We have planned to add a greenhouse to our gardens for over a year now and having looked at many designs we saw the plans for the Anna White greenhouse on google were free. This gambrel style grenhouse is 10ft x 12ft but we did many adjustments since we've an architect in house. We added stainless stell edging on both ends, reinforced many joints with steel angles, we installed plumbing and electrical outlets, lights, and included a very needed 16 inch ventilation van on the north end of the greenhouse, placed up high with a thermometer switch. On the opposite end we installed a storm door that have a large screen which can be opened and closed which allows for great cross air-flow from the floor level up to the vent fan near the ceiling.
We also built garden benches, which you will see in the video with screens to again allow for both air-flow and drainage of water. The floor is 3/8 minus crushed granite, about 1 yard was sufficient. The overall cost for everything was close to $2000, but if you decided not to add plumbing or electrical it could be a lot less. We also stained all the surfaces, glued joints, used plywood gussets on rafters and six 2 foot pieces of 3/8 inch rebar were hammered through the treated base plates for added stability, and strength against high winds.
Our foundation was again the treated wood all along the base plates, placed on concrete pavers in 12 places, on a bed of 3/8 crushed rock which we had laid landscape cloth down and leveled the 10'x12' area as best as possible.
Here's a brief video.