I have an add on bathroom and it is not on the foundation of my house. It is sinking a bit and before I do a bathroom reno and spend money on it ( and then have it shift and crack or w.e. ) I want to level the floor.

I'm sure if I need to like dig under the add on and concrete or if I can just do work from inside the house like take up the floor and put something under to level it there?

Im looking for a good job but hopefully cost effective and something I can do myself or have myself and some handy friends do .

Ill upload a pic after if anyone thinks it would help but any suggestions are welcome! Thanks!
( p.s. not sure what's under the bathroom floor or how the plumbing is tied into the rest of the house/basement?)

YOU need to first resolve the reason why the addition is sinking, and make corrective actions to stop it from sinking further.

Sounds like you have improper footing under the addition. What supports the addition? Post and piers, block?