My advice would be to buy as large of one as you can afford. You really want space to move around in during the bad weather. I bought a 20 by 30 high tunnel kit. High tunnels and hoop houses are basically low cost simpler version of fancy glass greenhouses but they are made of hoops of metal instead of wooden or metal frames and they have plastic sheeting instead of glass or polycarbonate panels. My kit cost around $5000. The plastic sheeting does need to be replaced ever so often, some say every 3 years and others say every 5. Mine is now three and doesn't look like it needs replacing yet. The kits are hard to put together, you need big equipment and lots of tools and strong arms. There is an insane amount of screws that you have to drill into the ceiling rafters - pointing up! pushing hard! and you can't pre-drill the holes. It took me months to get it done. Months!

It was well worth it. I love having it. I can play in it when the weather is harsh or wet or gloomy. I grow a lot of tropical fruit trees in it so it is like my own personal rainforest.

The good thing about hoop houses or high tunnels (they are the same thing, different names in different catalogs) is that it is easy to roll up the walls to ventilate them when the sun is shining and they heat up. The bad thing about them is that they don't always seal up tight during really bad windy cold weather. I can show you how I get mine sealed tight but it is the biggest problem with plastic sheeting structures no matter who builds them. Plastic stretches in the heat and shrinks in the cold.

For years I heated mine with a wood burning stove. I still have it as a back up heat source. I don't recommend this - you won't get much sleep on super cold nights going out and tossing a new log on the fire every hour or so all night long. I now use an electric poultry barn heater and it does fine and doesn't cost that much to keep everything toasty on even the coldest nights - But there are plenty of plants you could grow in an unheated structure! Roses, camellias, citrus trees - all of them can handle a light frost at night. Even if a bad winter came along I doubt it would flat out kill any of them, they would bounce back once it warmed up.

Thanks! I decided to get a cheap small one for now, but I do like the tunnels. Thanks for the info. :)

Not a greenhouse idea, but I highly recommend African violets for winter growing in your house. Those flowers in the dead of winter are just awesome, not to mention the fuzzy leaves :)

You can grow on a windowsill, but I have found that a plant rack with a light is great for blooming. You may find the plant light helps with the SAD,too.

I am awful at keeping anything inside alive...but, those violets sure are cute. Thanks for the idea! :)