The minimum size drain pipe for a commode is 3", but the drains for a kitchen sink are going to be only 1.5" or 2". It may be hard to find the space for the larger drain while keeping the proper slope to the soil stack.

You need to find the shortest path to the existing soil stack (which may be 4" cast iron in an older home), and determine whether you can connect in the new commode. In general, you want a path running parallel to the floor joists so that you don't have to do cutting and reinforcement of joists. The bathroom sink also needs a 1.5" drain, and the laundry drain should be 2" for a standpipe, or 1.5" if you want to let it drain into a utility sink.

Bruce

The separate apartment must have had a bathroom so a full sized waste pipe must serve that level. The issue is how far it is from the new bathroom. You might consider putting the new bathroom as close as possible to the old bathroom.

No one can tell you you much without seeing a plan with the existing plumbing noted.