Those calcs assume the door is close fitting and there are no draughts which will seriously affect things. As to how good the calcs are, a 2kW heater should keep it warm in there... so how was it last winter and whats the output of the space heater? The open door will permit significant heat exchange through air movement, so much depends on the temperature of the other building. I get the impression this is also unheated/uninsulated?

You didn't assess the floor losses?