Credits, Energy or whatever is included

There is a tax credit (not deduction) for insulation systems, possibly including the spray foam, but only if it was installed on your primary residence before September 30, 2025.  This outbuilding does not seem to have been your primary residence nor will it be, so you are not allowed to take the tax credit for energy efficiency improvements. 

All improvements to real property can be added to the cost basis.  This may reduce your capital gains if and when you sell.  (Real property is land plus anything that is permanently attached.)  So you should keep track of all the costs of renovating this outbuilding, because that can be used as a cost basis adjustment, regardless of whether or not you rent it out at some point.

If you do rent out the new accessory residence, you can claim depreciation as a rental expense.  Depreciation is basically an allowance for wear and tear, and the basis for depreciation is the cost of the accessory dwelling plus whatever you invested to improve it.  For example, suppose you bought the property 10 years ago for $200,000 and the accessory building represents 5% of the land.  Your starting cost basis for the outbuilding is $10,000, then you add all the costs of the improvements and renovations.  That is your total cost basis for the building and would be what you use for depreciation if you start renting it out.  Separately, you can either depreciate, or take as an expense, most other expenses of getting the property ready to rent out, like inspections to get a certificate of occupancy, appliances that you provide (stove, fridge, clothes washer), furniture if it is a furnished apartment, and so on.  There is a lot more about being a landlord you can read about before you make that decision. 

Lastly, some states may have financial incentives for creating accessory dwellings on your property.  I can't tell you which ones what qualifies for what benefits, you will have to check your state government web site.