Sunken Floor Joists

The goal is to attach your newly raised floor to the nearby floor to create uniformity and not two separates.
Sunken floor joists. Measure the distance between floor joists. Depending upon the conditions it is possible to strengthen or repair existing framing members such as floor joists or roof rafters by adding reinforcing material sandwiching the member on either side with plywood is sometimes worthwhile but the plywood must be installed correctly for greatest strength. True floor joist span calculations can only be made by a structural engineer or contractor. Cut new floor joists 24 inches longer than your measurement.
If a pier that the post is standing on has sunk into the ground you will need to lift the beam off the post and replace the post with a taller one. Check the architectural material and style used in the neighboring floor. Generally there are posts and beams under the floor joists. If your floor joists have been cut improperly to install pipes wiring or hvac ducts this could be causing your floor to slope.
If you have improper holes and notches from alterations and running service lines this can be the source of weakened joists. But always one of those correct me if i m wrong chief architect. In many cases you can remove your sunken living room subfloor and match the construction pattern of the adjacent floor. The sunken sections will be easy to spot but look for damage further along the joists away from the obvious areas so you can avoid potential issues down the road.
Check pattern and measure present beams joists. Eliminate any erect building metals dry cement etc linking your sunken area to the neighboring floor. Get the jacks under the floor. Thre ways to fix a sagging or sloping floor.
Observe the construction used for the adjacent floors.