Most states have an accessibility standards handbook. Many of them have one on-line, but even if your state does not, any other state's handbook would be a good place to start looking at general guidelines for design. What you're wanting to do is to not create future inconveniences. Look at doorway access, clear space inside the bathroom, proper fixture lay-outs for clearance (if and when they are of limited mobility), and any other obvious considerations. You'll likely find that too little space allocated to fixtures is your biggest concern. After that, most items almost take care of themselves

Have fun,

Woody