We want to get a single family house in the Reston, Sterling, Herndon area in the 500k-600k ballpark where we can add an accessory dwelling unit that will be rented out (Airbnb or long term rental TBD).

Even if liable, I'd rather be looking to a local surveyor then a possibly now out of town seller.Going from memory here but the survey affidavits we use require seller to affirm that no fences, walkways, buildings, accessory structures have been built on the subject property since the date of the survey nor have neighbors built/moved such fences, walkways, etc. near or along the property lines of the subject property, that no adverse claims exist or have been made as to possession, etc. 

I've specialized in Flips, Wholesales, and Buy & Hold Rentals, Accessory Dwelling Units (ADU's), and Short Term Rentals (Furnished Finders, etc.).

If the 2nd unit above the garage is a legal accessory dwelling unit then you should be able to use two leases. 

I know on the MLS here in our area it is no longer labeled ADU it is actually spelled out as Accessory Dwelling Unit and it is a yes or no checkbox...I hope that helps.

Excavation of basement in-law suite for an accessory dwelling unit.

As much as those things are valuable, it may be good to go minimal and cut off the accessories for a bit.

They do allow accessory apartments though, but that means owner occupied is required to rent out the accessory apartment.  

Important definitions include "use," "accessory use," "dwelling," "lodging house," "boarding house," and "family."

I'm a big fan of your company, I work for a local manufacturer of storage & organization accessories for kitchen cabinets.