When you build a new house, you must make a parapet for your roof, in order that you not cause blood to be shed in your house by not preventing one who falls from falling off the roof.
The obligation to build a parapet around the roof applies to all houses, not just newly built ones, and devolves on the individual whenever a house comes into his possession, not only if he builds it. The reason the Torah nonetheless chooses to couch this law in the case of a newly built house is in order to allow allegorical interpretations:
When you build a new house means "when you set out to make this physical world into a home for G-d." This house is considered "new" because spiritualizing the physical is a reversal of the order of creation. G-d made the physical world appear consummately physical; we reveal its inner Divine essence and make it a vehicle for the spiritual.
Make a parapet for your roof: In order to succeed in this mission and avoid being dragged into the materiality of the physical world, we must be sure to remain sufficiently aloof from the world. This we accomplish by setting appropriate boundaries, red lines that we do not cross. This shows that our involvement in the physical world is not for our own betterment or indulgence, but for selfless purposes. In turn, this selflessness opens us to greater insights into Divine reality and to higher levels of the Divine consciousness that we seek to disseminate.