An Etymological Dictionary of the German Language/Annotated/Söller
Söller, neuter, ‘upper room, garret, balcony,’ from Middle High German sölre (solre), masculine, ‘flat roof, floor in the first storey,’ Old High German soleri for solări, originally sŏlâri, from Latin sôlârium, ‘flat house-top, terrace, balcony,’ whence also Old Saxon soleri, Dutch zolder, English sollar, ‘open gallery or balcony, loft, garret’ (Anglo-Saxon solor). Corresponding to Old French solier, ‘granary,’ Italian solajo, solare, ‘ceiling.’ The word was borrowed prior to the Old High German period, contemporaneously with Speicher, Keller, Mauer, and Ziegel.