There are actually 3 types of 'extending the season' type structures to consider. You must already have the out of doors garden with raised or fluffed up 'permanent' beds with walkways that contain your perennials such as rhubarb, blueberries, tarragon, thyme, oregano...raspberries, horse radish, asparagus...? Fenced or protected against rabbits, raccoons?
The second is a completely enclosed structure that can easily be 8'X10' or within your building codes that you don't have to permit. 12X10 is our limit. This is insulated well, owns a heater (I use a little radiator oil heater that works even in the subzero weather), has a floor that allows drainage, air input and output, fans, easily accessible water source and a compost bin outside. Storage for chemicals. Shelving that is adjustable and great lighting! Amazing what you can produce in that amount of space. This works all year round no matter the season.
This could also be converted into vertical hydroponics. I've not gone that route. For me dealing with soil is enough. I get tempted sometimes noting the amount produced hydroponically...then I just go on with what I know and works.
The third is a green house with heat and the best skin for covering. You live in zone 8a? Gees, you'd hardly need to worry about season extension. I am working just to have what you already have!
For a real greenhouse I chose to use the soil beneath the roof. Not pots. Oh I use pots with potting soil as well... I make raised beds and improve the soil with decomposed organic matter. My raised beds have no lumber or bricks. I do this will all types of soils I inherit. Clay, caliche clay, lovely loam and now volcanic pumice. I am now seeding my 'fallow' beds with a green cover crop...Crimson Clover this year. If not seeding I am coating the beds with decomposed organic matter out of bales I get from the store about 2". I work in zone 1b! Alaska has the same zone, I am in Oregon at 4000+ elevation, central state.
We finally got a great skin that is anti condensation, actually clear and has a warranty of 4 years (make sure you KNOW what side is up and what side is down...this company has been put on notice for this particular problem). We have a pellet stove and a diesel construction heater. After a freezing night below 30 degrees F the greenhouse is 56 degrees about 5am. I am happy.
The perennials are blueberry, raspberry, asparagus, thyme, oregano, rhubarb, strawberries, boysenberries...I lost my kiwi plants in earlier cheaper temporary poly tunnels. This new skin all by itself is amazing for holding the heat within. I have also used floating row cover before the heaters and expensive skin which works so very well, I will always have that stuff around! For insects as well.
Putting a 'shed' greenhouse on part of your home will help you reduce electric bills in a big way, makes a super room to 'hang'...easy to maintain contact with your garden as well. Moisturizes the air in your home.
Remember, the light in the winter is not sufficient to grow anything. You will have to add artificial lighting...great to have anyway for starting starts or extending the season on tomatoes/peppers/eggplant...that is what I've been doing...potatoes are done (I am also able to leave the potatoes in the ground longer with heat if they freeze they turn to mush) (heck in zone 5 I left potatoes in the ground where I was able to run out and dive in with bare hands to pull out potatoes for dinner, no mush, no added heat, no greenhouse).
Air flow is critical. You have to have major fans blowing major air enough to blow your hair around to ensure proper ventilation. Put that in your plans for sure.
I hate to say this but in zone 8 I would think floating row cover would be enough to extend the season. An enclosed shed with lights would be perfect to give you starts and even do tomatoes during the winter.
Hope this gives you some ideas or more questions, Ben. What is your water source? What do you do for fertilizer? What are your favorite plants? I am thinking that you might love a place to lounge and watch your plants grow, get your skin moisturized?
