Make Sure There Is Space
In order to pass building code, your new shower stall needs to fit into a minimum of 30 by 30 inches. A circle with a 30-inch diameter must fit inside this space.
The recommendation by the National Kitchen and Bathroom Association is that the shower should ideally be at least 36 inches square to fit most users comfortably. Keep in mind that when installing a shower stall, you cannot come within 15 inches of the centerline of the toilet or within 10 inches of the center line of a sink in order to pass building codes.
Utilize the Corners
Sometimes there isn't enough room in a half bathroom to install a shower stall at the end or side of the room. But there may be enough space to install one if you utilize a corner of the room instead.
Corner showers take up less space than standard shower stalls. Some corner stalls also have special, neo-angle shower pans that reduce the amount the shower extends in the room.
Keep in mind that if you use this option, your shower stall will need to be a minimum of 32 inches on each side in order to fit the 30-inch diameter circle to pass code.
Make Extra Space
If you don't have enough space in the half-bathroom as it currently exists to install a shower stall, consider making extra space. Whenever possible, make extra space by removing the linen closet and moving the existing fixtures down to make extra room.
If you plan to install the shower in a corner, consider using the opposite corner for the toilet or sink to help open up the rest of the room. If you do take the linen closet, this option will often give you enough space to install a vanity or linen tower to make up for the lack of storage.
Once you've cleared enough space for the new shower, make sure it fits seamlessly into the rest of the existing bathroom. If possible, select a smaller version of the bathroom floor tile to use on the shower floor.
Run any wall tile around the perimeter of the room and into the shower as well. This will help the bathroom appear larger than it really is and make it seem less cramped.
It will also help give the space a cohesive design that will make it seem as though the shower had always existed in the space.