For the pier and beam foundation, try this website:
https://www.apawood.org/raised-wood-floors There are a host of building and specification guides you can download free of charge. This site is run by APA, the Engineered Wood Association. Might not make you an expert, but you will learn a lot. Also try
https://www.southernpine.com/span-tables/. This is a site run by the Southern Forest Products Association. They both contain a lot of information, guides, and charts for using engineered products and SYP. Very good information as they follow current building codes in their recommendations.
I was able to Google building codes in Missouri and find a site run by the state government with links to what building codes are enforced in each county and even specific codes in some cities. From there you can link to sites where you can either buy or download for free the applicable building codes. In my county there ARE no building codes, but I downloaded the 2012 International Residence Code used in larger counties so I can make sure I build to the most currently used code. I have only printed out a few pertinent sections because it is a 940 page PDF document. Like most code books (IEC, NFPA), it can be very confusing to navigate.
If you wish to make access ADA compliant, go to
https://www.ada.gov/ where you can download design standards and technical assistance materials.
Best of luck!