An engineering-based study requires a site visit (inside and outside) and a thorough review by an engineer and someone experienced in construction. 

@Gavin WelchNo problem they can be a great asset if you need some boots on the ground where you don’t know anyone and they are a trusted reliable source, quite thorough as well!

Take the time to build a strong foundation of knowledge and relationships, and don't rush into any deals without thorough due diligence.

@Leon Lai, I'd start by talking a thorough look at the zoning code regarding the lot and design requirements for an ADU, I'd also confirm that they are interested in building an ADU (attached to the house) as opposed to a DADU (detached backyard cottage like thing) as the requirements and considerations will be different.

@Doug McVinua I was recommended to use this one from a realtor, it does look thorough.

I do need to work on a more thorough lease.

Thank you so much for your very thorough reply.

We recently had a nightmare tenant and learned the importance of conducting a thorough background check before signing a lease. 

That was very thorough and educational.

I had an amazing agent who was my boots on the ground and I paid for a thorough inspection.What made you interested in investing in this type of deal?