We did this in our old house and it was the best thing about the extension. We had the usual front room, middle room and kitchen/side return on the back at first.
We extended into the garden slightly, filled in the side return then put a small utility in the middle. It was only one row of units/washing machine etc and then about a meter across but that was plenty in size. We hung a pulley clothes dryer from the ceiling which was also great. Downstairs loo with a sliding door at the end was also great in the small children phase.
I’ve attached an old floor plan so you can actually see what I’m talking about. Good luck!