If I was going to put my 20x30 1 storey on block piers, what is the best way? i have clay soil thats thick.
what type of footings would i need to dig for each block pier?
i want to raise the house ~ 36" above grade. the house will be on a slope with an ~3" grade.
When it comes to foundations and footings in clay soils your best information will be local.
For a 36" lift off the grade line you may be over the limit for a block pier unless you have no earthquake or heavy wind loads. Spot footings like concrete piers can move and rotate since they are not cross braced as wood piers could be.
You strongest and most permanent foundation is likely to be a poured reinforced strip footing supporting a poured or reinforced block stemwall. See page 2 of your plans.
would the same be true for solid concrete pier? I plan on installing a center row of piers also, so 15 total piers.
You could reinforce a block pier as well as a poured concrete pier (although the poured pier will be smaller). I expect there is a size of pier and footing that will make either one work in your soils and with your height off grade. I just don't feel confident that the piers shown in the plans are appropriate to the task.
That's why I think you should check with someone who can examine the soil and knows the local 100 year wind and earthquake design loads.