If you're going to breed on any scale I would say either garage or shed - you will find it more practical. I keep mine in our garage as it's brick built and we insulated the ceiling to prevent damp and draughts. A shed will also be suitable if it is dry and not draughty, preferably with an opening window.
Best for the mice is probably inside the house as it will stay at a comfortable temperature. If you have more than about 5-10 mice you probably want them out of the house because of the smell.
We have ours in an insulated building which no humans live inside. I anticipate having to purchase an airconditioner soon, although the winter temperatures did not bother our rats or mice at all. Even without heating and with snow falling on the building, breeding didn't slow down at all. I was most impressed!
Mine have been inside but are going to an insulated shed this week.
40+ mice do get a bit much indoors unless youre constantly cleaning out.
Ive basically sealed the entire shed, packed it out with polystyrene including the roof and boarded it up with thin ply.
It does have opening windows with very small mesh on the inside to make sure we have no unwanted guests :roll:
I did this with my ferret shed and its kept the temperature regulated really well throughout hot summer and freezing winter.
Mine have been in the house well to be precise in the kitchen, but now the weather has cooled off I have moved them into the conservatory, I now have loads of cages so need to find away of cooling the conservatory down in the summer I just cant bring the mice back into the kitchen I havent the room now I have loads more mice, also dont have room for a shed in our yard.