Hello KwangBeom,

Thank you for reaching out to us with this question. Yes, you are free to plan underground. There are no limits. Remember this is an ideas competition, so none of the urban/construction regulations that might exist in real life apply here. There are no height limitations in any direction, you are welcome to build a skyscraper or go deep underground if you think that is the best solution to the brief.

Good luck!
The arkitekturo team