I'd like to put a 1/2 bath in my basement but I'm not sure where my drains run. A 4" drain runs down from an upstairs bath in the Northwest corner (I'll call this drain1). A second drain from upstairs also runs down in my garage (South and West of the basement). I think the sewer main is in the Southeast corner. There is also a floor drain and washer drain there (East/SouthEast).

If drain1 runs East then South, I'll be OK. But if it runs South to the garage then East outside of the basement, then I'm out of luck. If that's the case, is there any chance the floor drain and washer connect to a 4" drain under the cement?

Thanks-

Yes.