It’s gas heat and water which I know I may have to pay for unless there is a creative way around that as well.Looking for some suggestions 

Probably they don't or you can watch them get heated up on the spot and make stuff up.Most of the national acts really seemed to be selling something else that probably won't help you.....database software for $30-100/month.   

Only "Absolute 0" is, if getting anally technical, actual 0 degrees as in 0 heat of any kind. 

A family who all want a yoga retreat?

Often ADU projects have comps where they compare the ADUs to little office spaces, personal yoga studios, etc.

If the lease requires Renters Insurance, do they have proof of the tenants current coverage4, Age of last update to the major systems (Roof, Electric, Plumbing, Heating)5.

Make a list of all assets and their components (how many roofs, refrigerators, heating systems, etc?)

The property had window AC's and a gas fireplace upon purchasing so we immediately had central heating and AC installed.

When it is time replace your central heating and cooling system with split units.  10.  

Also do occasional hot yoga (which will make you feel like a completely new person - and my wife loves it!)