I don't build them, but i've done quite a bit of research to. My parents have a small one behind their house as well.
Some considerations:
- Where are the utilities to the property located?
- How far is the nearest electric line?
- Is there access to city water/sewer? Have you considered the cost of a well & septic system?
- You will need/want the contractor to do a geotechnical report to determine the best foundation for the building.
- The less corners you have in the building, the cheaper it will be.
- Metal panels are obviously the cheapest, but there are options for different exterior finishes from stucco to brick to stone.
- If the acreage is in a community, make sure that the deed restrictions/HOA don't disbar a barndominium. I had a friend who's family bought a lot and when they designed the home & went to get bids from contractors, the price was way out of their budget as 3/4 of the exterior of the building had to be masonry. They had been planning on just doing a masonry wainscot around the perimeter.