With no direction from an Engineer (or Architect) Contractors have no idea and will price out the scope of work they think is best even if it's not the best option. 

(My fiancé with an architect with a refreshing vision) What made you interested in investing in this type of deal?

Who is their architect for all plans and permitting?

I'm an Architect and MRED Grad and have entitled and permitted over 200 units across Bay Area and Norther California, ask me anything about SB9 or ADU design / permitting

Your agent, your architect, whoever. 

They usually require experience and they are lots of docs needed from architect drawings to a lot of permitting. 

Depending on the size and scope of the project, an architect may need to provide a more detailed set of plans to get the necessary permits.

And if they are a hobbyist/part-time builder, then they should engage the proper professionals, i.e. a title officer, civil engineer, architect, contractor et al to assist in reviewing the project's feasibility.

I'd recommend a project manager or architect if your budget can afford one.

This way, you can ensure the design choices match your vision.Defining design choices often involves working with a designer or architect, but if you're hands-on, you can select finishes and fixtures yourself.