You need a mechanical permit for the HVAC, and you need a residential construction permit or EZ permit open in order to pull the mechanical permit.

A mentor will be invaluable in helping you navigate floor plans, budgets, and the logistics of permits and inspections specific to certain areas.

I would suggest looking into your local market first obviously you need to find out STR restrictions or permit needed.

ADA units are more expensive and require more from the permitting process to construction.

The permitting process and the availability of reasonably buildable lots is a concern.   

When it comes to acquiring lucrative coastal luxury vacation rentals on the Oregon Coast the most challenging components can be permitting and timing. 

So a nicely decked out  cabin I bet would rent real well for STR purposes  Not sure about permits etc..

I would check with an attorney or CPA on it.EDIT - Check out Scott Royal's site - https://royallegalsolutions.com/what-states-are-permitting-s...He used to be on here quite a bit but I haven't seen him in a long while.

You also should call your local zoning or permitting department to get information on areas you are interested in and how to be a legal STR.I 100% recommend making sure the STR could work as a LTR as well in case you have to pivot ever.

Said differently, you won't need a special ADU permit to create an ADU.I'm interested in potentially purchasing a single-family home in eastern MA (Norfolk or Middlesex county), and doing this exactly.