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Third Floor Additions: What You Need to Know

If you live in a densely populated metropolitan area like Toronto, you know that space comes at a premium.  For homeowners looking to add square footage to their existing house, the option to build out isn’t always feasible, due to smaller lot sizes and other physical restrictions.

However, there is another option – and that’s building up. In fact, many homeowners, especially those within central Toronto, are opting to add additional square footage by converting an attic into living space or by building a third floor addition on top of their existing home.

In Toronto, city by-laws for third storey additions vary from neighbourhood to neighbourhood. But, in many areas, the height restrictions are sufficient enough to allow for a third floor addition. This opens up a world of possibilities for space-craving homeowners who want to remain in Toronto’s core, but also need square footage to accommodate additional living space, such as a master retreat, guest quarters, office space, playroom or even a home theatre.

Of course, with any major home renovation, there are key considerations to be made and third floor additions have more than the average remodel.


For one, zoning by-laws can restrict the height of an addition and limit possible building options. If the addition doesn’t comply with regulations, a minor zoning variance can be obtained through the city’s Committee of Adjustments; however, there is a lengthy application and approval process, as well as associated costs.


Once the addition has been approved and permits have been secured, structural and mechanical considerations come into play.

Structurally, adding a third floor addition is more complex because, in many cases, the existing second-floor ceiling structure will not support the new floor. As a result, additional engineering and structural work are required to ensure that the addition is safe, sound and complies with municipal building codes.

In terms of mechanics, special considerations to the heating and cooling system will need to be addressed. If not properly insulated and outfitted with appropriate temperature zone controls, third floors can get very hot in the summer and very cold in the winter. In some cases, a dedicated HVAC system, separate from the rest of the home, may be required.


As a result, the cost per square foot for adding a third storey addition is significantly more than the average house addition, due to the additional structural and mechanical considerations, as well as increased labour costs from building above a finished space. A third floor addition will also require some amount of re-work to the second floor to accommodate stairwell access to the upper floor.  Realistically, the cost of a third floor addition can be 50% more than other house addition projects.

While there may seem to be additional roadblocks with a third floor addition, the benefits certainly outweigh the cons. If you are short on property space, building up does not take up valuable land and the need to re-landscape after extensive excavation work for the foundation is non-existent.

Also, in most cases, adding a third storey addition maintains the existing layout of your main floor, which is a major advantage to most homeowners, with minimal change to your second floor to accommodate a staircase and any mechanical or structural changes. It enables homeowners to add considerable square footage to their home without the need to address other areas of their house in the construction.

Best of all, a third storey addition allows homeowners to remain in their desired neighbourhood and avoid a costly move with land transfer taxes and other fees. For Toronto urbanites who do not want to leave the core for the west or east ends, this is especially valuable.

For a project of this magnitude, it’s best to work with an architect or a design build firm, like Inspire Homes, who is experienced in dealing with municipality rules and regulations regarding third storey additions. From concept to completion, we partner with residential clients to fulfill all aspects of their addition, including the design process, obtaining zoning variances and building permits, as well as the construction.

If you are considering a third floor addition to your Toronto home, please contact us to discuss your project, or visit our House Additions section for more information.

To view recently completed house addition projects in Toronto, Bloor West Village, Etobicoke, Alderwood and Mississauga, please click here.