Polish architect Jakub Szczesny  designed the Keret House in Warsaw into a gap between two other buildings. It is 72 centimeters at its narrowest and 122 centimeters in the widest.

 Szczesny: 

That is why at first it seems that the construction of living space within such premise is impossible. Keret House is to contradict that false image, simultaneously broadening the concept of impossible architecture.

via Ignant