Himmler visited Auschwitz I in March of 1941, and upon closer inspection of the camp, he ordered Auschwitz expansion. According to Peter Hayer, the Polish underground notified the Polish government-in-exile on the 10th of January 1941, that the Auschwitz concentration camp could accommodate roughly 7,000 prisoners and plans were made for the expansion to hold around 30,000 prisoners. Construction of Auschwitz II-Birkenau, called a Kriegsgefangenenlager (prisoner-of-war camp) on blueprints, thus the construction began in October 1941 in Brzezinka, about three kilometers from Auschwitz I.
Initially, the plan was for Auschwitz to consist of four sectors (Bauabschnitte I–IV), each of the sectors would have six subcamps (BIIa–BIIf) separated with their fences and gates. The first two sectors were completed, where sector BI initially served as a quarantine camp. Construction of BIII began in 1943 and the construction was halted in April 1944, the plans for BIV were abandoned.
Architect, SS-Sturmbannführer Karl Bischoff, was put in charge of the construction. His initial budget for the construction of Auschwitz was 8,9 million Reichsmark (RM). Bischoff first planned for each barrack to hold up to 550 prisoners, which he later changed to 744 prisoners per barrack. This should have resulted in the overall capacity of the camp to jump from 97,000 to 125,000 prisoners. There were 174 barracks planned, each measuring 35,4 by 11,0 meters, and were divided into 62 bays of 4 square meters each. These bays were then divided into ‘’roosts’’, which were intended for three prisoners at first and later changed to 4 prisoners. Personal space inside the roosts was 1 square meter and was used for sleep and to place whatever belongings the prisoners had. Robert-Jan van Pelt wrote that the prisoners were deprived of the minimum space needed to exist.
Construction of Auschwitz Birkenau concentration camp was done by prisoners themselves, as they were forced to live in the barracks which they were building. In addition to working on building the barracks, they were also faced with long night roll calls. This resulted in most prisoners of BIb (the men’s camp) dying of hypothermia, starvation, or exhaustion in the early months of construction. Of around 10,000 Soviet prisoners of war who arrived at Auschwitz I between 7 and 25 of October 1941, only 945 were still registered on 1 of March 1942. Those prisoners were later transferred to Auschwitz II, where most of them died by May 1942.