I just did this shed kit.
For "foundation", Simply use (2) ~14' 4x6"
Position like a skid. Typically no permit as, it's not "permanently" attached. I staked Skid Runners with 2' sections of Rebar pounded into ground at 4 corners.
Put your ~10' 2x6's floor joists cross-ways. I put mine on 1' centers just cuz..
Deck these joists with 5/8" sheeting. Did a 1 gallon garage floor paint to seal floor.
Costco has pre-cut lumber kits. My 10x14 was ~$2200
I used my own double pane windows, and insulated walls with 2" foam sheet, then finished interior with T-1-11.
My Kit :
pretty big really, twin doors allow riding mower storage and such.
Storage and a small loft.
I'll post my actual build pics soon. Proud of my custom mods.
https://www.costco.com/Crestwood-14'-x-8'-Wood-Storage-Shed.product.100281881.html