Before you buy/build a greenhouse, I highly recommend you read Eliot Coleman's Winter Harvest Handbook. He grows commercially in a cold winter climate, and the book will help you think through your plans. Save you a lot of stumbling around.

I was excited when I got this greenhouse. It is just the right size for my small backyard. The first thing we noticed is that the plastic cover is too small for the frame. We thought it might stretch out. It didn't.
Then, after putting it up and securing it with the small pegs that are provided, it blew over within the first 48 hours (not even that big of a wind). One of the legs broke in half and another bent in half. Luckily, my husband is handy with fixing stuff, so he fixed the legs better than new. We put it back up with massive stakes at each corner. All seemed well until I went into the greenhouse about 2 weeks ago, and everything in there was covered with small things that looked like fish scales. I couldn't figure out what they were for a while, and then it dawned on me to look up at the roof.
Almost every small plastic "window" between the green mesh was empty. I now have a SCREEN house with very large mesh, and I'm wondering if I can salvage it anyway. The plastic cover is shot.
This is an unacceptably short life span for this greenhouse.

I contacted the company (their website states that there is a 100% satisfaction guarantee), and got a rather rude reply, and the customer service rep didn't even bother to use capital letters in their response.

I filed a complaint with the BBB. The issue still isn't resolved. I DO NOT recommend this company!