026. See our local area pages for links to each source.
What a legal basement apartment costs
A full legal suite is usually the most involved kind of basement project. Our Open-Concept Basement Suite in Oshawa, a self-contained two-bedroom apartment of about 1,200 sq ft with a full kitchen, full bathroom, powder room and laundry, falls in a $90,000 to $120,000 range and took 8 to 10 weeks on site.
Costs depend heavily on what your basement already has: ceiling height, window sizes, an existing bathroom rough-in, and whether a separate entrance needs to be cut through the foundation. Our basement cost guide has the full breakdown.
What a basement apartment can earn
Our estimate of typical rent for a legal one- or two-bedroom basement unit in Durham in 2026 is about $1,500 to $2,050 a month, based on Durham rental listings and rent reports. Units first occupied after November 15, 2018 are exempt from Ontario’s annual rent increase guideline, and CMHC-insured refinancing for homes with a secondary suite can go up to 90% of the home’s value after the suite is built, for eligible owner-occupied homes.
Try our rental calculator to see how much of your mortgage a tenant could cover in your town.
Estimates only, for planning. Talk to an accountant and a mortgage professional about your situation. This is not financial advice.
From basement to suite in four steps
- Site visit and numbers. We check ceiling height, windows, entrance options and your panel, then give you a written price and rent estimate.
- Design and permit. We draw the layout to code and handle the permit application.
- Build and inspections. Licensed trades build it, and inspectors sign off at each stage.
- Register and rent. You get a finished, legal unit and the permit records that prove it.
