Knowing how cold it will be in that addition will help us give you a good answer. The best range for well llgnified, fully dormant cuttings in my opinion is 28 - 40. If they're not fully dormant or still even a little green then 35-42 would be better. Getting up to 45 wouldn't be a problem. An oil filled electric heater set for 35 degrees works well for me in my garage. When it goes down to 5 degrees it's able to keep the space at 30 - 35 without getting the figs close to it above 42. If too hot is your problem then you need a way of cooling. I know people who've added a thermostat to a chest freezer so it keeps a constant 35-40, wherever they set it.