half underground drainage question.

So I am considering my Earthship half underground so you step down into it a couple of feet. The windows will be floor height from outside but normal height from inside.
Our plan is to dig a drainage sump at the lowest point inside, I think if I go deep enough I want a double purpose to drain and also have an emergency water source. I know if its ground water it would need filtering and then purifying but it is mainly for drainage.
I want to earth beam the top and hide the entrance and windows/skylights. The reason we want to go down a few feet is to make the entire thing low impact and almost invisible. To grow appropriate things to hide all windows/skylights and entrance but still let enough light in.
All the walls earth rammed tires, then a breathable damp proof course over the entire thing before Earth beaming.