Have you considered making your deck into a permanent 'outdoor' room? Something that will vastly increase the value of your home as well as saleability, be able to grow vegetables or exotic plants in beautiful pots and raised planters year 'round, add a few pieces of comfortable furniture so you can entertain or go hide from the world in an elegant, glass outdoor room that could be showcased in 'Sunset Magazine'?
I don't think you are going to want to do anything temporary. Putting a greenhouse up and taking it down (8'x20') every year will be a big pain. Wood and water don't mix. As a Landscape Architect/Project Manager for 30 years in the Pacific North Wet, grin, I always talked my clients OUT of wood decks attached to their home. If they HAD to have a deck, we made it using 'Treks'...I think that is how it was spelled...which is recycled plastic bottles made into dimensional lumber. A bit more costly to purchase and you have to pre-drill for all your screws...but! No Maintenance. Lower home insurance costs as wood gets slippery, dangerous. It comes in a lot of colors but PLEASE, please stay with dove gray! Any other color will look like you have a moldy or dirty deck. Dove gray is the best color to use for hardscape in Northwest gardens. Same with pavers, concrete, CMU (concrete modular units) for garden walls, retaining walls...even wood arbors, screens, fences, walkway edging...gravel is wonderful especially in the PNW!!
You want the color in your landscape to be the colors in the forest, not Disney Land, grin!
How high off the ground IS your deck? Is it the same elevation as the main floor or do you have to step down from your home onto the deck? If you are only a few feet off the ground and/or the deck surface is not the same elevation as your home...I'd talk you into considering building an outdoor room, with garden walls, planters built right into the walls that are for sitting, potted plants and areas for raised beds big enough for trees or vegetable gardens. Glass french doors opening from the home that can STAY open allowing the heat from your outdoor room, green house to heat your HOME. Lower your heating bills drastically. The flooring should be 7X9X2 inch dove gray 'Roman Cobble Pavers' or similar. No mortar allowed. Pavers are framed with dimensional lumber, PT The walls would be 18" in height from the pavers to the top of the wall (which would include a 2" thick concrete cap stone, about 2' in width). The walls would be either REAL stone (in gray) and pretty expensive and not so DIY friendly...or CMU's. Concrete lego blocks, big grin! Every year this product gets better and more natural looking. I used an 'ashlar' pattern with 3 or 4 different sizes of blocks. It takes a bit more IQ but worth it! You should get bids from Landscape Design Build Companies to see what they'd charge. Let me put it this way. It isn't something for DIY...this is your HOME, your investment. Unless you've done this kind of thing before, now is NOT the time to learn. Landscape companies are COMPETITIVE. You'll get more for every buck you spend with a good landscape company rather than using an Architect, Engineer, Electrician. A good landscape company DOES IT ALL. Yes, even low voltage lighting (essential in the PNW...). I could recommend a few depending on where you live. Not sure if that is 'kosher' on this site but I could at least tell you what to look for in a landscape company.
An outdoor room that provides passive solar will vastly increase the value of your home. Done in concrete, pavers, 6X6 posts, proper glazing, french doors or the big, big bi-fold doors that fold out of the way allowing the outdoor room to come inside. It should be designed so that the glazing is oriented to the sun, allows light and heat in, insulation so you don't lose heat, ventilation to release heat when necessary, a wood stove or open fireplace for sure...this would be more for human ambiance and comfort. A room-sized cadet heater for sure.
I think you need to think bigger. Break it all into manageable chunks so you can afford it. A wood deck in the PNW does nothing for your home. It has been shown to be a liability and actually reduces the value of your home. Concrete and stone is the way to go in my experience with lots of glass and BIG, THICK POSTS. No 4X4's. (4X4's are in proportion maybe for a kid's play house but I won't use them if people can see them in connection with the home or landscape). I'll try to send a few pictures if you are interested.
Oh I just realized you just might be renting and that is why you want a portable 'greenhouse'...smack my forehead and grin. Goodnight!