What it does
Prevents the major setbacks that follow demolition-based corrections.
Where
Walls, doors and staircases of an existing house
Cautions
Explanation and notes
The rule is stated in the classical texts (Vishwakarma Prakash or Samarangana Sutradhara). In one case a door was opened at a wrong place by breaking a wall, a double blow, and the owner's four sweet shops across the city closed one by one until not a single customer came.
During the battle with Andhakasur a drop of sweat fell to the earth and produced a being whose rampage made the earth shake. Brahma had him lie face down and promised that he would be worshipped with offerings and that not an inch of land would be built on without remembering him.
Every house, however small, carries the Vastu Purush's eyes, nose, ears, hands, feet, heart, navel and other parts, including the marma and ati-marma (vital) points. A column, pit or stud on these points harms the occupants, just as a speck of dust in the eye or an injury to a vital organ harms the body.
In the Jain account, during the bhog bhoomi of the third era kalpavriksha trees granted every wish, until rising sin made them stop giving. Adi Prabhu then taught the six means of livelihood, asi, masi, krishi, vidya, vanijya and shilpa, and gave the complete knowledge of Vastu to his fourth son Vishwakarma.
Vastu knowledge passed by memory until it was written down in texts such as Samarangana Sutradhara, Mayamatam, the Skanda, Agni, Vayu and Garuda Puranas, Vishwakarma Prakash, Aparajita Prichha, Praman Manjari, Vastu Rajvallabh, Prasad Mandan, Vastu Mandan and Shilpa Ratna, each with its own focus.
Start with Vishwakarma Prakash, the simplest, available in Hindi with the Sanskrit shlokas and their meaning (translated by Krishna Kumar Jugnu of Udaipur). Then read Mayamatam, both parts of Samarangana Sutradhara, Manasara and Raj Vallabh.
Raj Vallabh states that a temple with a large river flowing to its South becomes not merely a tirth but a mahatirth, because for a temple the water element stands for peace. Rules made for homes must not be applied mechanically to temples.
Vastu results depend equally on four factors: dravya (the money used), kshetra (the land), kaal (the muhurta) and bhav (the feelings of the builders and residents). A perfectly planned house still fails if any one of them is wrong.
Before construction of a house or factory begins, the muhurta for the shilanyas or start of work is fixed from the rashi of the owner. If the Moon falls 4th, 8th or 12th from his rashi, money stays short however good the Vastu and the door.