I want to turn a half bath into a full bath in my basement. Currently the washer and dryer along with the water heater are in the half bath along with the commode and sink. I want to add a bath tub and the lay out of the room is such that the tub installation would probably require less work if it were placed where the sink is currently located and try to utilize the current sink drain. I was going to move the sink and vanity to where the washer and dryer currently are use the plumbing for the washer for the sink. My concern and question is this: The bath is on a slab and it appears that the upstairs sink shares the drain pipe for the downstairs sink. Should I build a small riser to place the tub on to accommodate the drain and if I try to use the drain the sink is currently using, will there be a drainage problem from the upstairs sink? Would it be better to cut a channel in the slab to install the tub drain than to try to build that small riser?

I can’t see the route of the drain because it disappears into the bottom plate that is sitting on the slab

Any suggestions?

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My concern and question is this: The bath is on a slab and it appears that the upstairs sink shares the drain pipe for the downstairs sink. Should I build a small riser to place the tub on to accommodate the drain and if I try to use the drain the sink is currently using, will there be a drainage problem from the upstairs sink? Would it be better to cut a channel in the slab to install the tub drain than to try to build that small riser?

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A partial blockage would back up into the lowest point, your new tub, no matter if the upstairs lav were draining into the downstairs lav vent or not. If it were a bathtub instead of a lavatory that was upstream from the new tub I would be concerned but the discharge from the upstairs lav will be much less so it should pass without backing into the tub. I would connect the new tub under the slab and not platform the tub above the slab. Good luck, Tom
You haven't mentioned where the washer station will be placed. You will have to run a new vent on the washer when you cut in the drain.