Heating solution for granny flat

[Ian Gilligan]

Hey all, 

I have a 425sqft add-on (was added on to the main house years ago; isn't reached by HVAC from main house) that I need a heating solution for. About to be occupied by a new tenant. There's already a window AC unit that functions great; I'd rather not lose that unit and install an expensive mini split... 

Any baseboard heating solutions or anything of the sort you can recommend? Need something more permanent than a plug in space heater or something like that. 

Really appreciate any thoughts.

Best (and Happy New Year!), 

Ian