I had a darkroom in a closet, once upon a time. No plumbing, of course. Washed stuff in the bathroom.
A lot of good advice.
The basic space you need is the table to hold the enlarger and three processing trays. If you have a separate wet area and it is dark, you just need the darkroom to hold the enlarger. It can be 4' x 5'?
For people with very limited space, a kitchen, laundry, bath or mud room is a wonderful place to house the processing equipment, be BW processing trays or a Jobo for color processing. You can put them away in closet when you do not use them.
My 'dark area' is in a room about 6' x 12'. However, the actual space I have for printing is just enough worktop space for the enlarger and three trays as the room is used for other things as well. The running water is in the bathroom which is the next room along.