Tying new half bath to existing drain

[nosajha]

I'm in the planning stages of a half bath upstairs and need advice on how best to connect the new drain to the existing drain.

The existing drain is a 3" ABS line that connects to the closet flange of the downstairs toilet, drops vertically a foot to an elbow, runs horizontally 2 feet to another elbow (which has a 2" heel inlet connected to a vented horizontal branch), and from there, horizontally another 3 feet to a vented stack.

Can I remove the elbow under the toilet, replace it with a combo, and connect the half bath there?

New bath fixtures will have new vents and drain would be basically an 8' vertical run from upstairs to a combo with cleanout, then a 12' horizontal run to said combo under the downstairs toilet.

Code here is International Residential Code.

I apologize for the lousy drawing I'm attaching but I hope it helps illustrate the above somewhat.

[hj]

The IPC is very lenient, but I doubt that even it would allow that connection because it destroys the venting for the downstairs toilet and could cause its own set of problems.

[nosajha]

So is there a way to make a connection there and preserve the toilet venting? Would it work if I added a vent at the toilet? Or should I run the new drain all the way over to the stack coming up from the building drain?

[hj]

Either one, but the toilet vent would have to be done properly.

[nosajha]

Ok. I'll leave that toilet alone then and go over to the section coming up from the floor. That'll give me the opportunity to add a cleanout there anyway.

Thanks for your help!