I would like to do an attic grow myself someday so I have had some thoughts on it.

There are a couple of important aspects you need to think about; a tent, ventilation, air movement, insulation and odour control. What sort of lights are you using ? HPS ?

So first off I would advise you get a tent which is big enough to suit your needs and no bigger. You can grow 4 plants in a 1m square space in a typcal grow, not SOG etc.
The tent will serve a number of functions so they are good to have. The tent allows you to have more control over the plants environment.
When you put up the tent there are round vents where you can attach flexible ducting. Use the bottom duct as your input and the top as your output. When the tent is in the attic postion it between 2 room below it. Cut a 4inch hole in the ceiling of each bedroom and fix a vent to the ceilings to make the job look better. One bedrrom will supply warm air to the plants and the second will sent the hot air from the tent back into the second bedroom. You will need to use a good carbon scrubber of course on the output. In the winter you can keep the heat from the lights in the grow room which will keep your bills down. Keep the runs of the flexible ducting short. In the winter you can insulate the tent with lots of fireblankets or something which is non flamable and insulates. If you insulate the tent very well you could keep your heat signature down too. If your roof is the only house with no snow on it people might notice.

In the summer you could remove the ducting from the 2 ceiling vents and run them to the eaves of the roof. The far edges of the attic sometimes have vents. If not make one and take in your fresh air from outside and pipe it out the other side of the house. You can pipe it out the same side if you want just dont keep the ducts too close so not to recycle old air.

When the lights go out in the winter you will need a constant flow of warm air in the tent or the plants could freeze and die. You could have a small heater in the tent attached to a thermostat socket control. This will turn on the heater when the temperature goes too low. You need to be extra careful with heaters in the attics. When the heater comes on you might not want the extractor to come on too often in the night as you will be losing the nice warm air in the tent. You will have to figure that out yourself. Although the warm air will be running back into the house so how bad. Just expect your power bills to be much more than usual in the winter. Personally I have my extractor on 24/7 but I dont worry about the heat loss as my tent is in my bedroom. I have to sleep with an ear plug in one ear heh heh... Thats why I wish I had a house with an attic !

So thats your best option in my opinion. Its hard to keep good air coming into the tent. In the winter use the air from your house and in the summer bring it in from the corner of the roof.

Keep an eye on condensation too. Keep a fan running in the tent and remember that the higher the humidity the less effective a carbon scruber is. Perhaps you should try LED lighting as they produce no heat. But be sure you get a good one. I grow LED and I got 5 ounces of top quality weed off 2 plants. You can see it in my grow.

As far as power is concerned I would not use the lighting circuit. They are rated at 10Amps whereas the sockets are rated at 20amps. You can see the trip switches on the fuseboard. I am an electrician so I could simply find a socket cable in the attic, cut it and make a new socket up there. If you are not comfortable with electrics the use the extention lead. I would break a hole in the ceiling in the corner of a room for the cable to go through to get down to a socket. Take the plug of the extention lead and run the cable down and reattah the plug top.

Remember to always keep electrics above the waist level in the tent. Water on bottom, electrics on top !

Good luck with it.

You can check out my grow in my link