Basement design is an animal all its own. It can be daunting for sure but as with everything, knowledge is king. Knowing the options and removing the guesswork can help make sure the decisions are the best for the budget and end game. When clients ask me to lower the floor and increase ceiling height I start by reviewing the site, and looking at the overall home value. Spending 50k on a full underpin may not make sense on a 300k house with ample above-grade floor area. Doing a combination of underpinning and benching for 30k in a downtown semi-detached house, however, can increase the usable area and increase the property value by as much as 100k if done correctly.
First things first, you will need to engage a structural engineer to both complete the required drawings and prepare the submission package to obtain the required permits. My process is to first complete my desired layout, then I forward that on to my engineer to design the structural requirements around that.
If you are in a semi-detached or row house you share a common foundation wall, which means as a requirement for the permit submission you’ll need to obtain a party wall agreement. This is a standard form in which the adjoining neighbours are notified of the work you intend to complete.
Here are the two common methods for lowering your basement floor, which can be done independently or in combination with each other.
In this method an engineer plots out a finely planned sequential plan in which a series of holes are dug-out from below the footings of your home (the footing is the concrete or brick pad which spreads the load of the foundation walls onto the soil) The most used method of underpinning is mass pour method where we excavate sections in sequence to a pre-established depth below the footing and place concrete on each pit. The method is repeated until the entire area has been underpinned. This is the more desirable method because once complete the finished interior basement wall is flush top to bottom. It’s also very labour intensive and can only be performed by a very experienced and skilled trades which makes it an expensive undertaking.
Benching is a simpler process in that it doesn’t require you dig below the existing footings, which makes it a more cost-effective option than underpinning. In this method the existing footing and soil remain untouched. A new foundation is poured inside the existing one down to the new desired depth. For every foot you go down in depth, you must add a foot in width to the bench. If you are dropping the floor 18”, you will end up with an 18” wide bench along the base of your new wall, which will affect the final design of your basement.
There are ways to work the design around the bench, which is why for my clients I sometimes consider a combination of underpinning and benching as a viable solution to save on budget. Consider adding storage atop the bench like bookcases or wardrobes or even adding an upholstered cushion to create more seating.