Short answer: An ADU (accessory dwelling unit) is a second, self-contained home on the same lot as a main house. In Ontario the legal term is additional residential unit, and the Planning Act lets most urban lots have up to three units, one of them in a detached building such as a garden or laneway suite.
Key takeaways
| Question | Short answer | Source |
|---|---|---|
| What is an ADU? | A self-contained second home on the same lot (kitchen, bath, sleeping area) | Ontario — Add a second unit |
| Ontario term | Additional residential unit (ARU) | Planning Act s. 16(3), s. 35.1 |
| How many units? | Up to three on eligible urban lots; one may be in an ancillary building | Planning Act |
| Permit needed? | Yes, building permit plus electrical permit | Ontario — Add a second unit |
| Can it be modular? | Yes; same permit and Building Code rules | Ontario — Building a modular house |
Types of ADU in Ontario
- Basement or in-house suite — a second unit inside the main house.
- Garden suite — a detached unit in the back yard. See garden suites.
- Laneway suite — a detached unit on a lot that backs onto a public lane. See Toronto laneway suites.
What Ontario allows
The Planning Act (s. 16(3) and s. 35.1) stops municipalities from prohibiting up to three residential units on a serviced urban lot: for example a main house, a basement suite and a garden suite. Each city still sets zoning standards such as height, setbacks, lot coverage and tree protection, so the size and placement of an ADU vary by city. Our GTA garden suite rules comparison sets the main cities side by side.
How to add an ADU
- Check zoning and lot fit (lane access, setbacks, trees, servicing). Start with the free feasibility check.
- Prepare construction drawings that meet the Ontario Building Code.
- Apply for a building permit and an electrical permit.
- Build, site-built or factory-built, with municipal inspections at each stage.
More in our ADUs in Ontario overview and the full guides hub.
FAQs
Sources
- Planning Act, R.S.O. 1990, c. P.13, s. 16(3) and s. 35.1
- Government of Ontario — Add a second unit in your house
- Government of Ontario — Building a modular house
Disclaimer: Figures cited are market estimates provided for information only and are not a quote. Municipal rules, fees, and programs change — always verify current requirements with your local authority. Modular X provides exact pricing only after a site-specific feasibility review.