Third Floor Additions: 5 Things You Need To Know

Updated: [February 2023].

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.

Here are 5 key things to know when it comes to adding a third floor addition:


1. Zoning Rules

Zoning by-laws restrict the size of a home, height of an addition and may limit possible building options.  Zoning rules may also regulate roof lines and setbacks from property lines, which may affect the design of the third storey addition.

If the addition doesn’t comply with zoning by-laws, a minor zoning variance can be obtained through the city’s Committee of Adjustment; however, this is a lengthy application and approval process, as well as associated costs.


2. Structural Assessment

The existing house will need to be assessed to ensure it can support a third floor addition.  Structural assessment will include the foundation(walls & footings), and main and second floor walls.  Additionally, soil below the foundation may need to be assessed for capacity to support additional weight.


3. Mechanical Considerations

Are the existing home’s mechanical systems adequate to support additional living space?   If the existing furnace has difficulty heating and cooling the 2nd floor, how will it maintain a 3rd floor?  If water pressure is low on the 2nd floor, how much lower will it be on the 3rd floor? How will heating, plumbing and electrical be connected to the existing home?

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.


4. Logistics

Most 3rd storey additions are built onto homes on small properties.  This means there is little space to store materials, place waste containers, etc. It also means that using time saving equipment and machinery may not be possible due to space considerations.

A third floor is also more difficult to build due to the height – a longer distance for materials and workers to travel to build the additional space.


5. Cost

There are many factors that affect the cost of a house addition, which we have discussed in our very popular article: House Additions: 7 Things That Affect Cost.

The cost of bulding a third storey addition is significantly more than the average house addition.  This is due to the additional structural and mechanical considerations, as well as increased labour costs due to the height.  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 costly, on a per square foot basis, than other house addition projects.

Many people are coming here looking for a Per Square Foot (PSF) cost of a 3rd storey house addition.  We hate putting square foot dollar figure on house additions, because as we’ve said before, Cost Per Square Foor is Inaccurate and Dangerous.

But if you’ve read this far, we’ll give you a range.  And since we preach how per square foot pricing is inaccurate, we'll give you a dollar range for a typical project.   Most third storey additions consist of a master bedroom, with an ensuite bathroom and an average size of 500 square feet.  The range (in 2023 dollars) is $350,000 to $500,000, inclusive of design, permits, construction costs and HST.

Make it 50 square feet smaller, is it going cost 10% less?  No it's not.  It will cost less, but not 10% less.  Which is why we're not talking per square foot pricing for these projects anymore.

You're probably thinking, why is a 3rd storey addition so expensive.  We answered the questions here:  Why Is A Third Storey Addition So Expensive?   We've also explained how construction costs are skyrocketing in our blog article Construction Costs 2022.


Benefits Of A Third Storey Addition

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 which is experienced in dealing with municipality rules and regulations regarding third storey additions.

Are you located in the Greater Toronto Area and want to discuss your third floor addition project?  Click here to Schedule A Consultation. 

View a sampling of our recently completed house addition projects in Toronto, Bloor West Village, Etobicoke, Mississauga and Richmond Hill. Click here.

Do you have more questions about house additions?  Request your copy of our 100% free e-book The Ultimate Guide To Planning A House Addition Project.