Having a space connected to the house that is dedicated to plants is very nice, but the infrastructure of the home is what makes the climate palatable (and obviously the sunlight) Regulating temperatures is a drain on time (check the weather- windows open or closed, interior doors open or closed?).
If the structure is exposed to the sun on 5 sides without insulation and opens to the exterior (where temp could be 90 or 40, opposed to opening to a home where its usually ~70)- so much can go wrong so fast especially as seasons change.. It is not going to buy you much time. That one October 87 degree daytime high, or 22 degree nightime fluke (and we always get at least one) all that work was for nothing. They basically make temperature changes exponentially greater.
I have had better luck planting things outside and using these:
https://www.mrmiddleton.com/ki...
I would NEVER pay $50- I think I bought mine at the christmas tree shop for $20 ea.
but the fact they are narrow (so roots still benefit from moisture/rain) gives you a lot of freedom...
"tender perennials" like thyme and rosemary, parsley, cilantro, and cruciferous plants like kale all flourished through winter with these things, but they like cool weather. In late February/early March, If I put them over a section of cleared ground and heaped leaf mulch around the base, I got much faster germination and better results with other cool plants like sugar snap peas and lettuces. I could go out and play or stay inside - but only because the stuff that lived under them was pretty cold hardy, and they are narrow enough that I didn't have to water.
Once it starts to get warm, you can move them over your tomatoes/cukes/basil and get a jump on them too- not more than a week- but they will fruit earlier or more abundantly if they have a pleasant childhood.
I'd buy some of these, or DIY something similar WAY before I ever considered spending money or donating garden space a real greenhouse- It will be too cold in winter, and too hot in summer- you'd basically be trading 3 weeks each spring/fall for 6 weeks in the summer. Same difference
