You mentioned waiting to save up all the cask to do it all yourself and that sounds doable to you but the seller may not be willing to wait for you.

Is the insurance you mentioned the homeowner’s insurance?

I spoke to my real estate agent and he mentioned that a law was passed recently in which you can get a permit to build a 1bed/1ba on a lot that meets the required sq ft.

 $210k is new build or a lot of build number, but you mentioned doing an addition.  

On the 2nd go around the seller can void the contract.But as a general rule if the contract does not mention they can not void, then they can void the contract once you open the contract back up for negotiations, because now you no longer have a ratified contract, but you are now negotiating again.

He bought this property as a fix and flip, and mentioned that there may be a 6 month seasoning period before a buyer can purchase the property with traditional financing. 

The reverse exchange as mentioned remains an option. 

Like you mentioned, @Jay Lohn, it seems like a reasonable investment in tenant retention.Since we're currently owner occupying one of the units, there are some pretty attractive improvement loans that we could take advantage, so may now be the time to do it.

(I although they may if they wish)Its worth a shot like @Chad Urbshott mentioned I have bought a few juniors in my day and negotiated with the Senior to keep payments current until such time as I could sell or refi.. 

I googled and something is mentioned aboutReturning  (Lesee) Non Employee in relation to like cars =LIt dawned on me gosh now are 'wholesalers' now representings properties for rent as 3rd parties or something, too?!