If you just want to develop roll film (35mm or 120/220 sized medium format), you don't really need a darkroom. So long as you have a completely dark space in which you can load the film into the tank, you can do all the processing in a home laundry or similar facility, providing you ensure there is good ventilation and you clean up thoroughly afterwards.

For loading and developing large-format sheet film, and for wet-process printing, it really helps to have a dedicated darkroom. It doesn't necessarily have to be a converted spare room. I've seen darkrooms built into the corner of a garage and in caravans. Remember to pay attention to ventilation. You want good air exchange, with air flowing from behind you, over the chemistry and then vented outside. This generally involves labyrinths of some kind to allow the air to flow freely while keeping the light out.