I work as an acquisitions associate with BPG Holdings, a local real estate investment fund in Alexandria, and also work as a realtor for our fund's brokerage team, assisting clients with investment properties in the D.C. metro area.I've recently created a series of reports focused on the city of Alexandria and its sub-market development prospects, which I believe alongside other macro trends will make the city an amazing buy-and-hold investment for real estate investors.

I need advice for creating the lease to include guidelines and wording on sharing responsibilities in the common areas. 

Any law that tries to do otherwise conflicts with sb13 and ab2533 (unless the unpermitted units was created post dates of protection by those laws).i agree he should get it permitted especially if close to the price quoted, but issues due to unpermitted units that are protected be sb13 and/or ab2533 present minimal risk and if it does happen get HCD to assist in any issues. 

Now the value set be appraisers typically is coming in between $50k and $100k   I invite you to search BP or create a post determining values that appraisers are giving to ADUs.  

2.Create a direct booking website and set up an escrow account where guest payments are held and disbursed to the property owner and cohost according to agreed terms?

Can I just fill out a grant deed myself and create a loan document and have them both recorded together?

And NEVER put a property in an S Corp.It's like getting in the weeds and creating LLCs to start a business...just go get a customer to pay you money to do something and figure the rest out as you go.  

Check out FilePlace, where user-created forms are available for free.

Quote from @Jack Krusinski: When my rental properties are in a situation like this, I make an effort to create a structured and aggressive payment plan to help tenants catch up.

Your dilemma boils down to two priorities: maximizing your investment growth versus creating financial stability for your personal residence.