How you prepare your site and what type of flooring and footing structure you'll end up using, are predicated on your intent and desired use for your green house. For some, their greenhouse is a place to sit, meditate, admire, and only "tinker" with their plants. For others, it's an active grow house year round with an active bee colony for pollination.

Keep in mind, depending on ventilation, greenhouses naturally trap heat and humidity. How, and to what extent, you plan to utlize those two elements will weigh heavily then on what type of flooring and footing system you should use.

Drainage
In all cases, ideally your greenhouse should be sitting on slightly higher (elevated) ground than its surroundings. You want outside rain and water to flow away from your greenhouse, not drain into it. If needed, consider installing and burying a gravel covered "french" drain (mesh covered 3 to 4 inch "perf" pipe) around the perimeter of your green house — and channel away any outside ground water. Note, in some arid climates, you may want to actually catch water run off and encourage it to penetrate the ground inside your greenhouse.

Footings and Foundations
The base structure for the walls that your greenhouse will sit on (and attach to), is called a "footing" or "foundation." It can be of poured concrete, concrete block, tile pavers, brick, or pressure treated lumber — usually 4x4s, 5x5s, or 6x6s. Heavier lumber will provide a more stable and lasting footing.

Remember, footings must conform to your desired greenhouse dimensions, must be "square" (check by acquiring equal measurments when running a tape daigonally from corner to corner), and should be level (a long contractors level is most accurate).

The earth below and around the footing should be as compacted and stable as possible (preferably on locking gravel) to avoid settling and and movement from season to season.

Concrete Pad
Full "slab on grade" (concrete pads) are very clean, solid, and a great (permanent) solution for many. Avoiding outside drainage from getting in, requires a perfectly flat pour. We also suggest that your slab not extend beyound the outer dimensions of your greenhouse — to avoid outside water from pooling and then seeping in under your greenhouse. In addition, concrete does not absorb any internal moisture, but does capture and hold heat well, so extra ventilation maybe required (to keep condensation in check).

Pavers or Brick
Although somewhat expensive, paver (tile) flooring systems may provide the best of both worlds (concrete vs. gravel) for you desired use for your greenhouse. Pavers (depending how titghtly spaced) can absorb a fair amount of internal moisture, while providing a solid and somewhat easy to clean flooring system.

Gravel
Gravel floors are a very inexpensive and a quick solution that provides excellent internal moisture absorbtion while keeping your feet relatively dirt free.

Earth
For some... portions of their greenhouse floor will be active growing soil for plants. Usually in this case, portions of the floor will include gravel or tile pavers for key walking paths.