Things to remember,
1. Construction is custom. Builder "A" may take 2000 man hours to complete and have a median rate of $90/hr across trades. Whereas, Builder B, may have a lower overhead cost and be able do it faster.
2. Site development, is custom, grading, soil prep...the list goes on. This can greatly affect cost greatly.
3. Design influences cost. If you have a simple design with all utilities on one side of the ADU, this will be cheaper than having to run drains, electrical, gas, to both sides. This is just one example. Hire an Architect with the end in mind, who understands the construction process, and how design decisions affect construction cost.
4. The best approach is to use a bottom-up analysis rather than a top-down of estimation of $250-$350 ft. Scope out each phase of development, after plans have been approved. Try to collect estimates from each trade, foundation, framing, plumbing, electrical, hvac, insulation, dryall, painting, flooring, and other finishes. Doing this will help you understand the true cost of construction. You may want to have your contractor deliver you a shell, and sub out the finishes, that gives you more control over the selection of the finishes and overall cost.
Obviously, if you are just building a unit or two, this is probably a big waste of time. However, more sq footage and units, this approach is definitely worth reviewing.