This is where all of the magic happens in my yard. Every plant in this picture was started from seed by me, with the exceptions of the Salvia 'Blue Hill' and the daylily.
Kit from Costo.com 10x15 and all glass. Automatic vents. I highly recommend it. It came from a company based in Austin, TX. My husband put it together---it wasn't easy, but he managed it pretty well.
Yes, I overwinter some things---although space quickly becomes an issue with large established plants. I've had it 3 years now. I start over 1000 plants in it every year---they all get transplanted and well established in 4 inch containers before going into the ground. Lots and lots of annual flowers. It's hard to direct sow outdoors here in the spring, bc of all of the slugs---they eat most of the seedlings. So I get things growing fairly large before they move outside.
It's an electric heater with a thermostat and a fan. 10x15 is too small. But I'm pretty efficient with the space. I only use the heater in Jan, Feb, March.