Before you buy a bathroom suite it is important to ensure that all of the components will fit into the existing room - certain components might be a slightly different shape or size to your existing suite.
Drawing a scale plan of your bathroom on graph paper is a great way of checking everything will fit. Cut out separate rough drawings of the bath, toilet basin or any other components and move them around the plan to achieve the best layout. Some bathroom manufacturers' catalogues have special planning sections with pop-out diagrams of their products, although these tend to be drawn to quite a small scale. Just because something will fit in a gap doesn't make it useable
Soil pipes (the large pipe that takes away waste toilet water) can be difficult to move so you may have little or no choice in the positioning of the toilet. Assume that it is staying in the same position and work your design around this assumption. In comparison, most other items in the bathroom can be easily re-positioned. To see if your toilet can be moved you'll need to call on professional advice.
When planning your bathroom, your wash hand basin should, ideally, have a mirror behind it so try to avoid positioning it in front of a window. For very small bathrooms why not consider covering a large area of one wall with a single, large, sheet of mirror. Sheet mirror can be bought from most glass merchants who will cut and drill the mirror to your required size. Large mirrors such as these can trick the eye into making a small bathroom look huge.
For modesty's sake try and avoid positioning a bath under a window! If you have to site a bath in front of a window, use a thick bathroom blind or curtain to avoid giving your neighbours a "shadow show"
If you are incorporating a shower cubicle in your bathroom planning, try and ensure it will be placed in a corner - our showers section has more details on positioning a shower.