That's a good point. Buy an old RV for a year, place in it onsite, and live in it as necessary until the structure is inhabitable, and then sell the RV, hopefully at not too big a loss. It can be a towed RV even, whatever those are called.
And then sometimes the house is also sheet-metal-clad and looks pretty barn-like—it's just better sealed and insulated and has a few more windows :-)
Also, I suspect that if you're building a smaller cabin-type house in the woods, you don't also want a barn. Maaaybe a shed.
My dream house would have some interior unfinished space that opens to the outside for use as a shed/workshop. Basically think of a large unfinished mudroom with storage for tools, a lawnmower, shelving, a workbench, and maybe some larger stationary power tools like a band saw and drill press.