You’ve probably been living with it for a while. The hallway runs downhill toward the back bedroom, the pantry door swings shut by itself, and there’s a gap under one corner of the baseboard. Most people notice the door first and the floor only once someone points it out.
Start with an elevation survey
Before anyone quotes a lift, someone should walk the house with a level or a laser and record readings room by room. The result is a map of the floor: a high spot here, a drop of some fraction of an inch over there. That map shows whether one corner settled, whether the middle heaved up, or whether the whole back half dropped. Each of those calls for a different plan.
Ask for a copy of the survey. When the job’s done, ask for a second one so you can compare.
Slab or raised?
On a slab, the low sections get piers driven or pressed underneath, and jacks bring the slab up. The slab foundation repair page covers that in detail. Under a raised floor, crews usually jack the beam, then add or swap shims and piers until the floor reads close to level. See pier and beam repair for what else they check down there.
How level is level?
Old houses carry decades of small adjustments: trim cut to fit, doors planed, cabinets shimmed. Pushing the floor back to dead flat can throw all of that off. Most crews lift to the point where the house is stable and the slope isn’t noticeable, then stop. If you have a specific target in mind, bring it up before work starts.