I kept my 25 gals in a large brooder I built in my garage with a 250 w lamp raised up about 3' for the first 3 weeks or so. My garage temp varied but was around 50-60 F on average. After 3 weeks I changed to a 125 w bulb, and at about 5 weeks I'd leave the bulb off during the day for a couple hours till I got home from work. Last week (6 weeks old) I moved them to a coop and run I just finished and hung the 125 w light in the coop at about 3'. I've been turning the light on at night since it's been getting down to about 45-50 but I've been cutting it off when I open the hen hatch to the run around 6:00 AM before I go to work. This is the coldest part of the day but it warms up quickly when the sun rises. When I check on them in the morning a couple will be laying under the light but most are scattered around the coop with a few on the highest roosts. I'm planning on removing the light completely next week since they'll be 8 weeks old and should be good-to-go, but I'll keep an eye on them the first night to make sure they're not under any stress. I think a few prefer the heat and most don't need it. This is my first flock so I wanted to take it slow to minimize any stress. Like a lot of people have said on here I think at 8 weeks they'll be fine without any supplemental light. I live in the low desert so heat will be more of a factor in a few weeks than cold.
After raising this first flock I can say without a doubt I would never raise chicks in the house no matter the number of birds or the temperature outside or in the garage. I can't imagine keeping up with the dust, etc. A 250 w bulb will provide all the heat you need for the first 3 weeks in a draft-free brooder whether in the garage, coop, or barn. And like others have said you only need to keep one spot warm at around 90-95 F for the first week. Since raising these gals I'm building a brooder/coop combo for the broilers I have due in March. In the future after my layers retire I'll raise the next batch in the broilers' brooder/coop. Even in the garage the dust and mess was a bit much and I had to do a thorough cleaning/sweeping twice a day after the first 3 weeks or so. Just my opinion but I think chickens were made to be raised outdoors for sure.