If you progress someday to apartments, then you'll need a GC to juggle the work load and timing of repairs, schedule inspections for signing off permits, etc.

See link.You are required to have a building permit to create another bedroom.  

My question is, is it worth getting a legal permit for it or just run it on the down low?

Challenges include timelines of inspections, pulling permits and the time crunch especially with the current Covid-19 challenge.

I just discovered that the rehabber has not closed the permit before selling the property to me.

This is part of the contract for the form of payments. " (1) Tenant must pay all rent timely and without demand, deduction, or offset, except as permitted by law or thislease.(2) Time is of the essence for the payment of rent (strict compliance with rental due dates is required).(3) Unless the parties agree otherwise, Tenant may not pay rent in cash and will pay all rent by (select one ormore): cashier's check electronic payment money order personal check or other meansacceptable to Landlord.

The architect should have contacts to help with this if they are experienced in this product type.Fees: Consult your local jurisdiction to find out what the development impact fees and permit fees for your project will be.

I am currently converting my garage to an ADU with permits.

I would call the county inspectors office first, tell them the scope of work you are looking to complete with a contractor and ask if a permit will need to be pulled.

I can't find a permit on the barn structure.