St Helen’s House and the adjoining building known as 'B' Block, also as the Pearson Building, in King Street had been used by Ordnance Survey staff during World War II. In 1945 this organisation vacated the buildings including Big School (the large school assembly hall) and allowed Derby School to return to its home after five years at the beginning of September at the commencement of the Autumn term of 1945. Derby School began its final period here of just over two decades before moving finally on to the Littleover site in 1966.