16 Building a Concrete Foundation Four: Our recommendation is that any double glass greenhouse or any greenhouse over 12x20 should rest on a concrete footing. Even so, many customers will choose a concrete foundation for a smaller greenhouse for longevity and easy cleaning. If you are currently building a new home, adding a concrete pad or footings for a greenhouse is a great idea. It’s cheaper in the long run and hey, the builders are already there! Just a note to be sure that you have the exact outside dimensions of the greenhouse before you dig. You have a few choices when it comes to concrete foundations and oftentimes this decision should be in conjunction with your local city planning department.