I have been playing the multifamily game for almost 10 years now, and the GRM is one of my least favorite methods personally since it ignores so many important factors (steam boiler versus electric baseboard heat as an example). 

Are you in a market where water lines could freeze in the winter if there’s no heat?

If this is a "store bought" fire place (not a wood stove) then usually only processed presto logs are appropriate because they burn at a constant speed and keep the heat down.  

Electrical work, plumbing, foundation, windows, roofing, and AC/Heat.

Hey Chris, a few thoughts here.Considering that office is not attached to the house, I wouldn’t add any footage to the total home square footage; however, you could advertise that office space and amount of footage and amenities in your remarks.If you’re a Realtor or are using one for the sale, your local MLS may have fields dedicated to additional footage for other buildings on the property, such as ADU’s.The other point is that typically, perhaps depending upon location, a space needs to have heating to count in the square footage, which this office doesn’t have if I understand correctly.Consider contacting an appraiser to inquire about how they would handle this space on an appraisal, or a Realtor.

In June 2024, we completed installation of a heated and chilled pool.

Here is what we have found and I will probably catch some heat for some of the things I have to say, because it does not sound "investor-friendly".

Probably need to add at least heat for the ADU to be considered livable space but that isn't a huge deal, if you can make it work great. 

under mount granite or quartz counter tops.. water closet  heated floors ..

If the home is 100 yrs old, I'm assuming it's a definite gut which is going to require new plumbing, electrical, heating, cosmetics.I spent about $140K reno on my multi family in the heights on NY ave and there were massive problems with it but it was a gut to 3 units & adding 2 bathrooms, 3 brand new kitchens 4 bathroom reno in total, all new flooring, sheetrock , insulation, framing, doors, electrical, plumbing, paint, parts of roof etc.