Most jurisdictions post their building codes on their website and also make them available in their administrative offices. You can also get details about your local building codes by calling or meeting with your local building official, plan inspector or site inspector.
Most jurisdictions base their local building codes on the International Code Council Standards, which you can find for free online by searching for the International Residential Code or the International Building Code. The Americans With Disabilities Act also calls out some specific requirements for building ADA-compliant wheelchair ramps. Most local jurisdictions in the United States use these standards.