1. What do I want to grow in my greenhouse? If your goals are just to keep some tropicals alive through the winter and start some bedding plants in the spring your needs are going to be different than if you want to grow vegtables, particularly the much sought after tomato. Reproductive fruiting plants like tomatoes and cucumber require more heat, light & control than vegetative plants like lettuce and herbs.
2. How much do I want to grow in my greenhouse? Once you have and idea of how much room you need to grow what you want, you should now consider adding more sq footage. Greenhouses tend to fill up fast and it is easier and cheaper to build more space at the beginning than add it later. You want to have enough room to spend some time out in the greenhouse in the middle of winter, so consider enough room for a table, chair or bench.
3. How much money do you want to spend for yearly upkeep? You may desire a 24 foot x 48 foot fully heated/cooled year round greenhouse but really only have a budget for a 12 foot x 12 foot season extending house. Depending on your location, the biggest cost will probably be your heating bill. There are several resources that will help you determine ahead of time what your potential heating costs will be. The next biggest cost is the ventilation costs. Naturaly vented designs are cheaper to opperate than fan cooled greenhouses, but generally have a higher initial cost, and can't cool as well as a correctly designed fan cooled house with evaporative cooling.

If you keep these three things in mind planning your greenhouse will be much easier. Now that you have this basis to start from, you can get to fun part of planning your greenhouse. The choices of style, framing, and glazing (covering) for a hobby greenhouse are endless. When you start adding choices about heating, ventilation, humidity, etc., etc.. it can become mind boggling.

Still have more questions try our Frequently asked Questions page for more information on building a hobby house.

Read about other hobby gardeners experices building their greenhouses on our If I could do it again page

Before you get too confused, consider finding a couple of good detailed resources. Two that I would recommend are a web site by West Virginia University. It goes over the basics and is an easy to read and navigate site Planning and Building a Greenhouse
A print resource that is excellent if you are considering buying a kit or building you own greenhouse is Greenhouses for Homeowners and Gardeners.