Most people will tell you an attached greenhouse is cheaper to build than a freestanding one. That is true only if a building permit isn’t required. In many places, local building codes consider an attached greenhouse an addition, like an extra bedroom, and mandate the same foundation type as the house (usually a concrete wall and footer) to prevent the greenhouse from shifting and damaging the home. Normally, the project requires stamped engineered construction documents, and a more robust foundation. These can easily add a few thousand dollars to the cost of the project. Even if a permit is not required, attached structures require more careful building, which often means more expense, not less.