Back Mortgage and new builds

[Jacob Delano]

I met a neighbor who id an older gentleman and has been renting out his 3 vs for 20 years. He is on a 1/2 acre and is interested in building another 3-5 bedroom apartment next to the existing unit. 

This is my first post and would be my first deal. But I want to put skin in the game by getting a construction loan for the build. His property is worth about 300,000 now. He has 200,000 in equity but his house on a back mortgage. The property is in the SLC area. 

my question is how can I structure this deal fairly and ethically for both of us? What financial tools do I need to use? I hope I haven't left out any pertinent information, so please just ask any questions you might have.