Thats the home gym.  It has a green (Living) roof and had a sculpture on it so from the house and yard it looks like a garden.  The photos you’re asking about where it looks like a bunker we’re taken from a drone... you can’t really view the  house from that angle/view in person because the property slopes down steeply towards the ocean.

This is is a vacation home for the clients and I’ve been working with them for about 1.5 years on their new house. Construction started last fall and should be completed late 2020 or early 2021.