How big? What are your available resources? What are your ground conditions (clay, you said). Rocky clays, or straight clay???
and you are in North Carolina?
I can't answer your questions, but I can run you through the thought process in building my barn, which has a concrete stem wall as its footer, and recycled concrete base for 2/3 of its floor, concrete for 1/3. My hen house is attached to the back, on 4x4 piers set in concrete,, using 2x6 joists 16" on center under a hardieboard floor.
I have a mix of sandy loam, sandy clay, and clay soils, and average 60"+ of rainfall yearly. I was also building onto a hill.
So, considerations. Concrete around here runs about $135/ cu yd. That's cheap - a 10'x10' section, 3" deep (typical floor depth in a 1 story house) is slightly less than a cubic yd of concrete. (Technically, it will cover 106 sq ft). We'll say 96 sq feet instead, the size of three 4x8 sheets of plywood. That's size for a flock of 24 birds.
You can't (prices just spiked) buy good exterior grade 1/2 plywood or OSB, joist hangers, pressure treated 2x6 framing, and exterior grade nails/screws for that price. The concrete floor is also probably faster to assemble.
BUT - without reinforcement, the concrete will crack. You need either rebar or wire grid reinforcement to help it in tension, as concrete is only strong in compression. That adds cost. You also need to level the whole area you are pouring, and frame it and stake it before pouring your concrete, that's cost and (depending on ground) a lot of time to level. Or you build formworks and use concrete to bring the whole thing to level (as I did). Setting a light timber framing on posts/piers on concrete blocks requires much less time preparing the site.
ALSO, clay soils expand and contract dramatically as they wet/dry, much worse than sandy soils. Its even worse in freeze zones. While you could pour a monolithic (all one piece) slab on grade as has been popular in FL for a century, your slab will be subject to uplift stress from soil movement, which will destroy it in time. To get around that, you need a footer, which has to be below the frost line. In NC, that's 12" - so now you are talking about digging a 15" deep trench, likely 1' wide, around the whole perimeter of the structure, bending and tying steel rebar reinforcement in it, setting it on chairs. More labor, more costs, more concrete. You now need another 1 1/2 cu yd of concrete. Suddenly, costs don't look so great.
and then you call the Concrete Ready-Mix company, and they tell you that there is a "short truck" surcharge for anything under 6 cu yd, that their trucks normally carry either 10 or 12 cu yds of concrete for a job. Your price went up again.
(Or you can rent a mixer, $65-90 a day, and transport bag after bag after 80# bag of concrete, mix, and pour on site. 45 bags of 80# concrete are a cubic yd, and run $5/bag or $225/cu yd - I'll let you work out where one becomes more cost effective than the other)
Because my barn is roughly 17x40, I poured an entire truck of concrete, 10 cu yd, into footers 1' deep, 1' wide, and raised 8" above the highest point of ground contact (FL Hurricane code). I then brought in 17 tons of recycled concrete base (which only cost me about $400) to fill the inside of that concrete pour up to 4" from the top of my concrete to level the ground inside, rented a plate compactor to tamp it all down (1 day, $85), then poured another 6 yd of concrete in the "back room" to create a concrete floor in my shop.
It was cheaper than levelling the ground, digging much deeper footers, setting 4x4s for framing, then concreting in place as a pole barn (and much more hurricane proof) due to scale alone. Having a continuous concrete perimeter also allowed me greater choices in wall framing, and roof framing, which helped with overall build costs. When I built my 80 sq ft upper story hen house (my ducks sleep below), it was FAR cheaper to use light structure wood framing then concrete, because I didn't enjoy those economies of scale, the amount of ground to level was much smaller, etc. Same for my 8x10 shed.
Hope that helps you do the numbers for your situation.
