Many municipalities have ordinances available online. Whatever you build, be very careful about side and rear yard setbacks, and distance to surrounding homes. Generally accessory buildings under 200 square feet have the fewest restrictions, and can often be built on piers. Use the concrete tubes from your local building supply store, and place one every 6-8 feet to prevent sagging. The pier holes will be below frost line.
The good thing about piers is that you can have good ventilation under the floor by raising the building, helping to keep the coop dry inside. The bad thing about piers is that the raised building makes a great place for skunks and woodchucks and other critters to establish a home, so you also need to bury wire fencing that is firmly attached to the building. The precast foundation blocks are worthless without unimproved bases (like piers) as they will sink in with every frost break or wet season.
You can build on concrete slab, but that is moist and holds chill, not ideal for birds. You can build masonry walls with footings below frost line, but that's expensive. Piers and heavy floor framing (timbers across the piers, attached via bolts set into the concrete when pouring) and then the building itself built atop the timber frame, will make for a long lasting, dry building.
My dad built a coop 30" off the ground on piers of old telephone poles. It was there 20+ years before a developer leveled the site and was still sound and dry.
We built 2 14 x 24 raised platforms on 4 x 4 piers and built hoop coops atop the platform. The older one is 11 years old and sound as can be.
You can also place timber skids directly on gravel beds. This won't last as long as piers, but is fairly easily reversible if you plan to remove the shed in a few years.
This video is one of a series. I only looked at the first couple, but you can get some ideas here. As with everything else, treat internet info cautiously.
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