Auschwitz is the German name for Oświęcim, a small Polish town about 60 km (35 miles) from Kraków in southern Poland. About 1.5 km (1 mile) from the town was an abandoned Polish army compound with 22 red brick barracks. This would form the basis for the original Auschwitz concentration camp, later called Auschwitz I, which opened in June 1940. Later, the huge additional camp Birkenau or Auschwitz II would be built 3 km (1.8 miles) away. Here's a Google Earth image. Thousands of people living in and around the camp areas were evicted.

So, to answer your question: the buildings that were converted into Auschwitz I used to belong to the (defeated) Polish army, the houses around it that were expropriated belonged to private citizens as did the ground on which Birkenau was later erected, which was part of the hamlet of Brzezinka (the Polish name for Birkenau). The Brzezinka villagers were evicted and their houses demolished.

I hope this is the answer you were looking for. If not, feel free to ask additional questions.