Creating a basement under an existing basement, commonly known as "underpinning," is a complex construction project that involves structural modifications and excavation work. The cost of installing a 400 square foot basement under your existing basement can vary significantly depending on several factors, including your location, the complexity of the project, and the specific requirements and conditions of your property. However, it's important to note that constructing a basement beneath an existing basement can be an intricate and costly endeavor due to the extensive work involved.
To get a more accurate estimate for your specific situation, it is recommended to consult with local contractors or construction professionals who can assess your property and provide you with a detailed cost estimate. They will consider factors such as labor, materials, permits, excavation, foundation work, structural reinforcement, waterproofing, plumbing, electrical work, and any additional requirements specific to your project.
Keep in mind that the cost of such a project can be substantial, ranging from tens of thousands to hundreds of thousands of dollars, depending on the complexity and scope of the work involved. It's important to gather multiple quotes and thoroughly evaluate the expertise and reputation of contractors before making a decision.