the building is fully insulated, like a house. So it's heated enough not to be a problem.
We didn't really "add on" as there were two buildings there to start. We took down the older side and rebuilt it much taller. Old one was so low DH would hit his head on the roof supports.
The other half we had converted into a greenhouse/chicken coop/summer kitchen several years ago. Last summer we built a separate chicken coop and opened that end up for more room for the Summer Kitchen. The new coop is that smaller red building on the side. We built it to go next to the shop, but did not foresee that the chickens would like hanging around on the cement apron - we got tired of stepping around droppings so decided to put it back to the other side of the courtyard.
We went taller mostly in case we decide at a later date to use this as a woodworking shop- lots of room to swing around a 8' board without taking out lights.