How do I figure the cost basis of a wired metal building that is on the premises of house that we purchased in 2012 for $140000? Assets/tools inside.

there are no "perfect" ways to determine FMV at the time of purchase.  Real estate bills that set FMV may not be reliable because a detached building that is not suitable for living quarters is often assed at a fraction of its true FMVbe valued. you can use your own judgment or hire a real estate appraiser.  either one could be challenged by the IRS but you have a better chance of winning if you use an appraiser. 

The term "office" can be used loosely. If you work from home, doing a type of construction, that will also qualify.

Your cost is what you initially paid for it plus any improvements made. 

Please see this LINK for more information.