By Erik J. Martin
Your home has a finite volume. You can remove or add walls to reconfigure areas, but unless you expand outward via a room addition or bump-out, you’ve got to work with the space limitations you have. That is, unless you want to look below the main level for spatial growth opportunities. That means creating a basement where there isn’t one or enlarging an existing basement.
Basement building/expanding can dramatically increase your usable square footage and provide new possibilities for your home, serving as the ideal spot for wish list items like a lower level in-law suite, home theater, additional bedroom, kids play area, home gym, wet bar, or gaming room. These efforts can pay you back handsomely when it’s time to list your home for sale, too.
But be forewarned: This intrusive and complicated project will require a lot of planning, likely be disruptive to your family, and probably be the most expensive home improvement endeavor you’ve ever undertaken – costing anywhere from $20,000 to $150,000, depending on your desired size and what’s required.
As long as the original structure of your home’s foundation is up to code, expanding an existing basement by digging horizontally or converting a crawlspace to a basement by digging down vertically is possible.
Many homes are not amenable to these strategies, including homes built on slabs, residences with septic fields or wells, homes built on sandy soil, properties below sea level, and houses with recurrent basement flooding issues.
There are two common methods for creating or expanding a basement: underpinning, which involves concrete being strategically poured into holes placed underground; and benching, which extends the perimeter of your foundation.
With underpinning, the depth of your existing foundation is increased by lowering the footings so that all of them rest on supportive soil. The soil must be checked first to see if it will comply. With benching, you’ll lose some space around your foundation’s perimeter, but not too much.
There is an alternative way to expand an existing basement.
You can jack the house up and add a knee wall underneath the existing home to add headroom in the basement.
The best strategy will be decided by the experts needed to complete this project, including an architect and general contractor as well as possibly an engineer.
First, you need to check with your municipality to verify setbacks, codes, and local ordinances to ensure your project is even possible. Next, you hire an architect to draw plans. Then, you enlist a general contracting firm to manage the project on-site from start to finish; this expert will be responsible for handling permits and hiring and managing all the subcontractors needed.
Alternatively, you could hire a design-build firm that will include both the architect and general contractor. Whatever route you take, get several bids from recommended professionals, ask to speak with homeowner clients, and compare prices, services, and reputations carefully.
Take note that if you plan to add a new basement, it must have an additional exit beyond merely the stairs to the main home.
This will involve excavation to add an enlarged version of a window well with an escape ladder. Any past issues with water in the basement will also need to be addressed before proceeding. This may include implementing outdoor solutions like grading, retaining walls, or rain gutter rerouting.
Other matters will need to be evaluated, too.
Is your existing structure capable of handling increased loads on the foundation and framing? Are walls going to be removed to allow an open-concept design, which will require load-bearing beams to be installed? What is the expense of relocating utilities to accommodate the new or expanded basement? And is additional homeowners insurance coverage needed?
It’s more cost-effective to use machinery such as a Bobcat or excavator on this job.
But often, there’s not enough space to get larger equipment in, so excavation has to be completed with pneumatic and hand tools. You also have to make sure the excavation doesn’t compromise your existing foundation. Sometimes, a retaining wall is required, as well.
Expect this job to span four to eight weeks or longer; you and your family might want to relocate temporarily during some of this time, as the noise, utility interruptions, commotion, and dust will be annoying.