I want to understand:1. where this restriction comes from2. how to know which code your house belongs too3. where all the codes are described and how they are related4.

You may be able to convert it to a triplex under the 2040 plan by adding fire barriers but a triplex is commercial building code. 

Between purchase and possession, they've transitioned to a significantly more strict HOA.  

Hey Steve, if you sign up for one of Brandon's webinars (he puts out one a week) you will oftentimes get an email or a code at the end of the event that gives you a discount.

I also would like to buy/invest in another property, and am lost as to which areas specifically will be subject to this new strict STR law.

Due to population density and Building Code complexity, the fee is well earned as it's not a walk in the park to obtain construction permits.

There are more than one tax codes that are interconnected here and that is why you are confused.

Geez...that's like asking what do I need to know about construction, financing, valuation, municipal codes and permitting, Landlord/Tenant Laws, liability protection, marketing, tenant screening/lease contracts, running a business, market demographics/inventory/year-over-year appreciation/rent pricing - and a whole lot more.  

Quick takeaways to avoid this pain in the future:(A) Add strict language to your lease regarding unauthorized occupants, such as:6.

I know that area is not the best zip code to flip in, but depending on the lot size you definitely could have built and sold, or built and had it rented out!