You can grow in the attic easily if you have a window to throw an AC into.
With no AC, my attic tops 110 in the summer. With AC, it is 75, or whatever I set it to.

In the winter, you're going to get some heat from your HPS, but during the dark cycle a small heater will be beneficial.

I have no problems growing in my attic, but the electrical costs are much higher than when I used to grow in the basement.