Septic tanks are arranged under indoor stairs.

no! Septic tanks are usually built on one side of the house! Even the buildings in the community are the same! Never face the gate! Otherwise, the fate has melted, and there will be great disaster! Why do we have to do that？ Because first, I know something about Feng Shui, and second, I used to produce septic tanks! There are so many construction sites installed, and I haven't seen anything as built as you said!