I'd like to study everything from standard property purchase agreements, to lease/option contracts, seller financing, wraps, sub2, deeds, notes, etc.  

There are usually laws regarding minimum standards for plumbing, air flow, and heating, fire safety standards, etc that are affected by the number of unrelated people renting part of the same dwelling.

We want to rent out a basement apartment in our primary home on AirBnb (or long term rental with standard lease).

@Colin Caporal-  extension fees  of 1.25 to take the  6% rate to 8.15  is  standard and OK  ...but ..you should ask the  question  about  what  happens if the  house and loan arent  closed  by 8/15/2022....(   this  could easily  occur  and  you want to  have this  issue  addressed  now  versus later )  ...were you able to  get the  DTI issue  addressed

@Gabriel Ramos yeah, this seems to be a pretty standard renovation loan.  

The standard is that your intent must be to hold those properties for productive use in business, trade, or investment. 

First and foremost, whatever you are doing, you must keep the Federal AND State Fair Housing laws in mind: Treat everyone the same, using the same standards.

@Pete Largo, while I would say 10% is an industry standard, I see many, many deals that are far less than that.  

Apart from the standard legal terms of a lease agreement, is there anything that I should consider adding to the lease that would not typically be already written in the contract?

There are noise standards no matter your mental issues or the issues of your child.It is wrong to decide one tenant can be destructful to another because of a medical condition.