This may seem like a dumb question, but if I'm filming in a pre-existing set like my house or a friend's house,would what lighting I use depend on the look of the film?

What I'm trying to say is could I get away with using existing lights in the room, or should I add more lighting to the room? Or is using only existing light going to make my film look crappy? I'm shooting on a EOS 7D and a 60D if that matters.

The primary considerations are these:

1. Do you have adequate light to make a good exposure. If you can put your camera on ISO 100 or ISO 200, and your lens on F4, and still get the meter to read at 0 or above, you have enough light.

2. Does the light create the mood and tone you want for the scene? If the answer is yes, then you do not need any additional light. If no, then you need additional lighting.

So if i can get away with using practical lights (lamps etc) then I dont need additional?

I've shot numerous things with only practicals. I re-bulbed them to be brighter, but that's all I used.