start with a floor plan.

If you want to design a handicap accessible shower stall, here are some resources:

Houzz
1.) Go to Houzz Ideabooks and enter "Universal Design" in the search box.
2.) Go to Houzz.com/Handicap-accessible
3.) Go to Houzz Design Dilemma and enter "handicapped" in the search box.

Handicap Access
1.) The Americans with Disabilities Act (ADA) site has specific measurements for wheelchair turning radius, placement of grab bars, everything.
www.ada.gov/adast94.pdf
www.ada.gov/

Aging-in-Place
1.) Consult with a Certified Aging-in-Place Specialist. You can find one through the National Associataion of Home Builders (NAHB) website.
www.nahb.org
[Thanks to dallasstudent13 for this link.]

2.) Go to Houzz Professionals and enter Certified Aging-in-Place Specialist in the search box.
One that I know of is
www.brickwoodbuilders.com.
[Thanks to Houzz contributor Deborah Butler for this information.]

Bathrooms
1.) For decorative grab bars that do not look institutional, look at
www.faucets.efaucets.com
[Thanks to Houzz contributor Sense of Space for this link.]

2.) For accessibility products for bathrooms, go to www.arcfirst.net
[Thanks to Houzz contributor Megan McDevitt for this link.]

3.) Commercial Bathroom Handicap Guidelines.
www.ehow.com#ixzz2Og8qlff
www.houselogic.com#ixzz2OgAPANpb

4.) Here are some Houzz Ideabooks about Universal Design in Bathrooms:
741559 The No-Threshold Shower: Accessibility in Style
4492518 How to Design an Accessible Shower
2504909 Doorless Showers Open a World of Possibilities
5008400 Make a Powder Room Accessible with Universal Design
1061289 Bathroom Safety Features that Support your Style

Best wishes.