The Collection comprises 15,000 drawings relating to buildings in the North East of Scotland, of which 10,235 have been catalogued as part of SAPPP. The remainder have been hand-listed. The catalogued material encompasses 215 projects begun between 1904 and 1939 that were identified from the contract books as being the most significant. Additional phases of these projects undertaken by the practice after the Second World War and prior to 1970 were also catalogued. The Collection includes 114 manuscripts, the majority of which are project specifications and contract books, as well as an album of local newspaper cuttings, compiled by G B Mitchell himself, dating from 1904-1923.
All catalogued projects are held at Aberdeen City and Aberdeenshire Archives. Some unannotated mechanical copies of catalogued material and smaller projects dating before 1939 are housed at Historic Environment Scotland.