Q: I've got a 63 year old colonial with less-than-level floors. In our mudroom, the floor slopes from one end to the other (about twenty feet) with a drop of 3 inches over that distance, although the floor itself is pretty flat. And then we've got a dining room floor that has various sags and dips within it (though end to end it's level). What to do? How much?
A: Your fix-it options are limited, it's hard to find people to do this work, and it's expensive.
For the mudroom with the general slope to it, you may be looking at foundation or other structural problems. Since you indicate that the floor itself if fairly flat, we would have to guess that the foundation footer has subsided. Or, the sill (the wooden part of the house that rests on the foundation footer) may have deteriorated either due to rot, water, termites, or any host of things that could have happened in the last 63 years.
If you have foundation problems, you are lucky (in a perverse sort of way) because you can actually find companies that specialize in foundation repair. They will have to come in and jack up that portion of the house and place new footers. It's difficult to estimate to cost, but I doubt you would get out of this for anything less than $8,000.
If your sloping floor is due to a deteriorated sill, you'll have a harder time finding crews who specialize in this kind of work. Foundation repairs companies probably won't take the work, so you may have to resort to a general contractor, who may know of someone who does this kind of work. A good general carpenter, who possesses house jacks or is willing to rent them, is certainly able to do this work, too, but make certain that he has done this type of work before. He may be tempted to take on a job he can't handle because of the high fees involved.
Keep in mind that anything involving 20 ton house jacks will take time. You can't jack up a house in a day. It has to be jacked up slowly over weeks or even longer to avoid cracking wallboard, plaster, windows, and even structural elements.
As for the saggy, dipping dining room floor, that has its own set of problems. If you have access to the bottom of the floor, such as a basement or crawlspace, it is possible to jack up saggy joists until they are level and then sister them so that they remain straight after the jacks are removed. Another, cheaper fix is to put adjustable steel columns under the joists to keep them propped up. This steel column fix does require that the base of the column be secured to the basement floor and the top of the column be secured to the joist.
On the top side, another fix-it idea is you could lay down new hardwood over the existing floor. A plywood subfloor would nicely bridge any minor waves in the existing floor, and leveling compound would help, too. You'll have to make sure your joists can handle the addition of more weight: 3/4" plywood subfloor and 3/4" hardwood. That's a lot of weight. On the bottom, you could always sister the joists and add a few adjustable columns to strengthen the joists to handle the additional weight.
One last idea is just to come to terms with your sagging, sloping floor. Old houses tend to have saggy floors, and you'll find that many other old house owners have worse floors than you. My theory: as long as the peas don't roll off the plate, you're in fine shape!