titleman,
You are the smart one to have someone else build it for you. I built mine myself, and wound up hurting my back, having surgery, & spending more money on the surgery than the dock!! 3 feet is a significant variance, & during the low water period, may be sort of uncomfortable if you fish a lot year round. Mine is a hybrid dock, 1/2 fixed, w/ a floating 12 x 16 platform. I used 30 gallon blue barrels to float them. I put 12 floats under the floating part of my dock. Some advice I would give is spend the extra money if you do fixed & get 6x6 posts. You can buy dock hardware, and "certified" floats that are endorsed by the coast guard at
Boat Docks & Boat Dock Accessories Manufacturer | Merco Marine | Wellsburg, Wv - Merco Marine . they are eeeexxxxpensivvee. $100-$150 per float. You can spend a crapload of money on these things.