The option I've been playing with is doing a 20 x 30 (or 20 x 34) with the equivalent of about 12' porches (though they'd be part of the house's living space) along both 30' + lengths. I'm thinking I'd do the 20 x 30 section with a 6/12 roof (I'm talking about the 1 1/2 story, fyi) and then switching to what I think would need to be a 4/12 pitch on the porch-like sections. Tieing in the roof is really easy if done this way, but you would have to be sure to support the 20 x 30 section's side walls well just as if they really were exterior walls (with windows, doors, etc) to support the 20 x 30's roof load. That means that aside from some well-planned and supported open sections (or places with posts for support) those side sections would need to be restricted to 12' lengths with simple doorway access. What I've done with my housplan to make this work, since I too love open space (we built a 20 x 30 1 story addition on to our house and left it completely open as living/dining area --fabulous), is put the master bedroom, bath, Utility room, a mud room, guest bath, kitchen or dining depending on plan, pantry, coats closet, etc on those side sections where 12' isn't a huge dilemma. Then I've got the entirety of the 20 x30 (minus stairway to top floor) for an open floor plan to use as living and dining or kitchen, depending on my priorities and plan.
Just a thought I've been playing with. Then the extra foundation is just "extra" and not changed completely, the spans and beams of the 20 x 30 stay the same, etc, etc. You might need to beef up the exterior walls of the 20 x 30 on the long lengths to support the top of the shed-style roofs over the 12' sections, but someone else would have to speak to the engineering part of that. We were planning to do the doubled walls for doubled insulation anyway, so figured we'd just support the second 1/2 story of the 20 x 30 with the interior 2x4's so we'd have double walls at that joint anyway. I think the plan is 2x6 exterior all around...we want to overbuild wherever possible, including plywood sheating on the exterior and on the interior instead of drywall --no codes to deal with here either. I HATE drywall! We're also in a drastic area as far as weather conditions are concerned, so stout is good.
I hope I explained that so you can understand what I'm saying...With a plan like the above, you can get a 32 x 30 with just one additional section, or a 44 x 30 with both sides.