Lowering a basement floor to increase headroom takes a tremendous amount of planning and care. Because of the complexity of the process - plans and permits, engineers, excavators, masons and city inspectors - many contractors will seek shortcuts along the way. Unless you'd like to risk your house falling down, I'd make sure it's done by the book. The process is increasingly common in Philadelphia as families grow but square footage doesn't. And it's true - there's only one way to do it - with shovels, picks, buckets and a lot of sweat. The concrete and dirt is usually passed in buckets through the small basement sliding or awning window at street level. Unfortunately there is no magic trick or innovative technology - just a bunch of guys workin' like the old days.
As we dig, there are two primary methods to stabilize the foundation on either side of the house - underpinning and bench footings.
Foundation underpinning involves excavating under the existing foundation to the desired depth and pouring a new, vertical concrete foundation to support the existing one above - basically creating a wall under the wall. Excavating must be done in alternating sections, as the load of the foundation is carefully transferred and supported as each section is completed. Underpinning is more time and labor intensive than bench footings, therefore more expensive, but it provides the finished basement with the maximum possible floorspace.
Benching or bench footings involves leaving a perimeter of soil untouched - usually about 1 foot wide per foot of depth. This method basically creates a concrete "bench" around the basement, plenty of foundation reinforcement but a significant reduction of the floor's square footage. You can get creative with this space, though, installing actual benches or cupboards or built-in shelving above.
We recently started a basement lowering and finishing project on Delancey Street. You can see from the photos that we began by replacing the dated water heater and HVAC system and relocating them to the back of the room.
We're using a combination of underpinning and bench foundations - because there are existing mechanicals such as electrical panels, gas meters, water heaters, furnaces, etc., underpinning the entire basement usually doesn't make sense. So we're underpinning the primary living space but benching the walls that have the mechanicals.