Source : Webster's Revised Unabridged Dictionary (1913)

Outbuild \Out*build"\, v. t. [imp. & p. p. {Outbuilt}or
   {Outbuilded}; p. pr. & vb. n. {Outbuilding}.]
   To exceed in building, or in durability of building.

Outbuilding \Out"build`ing\, n.
   A building separate from, and subordinate to, the main house;
   an outhouse.

Source : WordNet®

outbuilding
     n : a building that is subordinate to and separate from a main
         building