The Collaborative International Dictionary of English v.0.48:

Summerhouse \Sum"mer*house`\, n.; pl. Summerhouses.
   A rustic house or apartment in a garden or park, to be used
   as a pleasure resort in summer. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

summerhouse
    n 1: a small roofed building affording shade and rest [syn:
         gazebo, summerhouse]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "summerhouse":
   alcove, arbor, belvedere, bower, cloche, cold frame, conservatory,
   coolhouse, forcing bed, forcing house, forcing pit, gazebo,
   glasshouse, greenhouse, hotbed, hothouse, kiosk, lathhouse,
   nursery, orangery, pagoda, pergola, pinery, retreat, seedbed