i recently inherited an older house and in the kitchen i noticed the floor sagged. quite a bit too. its about a 3- 3 1/2" difference. now when i went into the crawl space to find the cause i discover that at some point in time there was a small addition added onto the kitchen (3 1/2' x 12'). now when this addition was put on they removed about 1 1/2' worth of stone from the stone foundation to run piping and this is where the sag all runs too. now they did add a new cement block foundation on the addition but right where the rocks are missing in the old foundation and where the new and old floors come together it sags.. i was thinking to fix the problem i would dig 3 or 4 4' holes and make footers with sonotubes about 1' in from the the old girder. jack up the floor to level and then install a new 4 x 6 or 6 x 6 girder with 4 x 6 posts going from the girder to the new footings. my question i guess is would this be sufficient or would i have to replace the old foundation? its a tight crawl space. its about 18 inches from the floor joist to the ground. this is a rough drawing i did on paint just to give an idea of what I'm talking about.(not to scale)