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Basement Underpinning: A Quick Guide

An example of Underpinning
Basement underpinning is the act of lowering an existing foundation's depth by pouring new footings BELOW the current ones.
By: Baron Alloway

The need for expanded space has one unique problem. Land is a finite resource. Basement underpinning can expand livable space without increasing a property’s footprint.

When Toronto homes were first built, basements were not living spaces. Owners would use them to store materials, food, or general empty space.

As the demand for space increased, residents began to wonder if they could convert basements into habitable dwellings. They could increase the liveable area of a home without expanding its footprint.

The idea of a finished, habitable basement was great for investors, too. With a basement apartment, an owner could convert a single family dwelling into two units. One unit would exist on the upper two floors with a completely separate unit downstairs. Finally, a way to convert dead space into a revenue-generating asset.

This has only one problem: most basements were not dug with the idea of (comfortable) occupancy in mind. Most of the basements in older homes had a ceiling height of 5 feet at best.

Basements like the one here in 87 Gloucester St were originally used to store coal & wood. Windows at the front have been converted from coal chutes.

So, what’s the solution to a basement that is too low to stand tall in? The answer is Basement Underpinning.

What Is Underpinning?

Basement underpinning is the act of lowering an existing foundation’s depth by pouring new footings BELOW the current ones. The result of an underpinning can be stable construction below the grade of a home.

To understand how underpinning works, its important to understand a home’s foundation.

A house’s footing is the lowest constructed part of the structure. The purpose of the footing is to distribute the weight of the entire structure onto the soil.

A Diagram showing the general structure of footings and foundations. Source: OREA

Footings are typically 16-24 inches wide and 6 to 16 inches thick. If the footing were to fail, the entire structure above would shift. This could cause significant structural concerns.

On top of the footing rests the foundation. The foundation’s purpose is to send the weight of the home down to the footings, below the frost line. Foundation’s must resist lateral pressure from the soil so that they do not cave in. Most foundations are actually basement walls.

Underpinning is the act of extending the footing further into the ground. The effect converts the existing footing into foundation, resting on the newly poured footing beneath it.

This is done by digging down to the footing, and then carefully digging and pouring below it. Basement underpinning can add an extra 2-3 feet of height to the basement.

A diagram illustrating basement underpinning

Underpinning Alternative: Benching

Underpinning is not the only way to lower a basement. If you have enough room, a cheaper alternative exists. Benching is another way to add some height to your basement’s ceiling.

Unlike underpinning, the new footing does not go directly below the old one. Instead, benching is the process of pouring a new footing beside the old one.

Diagram showing the difference between underpinning and benching
Benching does not disturb the current footings but instead reinforces them. Source: VMB Group

In doing so, the weight shifts from the old footing to the new “benched” footing. This lowers the basement floor to the bottom of the bench.

Although benching is usually cheaper than underpinning, there is a major drawback. The technique creates a “bench” with a height equal to the old basement’s previous floor. Depending on how deep you’re going, the bench can be as wide as 1-2 feet.

Benching adds a permanent ledge to your basement. Source: wetbasements

While this design can take away from a basement’s useable square footage, homeowners have been creative with finding ways to blend the bench with the design of the house.

The Cost of Underpinning in Toronto

The cost of lowering a basement in Toronto varies. Residents must consider several factors. The original footing and foundation style, soil quality, and water characteristics of the neighbourhood are all factors that play a part in feasibility.

As of 2022, the cost of underpinning a basement in Toronto is around $50,000.

Benching is a more affordable option when looking to lower a basement. Leaving the original footing undisturbed eliminates a significant number of variables.

Is Underpinning Worth the Investment?

Houses with (proper) finished basements tend to sell for more than those without. The addition of useable space below the home can add significant value, especially if the unit can be converted into a second unit.

However, there are three reasons why underpinning might be an effective means to add value to your home:

1. Underpinning adds No Calculable Square Footage

Toronto Zoning requirements dictate that a home may have a total number of square feet. This figure, known as gross floor area (GFA), is calculated based on lot size.

Basements are not considered when calculating gross floor area. As a result, underpinning is an efficient way to add space without causing the property to become non-conforming.

2. Underpinning is Cost Effective

The cost to underpin an average basement in Toronto is $50,000. Benching can be significantly cheaper than this. In both instances, useable space is added to a property.

In comparison, ALTUS Group estimates the cost of an addition to be over $500 per finished square foot. This means that the same $50,000 would only yield about 100 square feet (or a 10 by 10 room).

3. Benching and Underpinning Can Fix Water Issues

When done correctly, the processes of benching and underpinning can give the owner the ability to fix water-related issues. Parging and damp proofing is installed in the new basement, along with vapour barrier. Underpinning can end water-related basement issues.

Illustrating how parging and damp-proofing can de-water a leaky basement. Underpinning can give you a second chance at this.

Conclusion

Underpinning a basement in Toronto is no easy feat. Expert advice should be sought prior to the commencement process. Engineers approvals are required in Toronto to comply with building code requirements.

Are you looking at properties with Underpinning? Contact Catalyst for assistance on your next project.

Baron Alloway
Baron Alloway is the Broker of Record at Catalyst. As a trained engineer, Baron brings with him an extensive by-the-book amount of knowledge to the Real Estate World. Contact Baron today and have him help you buy or sell your next home.

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2 Comments

  1. Will

    “The cost to underpin an average basement in Toronto is $50,000. ” Is this the estimated cost for the entire basement lowering?

    Reply
    • Baron Alloway

      Most contractors (in Toronto) will charge somewhere between $80 and $350 per square foot to underpin. The $50,000 was just a well rounded estimate to get to an unfinished, fully dug basement, from our rough calculations. For a more accurate assessment, we’d recommend getting quotes from contractors.

      Reply

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