Upstairs, we're going with electric baseboard heat. This has several advantages, chief of which is that forced air is not good in combination with allergies, and our son has allergies. The second advantage is the flexibility; you can heat just the rooms you're using (granted, in this small an apartment, this is not such a huge advantage). Electrical heat, of course, is not the cheapest. If we had unlimited funds, I'd probably go with water radiant heat under the floor on both floors. But we do not have unlimited funds, so we calculate our BTU requirements.
(Of course, most of the heat from downstairs will waft up to the apartment anyway, so the baseboard heaters are really just for convenience and comfort. In May we were still living in the tropics. I don't expect November to be easy for any of us.)
To calculate BTU requirements, we first measure the carriage house. I've been wanting to get that done for three months, and here's the result! A man, a plan, a carriage house, esuohegairr a canal, Panama! We're going to heat the front room, the bedroom, the kitchen, and the bath. Using the area of each room, we use hearth.com/calculator/btucalc.html with its most pessimistic settings to get a maximum BTU requirement for tropical people living in the wrong part of the planet, use the conversion factor of 1kW = 3143 BTU to get a rough kilowatt rating, then use the rule of thumb of 250 W per foot baseboard heat to find out where we can put our heaters.
The front room is 15' x 13' = 195 sqft = 7800 BTU = 2500 W = 10 feet.
The bedroom is 14' x 11.5' = 161 sqft = 6700 BTU = 2000 W = 8 feet.
The kitchen is 11.5' x 10.5' = 121 sqft = 5300 BTU = 1500 W = 6 feet.
Finally, the bathroom is 7' x 7' = 49 sqft = 2200 BTU = 750 W = 3 feet.
We don't actually want 8 feet of heater in the bedroom, because we only want about 6 feet under the window. And we don't want 6 feet in the kitchen, because the north wall is the stove and cabinets, and there's really only room for about 3 feet under the window (but we cook a lot; the kitchen will never be cold). To compensate, we'll put 6 feet into the hallway between the bedroom and the kitchen.
In the front room, we'll put 6 feet under the window, then the other 4 feet onto the north wall opposite.
There's good insulation in the carriage house attic; with the new double-glazed windows and a whole lot of caulk and Great Stuff urethane foam to stop all the drafts, I really think this is going to be a really cozy little building for us. If we cranked all the heaters to the max, our outlay would be around $300 per month - but that would bake even us out of here if the outside temperature were anything above 25 below.
I think this will work.