Most very serious propagators will probably own a greenhouse. They provide an excellent propagating environment, but it can be difficult and expensive to control the environment in such a large space and they are usually more important in controlling the environment for the propagator!
They are excellent for providing even, overall light levels to ensure good, strong growth.
Some form of heating is needed to keep them warm enough for successful propagation from autumn to spring, which can be expensive once the temperature is raised to 5-13C (41-55F) – temperatures needed for most propagation techniques.
To reduce heating costs, you can partition off areas of the greenhouse with temporary doors or screens of bubble glazing, use heated propagators or create propagating beds using soil-warning cables.
Where space is limited, a lean-to greenhouse is a good alternative.