My lender originally told me I can put 15% down but once he started the loan process, he came back and said something about the regulations changing with either Fannie or Freddie and that the new regs require 20% for primary residence.

This doesn't even begin to touch the legal requirements for 506(b) or (c) regulations. 

@James Edwards You'll need to enforce your rules and regulations.

Make sure to be aware of the different laws and regulations regarding section 8.

I am thinking that is the spirit of the regulation - They don't want you using your 401k to put cash in your pocket. 

The way around it is to use a PPM and set something up under Regulation D of the IRS code, but it's super expensive and you need a quailified attorney to set it up.

Become familiar with what the 6 ADU/JADU regulations allow and their limitations. 

The reason this is important is that it implies that duplexes purchased will be under rent control regulations (assuming a unit is occupied) regardless of if the new owner decides to move in (house hack) after purchasing the property. 

Recently there have been municipalities that have set guidelines (regulations) that prohibit denying a tenant for previous criminal convictions at least x-years ago. 

Regulations.