You can pay an attorney to do the docs once, them use them again and again (if you know how to alter the names, contact info, terms, and legal description). 

Therefore the site would have non-conforming rights to have 2 single-family dwellings at the site.A building permit was recently issued for a residential alteration at the site... xxxxx"There are two separate structures on the site (one being a duplex and one being a SFH) as she mentioned but I am curious if it would it be impossible to legalized the downstairs unit now since the zoning has been tightened. 

Is it possible to alter our current arrangement or terms?

I’ve even heard of a couple of horror stories of lenders leaving buyers at the alter on closing day.  

How do you know what to do, for example, for converting something into a master bedroom, or remove walls in a certain area, or any other type of restructuring or remodeling project where real estate is altered to fit a new purpose or design?

Tossing those guestimates in the middle of this will significantly alter the data output.  

You may have to alter per new tenant to suite particular needs.Organization, ultimately, will make or break you.

I don't know if this could come back on me if I end up not buying the property and they see that this alteration was made.

Have you set up your keyword alters yet?

If you purchase on MLS/ Pocket Listing, you can minimize risk by doing inspections and walk throughs prior to purchasing so the potential repairs can be readily foreseen but you will be paying extra for the comfort- Which will alter your return if you buy and fix or buy and rent- Significantly!