When they are footing the bill you are the first to know about any minor detail.When I analyze a property if landlord pays water/sewer etc.

Over lap any joints by a foot or so.  

All one story, will be average quality construction, and is in a section of the municipal City of Los Angeles that does not have architectural committees or anything else outside of normal building codes.Assuming no major issues with adding to the existing dwelling, trying to get a general idea of per foot cost.

Assuming it either allows it or that you can get a variance to do so, the build out cost should be about $20 per square foot.

Hey Joseph welcome to BP.You already have your foot in the door being an agent which is cool.Instead of driving for dollars, you can FLY for dollars.

how much do you pay by the foot/yard for each of the different materials?

Will be taking action on this, but just want to make sure that I'm not shooting myself in the foot for the future somehow! 

Also, the cost of purchasing a duplex is typically less per square foot than a SFH. 

The property was an out of business car dealership, which contained a small 90 sq foot office, in addition to a legally non-conforming 2bd house.

Start out on the right foot from day one.