Re: HVAC options for addition
Good grief how did you come up with those BTU's? Yikes. I can heat my whole 3200sqft home and full basement with 80-90k BTU's.
Depending on where you're located, 800 sqft shouldn't need more than 1.5 Tons cooling (18000BTU) and 40k BTU's or less heating.
IF you have oil now and no natural gas, a conventional heat pump will be most economical. No need for a mini split. You probably only need a 1.5 or 2 ton heat pump for that space with maybe a set of 5KW heat strips for auxiliary heat. Smaller system will be quieter and need less ductwork and fewer supplies.
Put the unit in the attic and make it a sealed attic with the roof deck spray foamed and use ice shield across the whole roof as a vapor barrier. or better yet use a steel roof and a radiant barrier on the roof deck.
1925 Two-Story Stucco Beaux Arts Neoclassical