The Collaborative International Dictionary of English v.0.48:

Countinghouse \Count"ing*house`\ (kount"?ng-hous`), Countingroom
\Count"ing*room`\ (kount"?ng-r??m`), n. [See Count, v.]
   The house or room in which a merchant, trader, or
   manufacturer keeps his books and transacts business; the
   offices used by the accountants of a business.
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

countinghouse
    n 1: office used by the accountants of a business