Hello... I live in a condo (ground floor) and am having water back up into the kitchen sink; the water is soapy and has a foul odor. All 12 residents in my building share a laundry room that is also located on the ground floor next to my unit and I know that the washing machine isn't causing this back up. They are never being used when I experience the water back up. My next best guess is that it's being caused by a dishwashing machine due to the soapy water. The water back happen at all hours of the day; sometimes late at night where I'd guess it's just too late for a neighbor to be doing dishes.

Any suggestions on what I can do myself to fix the water back up in my kitchen sink? If I call for professional help, what type of an expense is this going to be? A high or moderate expense?

Sounds like the main for the building is getting clogged and you just happen to be the low opoint in the system. Do you folks own or rent the condo and is there an association, this may not be your problem to fix.