An Etymological Dictionary of the German Language/Söller
Söller, n., ‘upper room, garret, balcony,’ from MidHG. sölre (solre), m., ‘flat roof, floor in the first storey,’ OHG. soleri for solări, orig. sŏlâri, from Lat. sôlârium, ‘flat house-top, terrace, balcony,’ whence also OSax. soleri, Du. zolder, E. sollar, ‘open gallery or balcony, loft, garret’ (AS. solor). Corresponding to OFr. solier, ‘granary,’ Ital. solajo, solare, ‘ceiling.’ The word was borrowed prior to the OHG. period, contemporaneously with Speicher, Keller, Mauer, and Ziegel.