I'm in the process of building a 12' x 24' greenhouse (cold frame) featuring polycarbonate walls, two solar powered fans (intake and exhaust) and sliding glass doors. Below are snapshots of my progress.

Site selection / batter board construction

After selecting a site for the greenhouse, I constructed batter boards. The string lines define the greenhouse's perimeter and a level reference for foundation work.

Digging the trench

Because I'm building on a hill, I had to dig a trench to obtain a level surface for the foundation. The trench walls will serve as forms for the cement footing.

Rebar construction

I created a simple rebar structure to reinforce the 5 inch deep concrete footing. Vertical rebar was placed in the corners to anchor the concrete blocks to the footing. Having vertical rebar anchors along perimeter of footing wasn't necessary as this greenhouse will be a lightweight structure. The shallowest end of the trench wasn't deep enough to hold 5 inches of concrete so a form was built to accomodate it.

Footing

Here are pictures of the completed footing. The next step is building the concrete block walls.

Block foundation

Here are pictures of the completed concrete block foundation.

Greenhouse frame

Here are pictures of the greenhouse frame.