A lot to think about. 

Apply for a dimensional variance on the vacant lot to allow a smaller lot and/or setback so it can be built upon.Your best first steps are to sketch a plan, possibly with an engineer's help, then contact the zoning department to discuss your idea with them to get informal feedback on your liklihood of success before you spend a lot of money.

Search the BP for ADU appraisals to encounter numerous examples.

It is maybe a couple hundred dollars to join and you can attend numerous classes and meetings.

This is going to vary significantly depending on numerous factors.Always happy to connect 

A friend of mine just built in a purple state and he said it was worse thing he ever been through and he grew up in household where they built numerous houses.

In neighborhood it is very important to present your self to the home owner next to you, this helps a lot to prevent misunderstandings. also this shows your professionalism.

Hi @Deepak Malhotra,Thanks for sharing your experience—there’s a lot to learn here!

A whole lot to be wary of- well, lagoon system and then those POH trailers. 

So a lot to know.