We're building on a 24-foot equipment trailer designed by Tiny House Builders and fabricated for us in Florida. We used uShip to arrange delivery—much cheaper than driving down in the truck to pick it up ourselves. The design we've come up with runs the full 24-foot length and extends over the sides of the trailer about two feet to keep the house from feeling too narrow inside. If we planned on moving the house a lot (or at all), we would have stuck with the 8'6" width of the trailer. At this size, if we want to move the house, we'll need to get a permit.
Yesterday, Day One of the build, we attached plywood sheathing (waterproofed on the bottom) and floor joists with 4-inch lag screws running through holes Bill drilled in the metal frame of the trailer. We ran out of steam the end of the day, so we'll install more floor joists today. The joists are high enough that we can build the floor on top of the trailer's wheel wells. This means the house sits high off the ground, but that works in the site we'll be placing it: the front door will open to a higher terrace level.