If you are planning on building a new deck, there are a number of factors that go into determining the size, location, and height; these factors will determine if you need a building permit as well. Specific regulations for deck locations can be found in the Zoning By-law. More detailed information about deck construction can be found in this "Information Guide for Residential Decks" from the Building Division.
Distance from Property Line: Decks may encroach up to 3.0m (9'-10") into the required yard setbacks, but must always remain at least 1.2m (3'-11") from your property line. If you live beside an Open Space zone (OS4 or OS5), you must increase that to a 3.0m (9'-10") setback. It's always best to verify the zoning around your house prior to planning your deck, to avoid costly errors. This can be done by calling 519-930-3510, or by visiting the 7th floor of City Hall.
New Subdivisions: If you live in a new subdivision, make sure you don't affect overland flow routes, or alter the way water flows across your property. When designing your deck, make sure you wait until the sod is down and, even better, that the subdivision is assumed prior to making changes to your property. Property lines may not be as they appear, grading may be subject to change, and other alterations to the land around your house may impact the height and location of your imagined deck, even though you're living in your new house.
Balconies, Porches and 'Enclosed' Decks: All of these are different from the decks described herein. If the words balcony, porch, or "enclosed deck" (i.e. walls around your deck) better define what it is you are planning, there are other regulations that may affect your design. It is best to contact a Municipal Law Enforcement Officer prior to planning your construction, to discuss what exactly the City defines your project as, and how this definition can change the requirements. Zoning officers are on duty Monday to Friday, 8:30am-4:30pm, and can be reached by calling the number to the right under "Contact Us" (Zoning) or by visiting the 7th floor of City Hall.