Building a greenhouse is not much different than building a shed or a house, as you should plan with maximum efficiency in mind. Besides the frame and the glazing, you also need to think of the grow medium as well as the heating and cooling system. The ideal greenhouse design will depend on your location, your budget, if you want to grow year round and what you want to grow.
The frame of a greenhouse can be made of any material that will support the glazing structure. However, treated wood should be avoided in most cases. The greenhouse usually has a very high moisture level and water dripping from the treated wood could make its way into the grow beds and poison the plants.
The glazing utilized on the greenhouse allows the sun to get in the greenhouse. Glazing can be as cheap as a few cents per square foot to as much as $4 to $5. The cheap glazing is usually plastic film that will only last a couple of seasons, provides little insulation, is very sensitive to wind and may break or puncture. Most expensive solutions are design to be much more solid, with an effective life span of 15-plus years and significant insulation.
Inside the greenhouse, you can use a variety of grow mediums. The easiest one is simply to use the soil under the greenhouse. You can also use grow beds if your soil is not good for planting. The grow beds can be made to rest on the ground or be raised for easier access. You can also use hydroponics where you grow food in water and use that to transport the nutrient instead of soil.
Heating a greenhouse can be done using solar or conventional heat. Conventional heat is easy to setup and propane gas and wood heating can also bring good carbon dioxide for the plant to use. If you decide to setup gas, propane or wood heating, remember to setup a carbon monoxide alarm inside the greenhouse that can be seen or heard from outside to prevent you from entering a carbon monoxide filled room. You can use the sun to passively heat the greenhouse. Setting up large drum of water painted in black will also allow you to capture and store that energy to be release at night, cutting down your heating cost.
For at home greenhouses, using a conventional HVAC system would be very expensive. Using vents on the top and bottom of the greenhouse is usually enough to create ventilation that keep the plants from dying. However, if you have a very hot climate you can, in addition to the vents use shades that let in only a portion of the of the light and heat inside. In a dry climate, you can also use a swamp cooler to cool the greenhouse and provide moisture to the plants. A swamp cooler is easy to build yourself and cost little to operate.