My current plan (number 12,547) pretty closely resembles the 20X30 1-1/2 story, and I may in fact use that plan as my basis. I plan a wing off of each side, as I want my main rooms to have windows on 3 sides for light and ventilation. The main house would have a big kitchen across the back, and also encompass the stairs, and various bathrooms and closets. One12 X 16 wing would be the living room; one would be the master bedroom. Rather than having cross gables, which would be a lot harder to frame and roof, I think I'll just go with a shed roof. The main gable would be 12/12 pitch; the wings would be 4/12. My question (you knew I'd get there sooner or later) is about the transition from the shallow pitch to the steep one. I plan on using metal roofing, and wonder how this is done. I know you would over lap the metal, but what about wind driving water up under it? If one sheet overlaps the other, you would have to bend one or both sheets of metal- How do you bend the ribbed material? Does it just flatten out? Is my thinking correct that the shed roofs would be a lot easier than cross gables, or am I just opening a different can of worms? I'll be near 60 by the time this house is done, and I won't want to ever go up on a roof again!

The roofing supplier I used sells transition pieces, each 10 feet long. They are formed from flat metal in colors to match the roofing. They come in a shallow Vee shape. The upper arm slides up under the metal roofing on the upper slope. The lower arm lays on top of the lower roofing panels and are screwed down as necessary. Foam rubber self adhesive 'seals" are placed on the underside of the lower panel just as is done at a ridge where there is no ridge vent.  It's sort of like doing a valley.
Was that understandable?
There may be other ways.  

That makes good sense. Do you think that would be easier than doing cross gables? I know the cross gable looks fancier, but our place will be out in the woods, so not many folks will see it, and I'm trying to economize, both in time and money.

It would be easier in my opinion. Probably not take as long, especially as the roof metal will have less cutting and waste pieces cut off. There's some cost savings, but I'm not sure if that is a significant figure when compared to the total cost. However, I understand how it all adds up.

When I was saying 'economize, I was also thinking in terms of the total structure- framing, sheathing, and roofing materials. Plus the low pitch shed roofs will give me something to stand on when working on the 12/12 pitch. On one side of the house, the shed roof will run full length, so there will be a porch on each end of the living room. Porches are important in rural SC!

Porches sound cool, in more ways than one.   

The twin sheds will likely be easier and less expensive to build. This is a time honored way to handle a projection on a simple gable house. That said, a cross gable, on one of the additions can give a "classic look" and have options for a more open ceiling treatment. It might be an upgrade option for the main living room.

A simple model of cardboard would help visualize this and determine if it is worth the extra effort and $$$'s.

Oddly enough, I'm not a big fan of cathedral ceilings. The house will be finished as an old-fashioned cottage inside and out. Ceilings will be 10', however. I plan a small second story, really only enough to access a second story open balcony, so I can sit up there and look at the mountains behind us. Due to the slope of our property, that deck will be about 20' off the ground. By bringing the sidewalls of the main house up to 14', I can then get my 4/12 pitch on the sheds and still keep my 10' ceilings. The upstairs will be big enough for a bedroom and bathroom if desired, but I'll be more inclined to make living space in the walk-out basement, where I can take advantage of the semi-underground temperature stability. A friend of mine in the same neighborhood and terrain measures 55 degrees in the winter, and 75 in the summer in his basement. I can live comfortable at those temps. What I haven't decided yet is whether to put my wings on a full basement (probably not), a crawl space, or on piers. Neither one will have any plumbing, so I have options. The main house will be 40' long, so a 20X40 basement is plenty.

What about starting your sheds off of the walls instead of the roof??Just a idea

I thought about that. I'd have to make the sides of the main house taller- probably 16'. It would look better- I think. At that point, though, might as well go with a full two stories, which would look nice, but it would be more expense in a direction I didn't really want to go. I don't plan to declare the second floor as living space (trying to keep my taxes down as I head into retirement) and a full second story not used as living space would be odd. I think the whole thing would then become more complicated as well. If the two roof pitches meet, the rafters for the shed roof wings can simply set on top of the main house walls, alongside the main gable rafters. Although the building inspector won't consider it, the shed roof sheathing will also brace the main walls from spreading significantly, probably enough to eliminate the need for a ridge beam, considering the area's low snow loads. As I said, I don't think the inspector will buy it, so there will be a ridge beam anyhow.