Decks, Patios, Porches and Docks - Mudroom Foundation

I am planning a mudroom extension on the rear of my house and have a question about the foundation. The size will be 8' x 12', with the short side attached to the house. Rather than a standard poured foundation, I plan to use post & pier using sonotubes. There will be 6 sonotubes in 2 lines of three, 8 ft apart. On top of the sonotubes will be simpson brackets that will accept two 12 ft 4x6 PT beams. On top these two beams, I will build the floor, using 2x6s or 2x8s joists running perpendicular to the beams, anchoring it to the PT beams with some type of hardware. The long side of the room will be atached to a 16x16 deck that is also built using sonotubes.

Does this sound like enough piers, or will I need some between the two rows (along the 8 ft joist span)? Also, are 2x6's ok for the floor joists (height may be an issue), or should I use 2x8s? Thanks for your input.

Although you didn't mention it, I am sure you have plans for rim joists and a ledger joist against your existing deck. Some things are given, but I had to mention it. Simpson makes beam attachment hardware for the 4x6 beam. Will you be using posts, or directly resting the beams on the piers? Your spacing seems to be fine on the piers, and 2x6 will span the 8' without problem. If you place the 2x6's on 16 centers you can use 5/4x6 decking which has a radiused edge, but on 24" centers will require 2x lumber. You also didn't mention a roofing system. I guess you already have that figured out.
Post back if we can help further.

Thanks for the info. Yes, i plan to build the floor on top of the beams, attaching the rim joist to the deck and the other side to the house. I will be using 3/4 in TG plywood for the floor, and putting vinyl flooring on top of that. You say 2x6s are ok for the joists? I am planning to put the beams directly on the piers. i build a shed in a similar manner, sinking j-bolts into the pier, and attaching a simpson bracket that raises the beam about 1 inch off the surface of the pier. The walls will be 8 ft using 2x4s, insulated, and t-11 (?) plywood on the exterior, drywall interior. The roof will be typical gable roof 12/6 pitch with asphalt shingles. Can you give me any information about vapor barriers for the walls and floor? And how about insulation for the floor? The shed I built has no drywall or insulation.

Thanks for your help!

Not sure of the clearance you will have under this mudroom, but I would apply a good 4 mil plastic on top of the ground. I have also installed chicken wire under the joisting and installed the R19 (R25 would be better if you can get it to fit, but 2x6 may limit you) insulation from above. That shows you how much I hate insulation. Make sure the kraft side faces the living area. The kraft paper in itself is a vapor barrier, and unless your local codes call for it, placing another plastic vapor barrier over the insulation in the walls would not be necessary.
I see that you are going to use 3/4 t&g for flooring. Do you plan on 16oc for the joisting? It would give better support with less deflection.
Sounds like the rest of the project is in #1 take off position.

Yes I'll be doing 16" o.c. Thanks for the chicken wire idea!