While it is true that you can simply stop doing the digital work, do you want to. Once you are deeply into something, do you want to be disturbed.
I will occasionally answer the phone in the darkroom. I can usually continue processing a print while on the phone. But, I am also happy to tell callers that I can't talk because I'm working in the darkroom.
My darkroom is 35 sq. ft. It has a dry side with Beseler 23c and a D2, The wet side is actually a counter top I built myself and a laundry sink. Because each side is only 5 ft. in length, I built a tray stack to process 11x14's. I could probably use it for 16x20, but it would be tight.
It has plumbing and a darkroom fan and was built by partitioning a utility room.
I paid for the lumber, laundry sink, adorama darkroom fan, white paneling and prehung door. I scrounged the cabinets at the landfill (someone was dumping 1980's kitchen cabinets after a renovation) and made the counter tops out of a damaged 4x8 sheet of Melanine type shelving ($19) and some nice oak trim.
All of that said, my first darkroom in the 80's was a temporary one in a bathroom. It worked fine for the time.