Make sure to have clear documentation of regular, on time rent payments and the agreement clearly laid out.

They should have the policy documents from many of the new home loans they have done.

Once you have that letter you can use an LLC formation document template to create the LLC and register it with the corporations division of your state or jurisdiction.  

Our counterparty has an attorney, who I believe would draft up the promissory note and personal guarantee documentation.

The City of San Jose has on their website that new addresses are required but separate meters are optional (link below).https://snapadu.com/blog/electric-panels-upgrades-power-sources-adu/https://cao-94612.s3.amazonaws.com/documents/City-of-Oakland.PGE-Joint-Information-Bulletin-METERS-FINAL.pdfhttps://www.sanjoseca.gov/business/development-services-permit-center/accessory-dwelling-units-adus/adu-faqs

If possible I'd even request it be documented on the invoice.

She seems like she may be in it for the long run, though, so keep documenting everything you can. 

You'll want to understand what kind or notices and documents you need to get executed to either up their rent or turn the unit.Solid long term tenants are worth their weight in gold in my book so if theres a way to up the rent and keep them happy I's say thats the best route personally.

When you buy the property, you are also buying the lease that is in place and the rights/responsibilities that go along with it, so it is important to understand what that document says.

Therefore, don’t let your referral sources gather documents for you or issue loan quotes.When is it illegal for a mortgage broker to pay a referral fee?