For about one year, I'll work in my newly constructed garage. About
24x28 with 14-foot ceilings. California regs are tough, so this
building is tight, thermally. I bought a 220v plug in heater to take
the chill out. 1kw produces about 4000 btus. A 400 sq ft garage, if
insulated, would require about 5000+ btus to heat comfortably. So you
can gauge the heat demand, a house built to modern code will need about
12,000 btu's for every 800 ft/sq of living space.
I chose a portable electric heater over a pot-bellied stove, kerosone
barn heater, pellet stove, or propane heater. Worried about fire,
fumes, oxygen depletion, soot, ashes, and the fact that my small
electric can be moved around close to me.
All bets are off on btu ratings if your attic is porous or the windows
and walls aren't insulated. When my full-sized shop is built, it will
be heated and air-conditioned using a Geothermal Heat Pump. With
radiant piping in the floor. But that is another story.