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.