Gardening

With winter not all that far away, I finally decided to set up a greenhouse. As much as I wanted to build one myself, the cost of materials was going to be about the same as buying a pre-made kit. I ordered a 6x14ft greenhouse kit, somewhere in the mid-range as far as quality goes.

I'm having a hard time figuring out where to place it, though. I have a few options in mind, and they all have their pros and cons.

The option I like the most is a concrete pad just beside my house. It gets good sun (this time of year, direct sun from about 8am to 5pm), it's a convenient location, and it's already a level surface. I'm not so sure about putting a greenhouse over concrete, though. I'll be using an electric heater in the winter, and I'm worried the concrete will pull a lot of heat away from the air in the greenhouse.

I have a few other areas on my property that would work, but they are all a very uneven gravel surface over extremely hard-packed rocky soil. I could dig out a level area for the greenhouse, but I would imagine I still would need to build a raised base, maybe a wooden platform.

Is putting a greenhouse over concrete a bad idea in the winter? What if I decide to put it somewhere else and just level the ground. Can it go over dirt and gravel, or would I need a base?

Any suggestions are welcome. This is my first foray into greenhouse gardening, and I'd like to start off right.