if i change my 20x30 single story plans to a 21x30 (concrete footers are a little off) can i still use 2x12 for floor joist?
(https://i999.photobucket.com/albums/af112/astidham80/concrete.jpg)
sitting on top of these footings will be 5 12" sonotubes to hold my simpson 6x6 post bracket out of the dirt.
i will run pt 6x6 post to level out the foundation beams.
if i add a foot in width the house will overhang the beams in places ~12
The 20' wide single story plan has a width of 20' to the outside of the beams. If you use the built-up beams that are 5 1/2" wide, the span of the floor joists is a bit over 19'. If you don't want to use engineered joists (I-joists) then you should be using Doug-Fir Select Structural 2x12s at 16" o/c as called out on the foundation plan. The grade and species is important. If you use Spruce Pine Fir #2 (a common grade) it will only span 17'-10". Note: if you did use the lower grade material it would not collapse on you, but it may have unacceptable bounce (deflection).
To go wider, put the joists on top of the beams and have them extend no more than the depth of the joist (11"+) over the edge of the beams. This will not count as a structural cantilever. The span of the joists is still the inside dimension between the beams. Doing this should allow you to go 21' wide for the single story plan and still use the Select structural 2x12s.
Thank you john