All the “ones” are dug first.

Basements, including basement apartments, are common in my area, and real estate is valuable. So, we often find that clients who live in older homes with short basements (5- to 7-foot ceiling heights), or even shallower crawlspaces, are interested in turning the spaces into full-height, usable rooms.

It’s possible to dig down just the middle of a basement and “shelf” the perimeter so the dirt below the existing footings remains intact and has ample support, but we more often excavate the entire basement, wall to wall, which requires extending the existing foundation walls down to new footings, a process called “underpinning.”

There’s no question that this is a lot of work and even carries some unusual risks compared with other projects. In fact, there have been some disastrous failures in our area when contractors aggressively undermined too much of a foundation, and the building collapsed—or in some cases, the building and the connected building next door, too. One jurisdiction has twice modified the permitting and approvals process to include more stringent controls in an effort to avoid problems.

We’ve done several of these projects and have more on the boards. Here’s how we do them, along with some lessons we’ve learned.

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