Choose wall-hung fittings wherever possible. It will make your bathroom much easier to clean and give the illusion of space, too. Try to keep walls clear of clutter and consider fitting underfloor heating, so you can keep it toasty without taking up space with wall-hung
A scale plan of your space is a good place to start. Measure carefully as accurate dimensions are key and include features that are likely to affect the design, including windows, chimney breasts and doors.