The kitchen layout does NOT work. Green is right that you cannot stand at your sink and load the DW. If you are going to do an angled island like that, it needs to be MUCH larger to work well and to adequately separate the cleanup and prep zones. You need at least 12" cabinet between the corner angle and the DW to be able to stand at the sink and load the DW. And that cleanup zone needs to be opposite the prep zone side of the sink if you are only going to have a single sink. It would be a better layout to put the cleanup zone on the top run next to the refrigerator, and then straighten the island out and put a prep sink on it to have actual prep space on the island. Right now, it doesn't meet the minimum guidelines for 36" adjacent to a water source for prep. And the clearance between the wings and the walls is tight. This is a one person kitchen. If you envision any one helping in here, then it needs to be completely redesigned. I'd personally swap it with the dining room so that it could be lighter. I dislike a dark interior kitchen, whereas a dining room can be a dark interior room because it's mainly utilized in the evening when there is no natural light anyway. The addition of a closet to the office may make it a technical bedroom, but I'd want more room to be able to realistically have a bed in it. And get rid of the angled entrance. It's only hurting the furniture placement in the room. I'm not a fan of the bathroom that ate the house syndrome, but I'd at least want a double vanity in the master. And I'd want the other upstairs bath to be accessible from the hall rather than just the bedroom. If the office has to count as a bedroom, then it HAS to have access to a bathroom. The two baths should also be back to back, as that simplifies the plumbing, and it allows the bedroom to have the choice corner location with windows in two directions. Most locations do not allow a direct stair from a garage to a basement living area. The stair would need to be in a mudroom type area with a fire door between it and the garage. I would redesign the surplus garage space to create a mudroom and pantry space from it and to contain the stairs to the garage.

Gazebos are typically free standing. You might try using that as a base and adding screening. Also, do you have Menards in your area? They have a lot of building kits for all sorts of structures.

I'm guessing that the additional 15x10 area may not be covered, so it will add a fair amount to the cost, but not enough to really make a difference in the whole scheme of things. The designer might have been thinking that someone would want an open area without a roof right next to the other area for grilling or something. Living in Georgia, I would screen it, and either have a patio for a grill, or put a built in grill in the screened area with a vent hood made for an outdoor grill.

A lot depends on the look you want. We're also doing a screen porch and will use a composite (not sure which yet) but the type that is interlocking and looks like T&G after install. You can't stain a PT deck immediately so it will have to weather as is for a while - how long I believe depends on how much weather/sun exposure it will get but I believe a year is recommended. Will you be screening underneath the PT flooring to keep bugs out? How will it be fastened? Will they do a blind system so you don't have to see nails in the boards?