Underpinning extends your foundation downwards. Short sections of the wall are excavated below the existing footing and filled with new concrete, one round at a time, until the whole perimeter bears at a lower level. Then the floor can be dug out and rebuilt where you want it.
Your foundation wall sits on a footing, and the footing sits on soil. If you simply dug the basement floor lower, you would be digging away the soil that footing is standing on. Underpinning replaces that soil with concrete before it is removed — but only ever in pieces small enough that the footing can bridge the gap on its own.
The perimeter is divided into numbered sections on the structural drawings. All the sections marked 1 are dug, formed and poured. Once that concrete has cured, the sections marked 2 open. Then 3. At no point is a section opened next to another open section, which is why the wall above never loses its support.
The last step in each section is the one people never see and the one that matters most. Concrete shrinks slightly as it sets, so pouring to the underside of the old footing leaves a hairline gap that carries nothing. That gap is filled with a stiff, almost dry mortar rammed hard into place — dry-packing — so the new pin is in genuine contact with the old footing and actually takes the load.
Once the perimeter bears lower, the middle of the floor is excavated to the new subgrade and the soil leaves the property. What goes back is not just concrete. Because the finished floor now sits deeper than it did, it sits closer to whatever groundwater is in your ground.
Skipping any of that saves a little money on the day and costs a great deal the first spring the water table comes up.
Underpinning changes the depth your foundation bears at. It does not, by itself, dry out a wet basement, straighten a bowed wall or make a room legal to sleep in. Those are separate pieces of work that are usually done at the same time because the floor is already open — which is exactly why they should be priced and drawn at the same time too.
That is set by the drawings, not by preference. The two things that usually decide it are how deep the buried drain line under the slab already runs, and what the soil and water look like at the new level. The designer works backwards from those. It is common for the answer to be a foot or two less than the homeowner first imagined, and it is much cheaper to find that out at the measuring stage.
In most cases yes, because the work is contained to the basement, but it is noisy, dusty work with a lot of material movement through the property. Dust separation, floor protection on the route the material takes, and honest conversations about which days are the loud ones matter more here than on almost any other renovation.
Done to the drawings it does the opposite: the wall ends up bearing on new engineered concrete at a deeper level. The risk is not in the concept, it is in the sequencing. Opening too much wall at once, or pouring the next round before the previous one has cured, is what causes movement — which is why the numbered sequence is on the drawings and inspected.
Bare. A level concrete floor, the walls as they were plus whatever repair work was in the scope, drainage in and tested, and the sump running. Framing, insulation, plumbing fit-out and finishes are the next trade.
Send the height from your floor to the underside of the joists and a photo of the wall. We will tell you what is possible before anyone quotes anything.