How to Dig Out a Basement Under an Existing Home: DIY Basement Project

I bought a 1950’s cottage about two years ago which had about 950 square feet on the main floor and about 200 square feet in the half basement, and I need more living space. The attic is not tall enough to convert into living space and my yard is too small to expand out, so I decided to research the possibility of expanding my half basement into a full basement. To my surprise, there is a way to do it. If you are in a similar boat then read on.
Foundation Issues

The first hurdle that you will come across when evaluating if it is possible to dig out a basement in an existing home is the condition of the foundation and the type of geology you are dealing with. If you have an older home, like I do, then the foundation may be in bad shape and need to be replaced or it may be fine. If your foundation needs to be replaced, then the best time to add on a basement is when you are re-doing your foundation.

The geology under your home is also going to impact whether digging out a basement under an existing home is possible or not. For example, if your home is located on solid, or semi-solid, rock this project is not going to be worth it. However, if you have a gravel or soil base to work with and are capable of handling a jack hammer, shovels and possibly a bobcat, then this can be a fun project for the family to tackle.

(Warning: If you live in California, Alaska or other seismically active areas of the world, or if you live in an area with a high water table or high risk for flooding, don’t try to add a basement using this method.)

The Process

The process for adding a basement to an existing home is actually pretty straight forward. You will need to get under your home, use floor leveling jacks and steel I beams to temporarily support the weight of your home, remove the extra earth, install new basement foundation and reattach the home to the foundation. Simple, right? Well it sounds simple, however, the process is very labor intensive and a lot of things can go wrong.

The first problem in constructing your new basement is getting the digging equipment under your home. Some people gut a room and tear up the floorboard to get equipment in, however, if you don’t mind hard labor you can use bucket and shovels to pick away at the earth below your house. If you hit a small patch of rocks or hard earth then a jack hammer can be a great tool to use and it is small enough to get into tight spots.

Bracing is the major problem that you will need to solve before you begin excavating your project. To avoid your home from collapsing down on you, settling or from damaging its structure, you will need to make sure your home is properly supported before and during your excavating. It is recommended that you use a house jack or floor leveling jacks to act as a temporary foundation while you work. You can rent these jacks from a local home movers or heavy equipment dealer, or you can buy them. I found a couple of companies online that sell these jacks for between about $400 and $1,000 each. As you can see this can be a very expensive project, however, it can net you a large chunk of equity value. In addition to the jacks you will also need steel I beams to support your home as you work.

Creating a stable foundation is another challenge that you will need to deal with during this project. Once the excavation is done you will need to get someone with experience to come in and construct a proper basement foundation. Like I mentioned earlier, if your foundation is in trouble already then this is a great time to kill two birds with one stone. The construction of the foundation is something you don’t want to do yourself as it will take the right equipment and professional expertise to build a solid basement foundation for your home. The cost of this will vary depending on your location and the size of your new basement, but figure at least $5,000 to $7,500 for a small to medium sized foundation.

Reconnecting your home is your last challenge. Building rules are going to be different in every area, but generally you will need to secure your home to its foundation. This is accomplished by using special flasking strips and tools that fit between the foundation and the wood structure. If you don’t know what to use as your contractor or local building inspector.