Greenhouses run the gamut from elegant conservatories to compact window greenhouses that fit snugly into a kitchen window frame. Whatever the size, similar suggestions for selection, design and installation apply. There are three major types of greenhouses to consider. The lean-to greenhouse is usually small, about 6 to 10 feet long. One of its long sides is formed by the side of the house to which it is attached. Relatively inexpensive to make and maintain, its major drawbacks are a lack of space for an expanding collection and a tendency to heat up and cool off more rapidly than is desirable.