Is it okay to change the room to a kitchen room?

If you just change the piping system, it has nothing to do with the structure.

It's important though, that the drain riser doesn't go straight down to their bedrooms, and I'm sure you won't do that.

As for the floor leakage, it doesn't matter much, when we do the design, the kitchen generally will not put the floor drain, that is, consider that there will be no water in the kitchen, just mop it with a mop.

Anything else, don't bother.

Although the wall was not knocked out during the renovation, and the building structure of the house was not changed, "the other party thought that it would leak, and we agreed to make a waterproof floor, and the other party thought that our drainage pipe would affect them, so we agreed to change the drainage pipe", which would change the design load bearing of the floor slab. I recommend consulting with the structural designer of the architectural design unit before making a decision.

There is generally no problem if you don't knock out the load-bearing wall! But in any case, you don't have any threats downstairs, such as water leaks, etc. If you do, don't worry about anything!

It's better to discuss things between neighbors!

If there is no change to the load-bearing structure of the house and no actual impact on others, how the house is used is the owner's own business.

If the other party says no, you should ask him to explain the reason first.

OK! Be patient, though.