If not that would be my first step, also be sure to get a phase 1 environmental done to ensure the ground is not contaminated.

Is it a great opportunity or a headache with environmental cleanup concerns or something like that? 

While it can be lucrative, units can also sit vacant for much longer - especially right now with tons of offices and retail reducing footprints.

I was unaware and have spent several thousand having property inspected, site plans, environmental studies etc.

Also, if you can't pay for an appraisal or finance an environmental study with a property of this size, you need to sell as soon as you can to pay back your friends and family because you're seriously under capitalized. 

However, our capital sources ask the question "asbestos" and other hazards on their environmental disclosure form(s).Thanks for replying.

In NJ, one thing people often forget to ask about is an underground propane tank/well which, if its there and has not been remediated properly can be a disaster to overcome (very expensive and NJ Environmental dept will be all over you).

Sometimes former gas stations and dry cleaners don't sell due to environmental hazard.  

If someone told you that your cost of hiring each sub on your own would save you half the cost of a general contractor, they are sadly mistaken.A 500 SF ADU for $100k sounds like a very good price to me - generally speaking, but would need all details of the project and the site to be sure.Assuming you have the footprint room on the lot, a multi family zoned property is allowed 2 detached ADU units.

Has color variations (depending on sun/shade), and animal foot prints (chickens, etc. that walked through when tiles were wet).