detached garage rental

It would be original cost of the garage plus costs to turn it into a rental.

Original cost should be in the contract to build.

Your client should have documentation of the costs incurred; hopefully detailed as to house vs garage.

If not, maybe the contractor can provide the breakdown.

If not,  your client may need to approximate the breakdown using square footage amounts to 'build a garage' at the time of construction.   Be wary of "guessing too high" syndrome.  

Then, as Susan says, ADD the costs of conversion.

Maybe it was built in 1765 and originally called a carriage house.  First you start out by guessing how much of the original purchase price should be attributed to the land.  Then you can guess on how to divide the remaining cost between the two buildings.  Cost per square foot is good enough for government work. 

A lot of work for small deduction.  Take the direct expenses of conversion and depreciate them as a commercial property.  I see it often an take a conservative approach.