She's now interested in a larger more expensive unit that will be vacant next month !!! 

Any developers care to share their opinions on building multiple duplexes vs fewer but larger multi family buildings?

RUBS is likely the most common when looking at larger scale complexes. 

Especially if you are doing larger projects where there is more likely for something to go "bad," you don't want one lawsuit in 'Project A' to affect any of your other projects especially when you might have different investor / limited partners for your projects.The general concensus is NOT to use C Corp for and any long term holds because of the double taxation (and I think there was something else about if you were to transfer the property...). 

Keep the renovation costs as low as possible and you can have the suite quickly pay off the borrowed amount and then have a larger heloc.

You start seeing copper supply lines, PVC drain lines, larger electrical services, modern breakers, sufficient electrical service to kitchens and baths, etc.

I think as you stated especially for larger players talks with your lenders up front could be very prudent..

Currently I own several duplexes and have dealt mostly in smaller multifamily.I want my future investing to focus on larger apartment buildings and mobile home parks.

Clearly there are tasks that we’ll all need to complete, and a new type of focus we’ll all need to find, while we look to continue to drive the health of ourselves and our businesses.

We would like to keep our first house to rent out the top and bottom while renting out the bottom of our larger future house.