Usually a set of engineer or architect plans would detail everything that is needed and the Contractor would price the project off these. 

Hi Jonathan,I am a licensed architect in LA, I might be able to help you with some of these questions.1.

I had an Architect/ Contractor just ask me Friday how he can easily make sure everyone bids items so that they are not left out of the Bid, I told him he needs to create a Bid SOW ( scope of work, for those that may be wondering) and to spell out everything you need them to bid on and have them sign it and agree that it becomes part of their proposal and that no proposal should be accepted without it.Delicia, I think that @Dennis M. has given you a good option,unless you need that money back right away, this way you do not get further in dept and you will collect principal and interest on the loan for the next 10, 20 years.

I spoke with an architect and general contractor (both highly recommended by a trusted advisor).

Hi Kyle,I've spoken to a few architects and contractors but haven't decided to work with anyone yet. 

Also look up the architect Steve Baczek, Peter Yost, and the builder Matt Risinger on YouTube.

Plus the addition likely had to get the blessing of an architect and engineer. 

@Ricio MontesYou will need to consult an architect on plans and some do specialize in ADUs

Architects, engineering, contractors, lenders, AHJ info, etc.

There is a ton of time working with engineers, architects, general contractors, etc and not to mention permitting times etc.