I get asked this question on occasion and my answer is, it would be easier to tell you when you don't need a building permit.
When do you need a building permit?
If you construct a new building, renovate, repair or add to a building, demolish or remove all or a portion of a building, change a building's use, install, change or remove interior, exterior or load-bearing walls, make new openings for, or change the size of, doors and windows, build a garage, balcony or deck or enclose an existing deck, build a utility shed over 108ft2 (10m2), excavate a basement or construct a foundation, install or modify any life safety or fire suppression system such as fire alarms, sprinkler or standpipe or fixed extinguishing systems including commercial kitchen exhaust hoods and the installation of electromagnetic locking devices, install or modify heating, plumbing, and air conditioning systems, fireplaces, fireplace inserts and woodstoves, install kitchen or bathroom cupboards with plumbing, construct a chimney, to finish a basement or convert a room to a bedroom, to install a swimming pool or hot tub, or erect a sign, you are required to obtain a building permit.
There is no way around it unfortunately, and a failure to do so could result in a stop work order, an order to remove completed work, or be subjected to the prosecution for contravention of the Building Code Act. If you are unsure of whether or not to acquire a building permit, just check with the local Building Section to see if you are required to obtain one. One good thing about the building code is that it keeps the playing field level by establishing minimum standards for construction that apply across the Province of Ontario. The building permit process protects the interests of property and building owners, as well as those of the community by ensuring the buildings in which we all live, work and play are safe, legal and sound. In other words: Without a building code, we would probably have a city with a bunch of houses that look like this.
Original source of Building Code Information is from the City of Belleville Website. Click the link below for original source.