In our climate (zone 4B), a greenhouse really helps to extend the growing season.
We decided to add a greenhouse on the south side of an existing shed on our new property. It's right by our driveway, has good solar exposure, and the dark brown wall of the shed soaks up the sunlight and radiates heat into the greenhouse space.
We hired a wonderful neighbor (Paul, shown in the photo above) to build the greenhouse out of mostly reclaimed materials. The windows came from another neighbor via our Buy Nothing group (documented in another article).
We added a raised bed along the southern wall, made of redwood and filled with composted horse manure from Paul's horses.
The interior of our greenhouse gets toasty warm on sunny days, and the back wall radiates enough heat overnight to keep our herb garden (in pots along the wall) from freezing months into the winter. We are growing lettuce in the greenhouse in January, when it is near zero degrees F overnight, with no heat other than the passive solar gain from the windows.