Can You Build a Basement Under an Existing House?

There are a variety of ways to add extra living space to your home, but what if you want to build a basement under an existing house? 

The answer is you can, but there are factors to consider. First, the house must be lifted, the ground excavated and foundational support built. Then you must pour a new basement floor and build the basement wall structure. There is also the need to properly waterproof and insulate the basement structure.

That's the basic process to consider. But here are some other important factors that must be addressed before deciding that a new basement is how to add extra square footage to your home.

Worthy investment?

While it's possible to build a basement under an existing house, you should determine if the project is worth the time and money.

Renovations can be expensive and can add value to your home. The idea is to not spend more for the renovation than the value added to the property. A new basement added to your home can cost as much as $200,000, depending on the difficulty of the project and the size.

Determine the scope and size of your new basement project as you decide how much to invest in the project. A basement can offer plenty of new square footage to your home, but how much do you want to spend? If the addition serves your family's need for space in a home that you intend to keep for years to come, it doesn't matter if the cost of the project exceeds the increased value of your property. 

Start with solid foundation

Your new basement needs a solid foundation, and many builders use underpinning to establish a series of concrete columns in a basement area to provide a strong support system under the house.

The builder digs holes beneath the house as deep as the planned basement. Wire rebar is used in the holes to provide support for the columns. The holes are then prepared for concrete that will secure the structure's load-bearing walls.  

Dig new basement area

The work of digging under the existing structure to create the basement requires large equipment. The dirt, large rocks and other debris that might exist under the house must be removed. Contractors can use large equipment if space allows, or smaller equipment with a belt system to remove the dirt, rocks and debris. 

This is a time-consuming and tedious process that requires care and expertise. In some cases, the digging is actually done by hand, which could be cost prohibitive.

Build the walls 

With the house already supported by the concrete columns and the dirt removed underneath to make room for the new basement, the walls and foundation come next.

The builder will construct walls that are as thick or thicker than the existing structure above. The foundation must be linked to the supporting columns and walls to improve stability. Any plumbing, draining and electrical work also must be considered at this stage to ensure a new bathroom, kitchen or other components are accounted for in this part of the project.

Waterproofing and Insulation

The new basement area needs protection. The foundation and the walls must be waterproofed, with a protective wrap around the exterior of the walls and under the floor. Insulation in the concrete walls' interior and in the basement ceiling also help provide protection. Prior to installing the drywall, the ceiling insulation will help maintain the interior temperature and offer sound-proofing for the basement.

Keeping water out

Drainage will be an important consideration for the new finished basement. Whether you add a new bathroom, kitchen or utility area, proper drainage in your basement is important. It also helps to prevent flooding. The most common option for flood prevention is a sump pump. If the new fixtures can be tied to the existing sewer lines, you may need to add a septic to handle basement waste drainage.

Finishing touches

Once all of these steps are completed, all that is left is for your builder to finish out the basement space to your specifications. Your flooring, cabinets, trim and other touches will make the space ideal for your home. You will want to tie in this new space with the design and style of your existing house.