The following is a list of common projects that require a building permit....
5. Construct an accessory structure larger than 10 square meters (107 square feet) in area, such as

Detached Garage
Workshop
Carport
Pool house or cabana
*Note: If you are constructing an accessory structure and it is attached to an existing building and/or has plumbing, regardless of its size, a building permit is required.
https://www.toronto.ca/services-payments/building-construction/apply-for-a-building-permit/when-do-i-need-a-building-permit/