answer:I’m trying to visualize from your description, and here’s what I have so far: Your kitchen sink drain routes into a concrete floor and from there somewhere through the concrete floor into the main drain at the same elevation as the floor. You would like to re-route that to run through the floor at the same point, but from there down to the collective drain for a bathroom on a lower elevation. My questions are: 1. Is the downstairs bathroom above grade? Or does that plumbing route to a sump that has to be pumped back up to the main drain? Draining your kitchen sink into a sump (if so) will put a lot more load on that pump than it had faced before. 2. Where is the vent for the rerouted kitchen drain line? You’d need to have a vent somewhere between the sink drain and wherever it empties, because if you don’t have a vent then that kitchen sink drain could either (or both) suck the lower bathroom traps dry, venting sewer gases into the dwelling, or blow the sink and tub traps back into the sink and tub, which would also be messy and unsanitary. 3. Is there also a dishwasher attached to the kitchen sink drain? 4. Does anything else share the same drain line at the main / ground / kitchen elevation, such as a washing machine or other fixture? You can’t afford backups in that line that might rise above floor level, which wouldn’t have been a problem with a vented sink drain above. Now I’d wonder if that line can be permanently capped and abandoned where it is. If any fixture shares this drain line, then I’d say that you need to abandon your plan or reroute all of the drain lines.