I have an attached one car garage where the floor slab seems to have sunken in one corner by about an inch and a half and varies lesser amounts in other areas. The slabs continues behind the garage into our stairwell (the house in on a hill and the main living are is one level above the garage).
We want to fix the slab in the stairwell and would prefer to make the fix in the garage as well. We have so far gotten a quote for "slabjacking" using polymer foam for around $1,500. Other options are to use a special bonding top-coat cement mix to level out the slab while also inject some of the mix into the cracks about the edges. That cement mix is not cheap ($15 for a 40lb bag) but because of the thickness needed I don't imagine it would require that many bags.
FYI, we moved into the house about a year and a half ago and haven't noticed any sinking. The house is 50+ years old so the sinking may have happened gradually.
Best Answer
I’m guessing that slab is on the ground floor .If so, and then from my experience it is much cheaper and easier to make a new than to fix the old slab. Firstly, you should smash the old one and then check groundwork under the slab. If it is not quality (well compacted) then you should pour some Breakstone, gravel and compact it as much as possible. Then you can work on a slab. It should be at least 15cm thick (round 6 inches if you are in US) and make sure that you use reinforcement; you can use either bars or net. At this stage you must make a decision if you want to make a slab „floating“ meaning separated from the walls, or you should place some steel bars from the foundations of the walls to the slab. This way they would be connected, act as a moonlit structure. But unless you have a problem with underground water, or anything that could cause negative pressure maybe you can go with „floating“ slab; in which case you should use something to separate slab from the walls while you are casting concrete, something like very thin Styrofoam placed on the walls.