Why are snakes and urn buried in the foundation of a house?

In the fifth Skanda of the Srimad Bhagavata Purana, it is written that beneath the earth lies the netherworld, ruled by Sheshnag. Ten thousand yojanas below the earth lies Atala, ten thousand yojanas below Atala, ten thousand yojanas below that lies Vital, and ten thousand yojanas below that lies Satal. All the worlds are situated in this order. These seven worlds—Atal, Vitala, Satala, Talatala, Mahatala, Rasatala, and Patala—are called Patala Swarga. Lust, enjoyment, opulence, bliss, and glory are present here. Demons, devils, and serpents all live there happily, enjoying pleasures and luxuries. Numerous cities shine within these netherworlds. They boast gardens and parks that surpass even the splendor of the heavens. In these nether regions, there is no division of day and night due to the absence of planets like the Sun. Therefore, there is no fear of death. Here, gems on the heads of large serpents ward off darkness. Vasuki, the husband of the serpent realm, lives in the nether regions.

The original serpent resides there. According to Shri Shukdev, Sheshji resides thirty thousand yojanas away from the netherworld. The earth rests on Sheshji’s head. When Shesh desires to destroy the world during the time of Pralaya, eleven Rudras with three eyes appear between their angry, crooked eyebrows, holding tridents. In mythological texts, there is mention of the earth resting on Sheshnag’s hood (brain).

Shesh Chakalpayaddevmanantam Vishwarupinam. Yo Dharayati Bhootani Dharam Chemaam Saparvatam.

-Mahabharata/Bhishmaparva 67/13

That is, this Supreme God created Sheshnag, the divine form named Vishvarup Anant, who is holding the entire earth along with the mountains and all the ghosts.

It is noteworthy that the thousand-hooded Sheshnag is the king of all serpents. He is a devoted devotee of the Lord, bringing him happiness as his wife, and often takes incarnation alongside the Lord to participate in his divine play. In the 29th verse of the 10th chapter of Srimad Bhagavad Gita, Lord Krishna has said – Anantashchasmi Naganam’ i.e. I am Sheshnag among the snakes.

The entire ritual of foundation worship is based on the psychological belief that just as Sheshnag holds the entire earth on his hood, in the same way the foundation of my building should also be established with complete strength on the hood of the installed silver snake. Sheshnag lives in Kshirsagar, hence Sheshnag is called by pouring milk, curd, ghee in the Kalash of worship and invoking him with mantras, so that he can be present in person and take the responsibility of protecting the building. A coin representing Lakshmi is placed in a pot representing Vishnu, along with flowers and milk, which are dear to snakes. Snakes are Lord Shiva’s ornaments. Lakshmana and Balarama are considered incarnations of Shesha. This belief continues this tradition.