Is vastu poojan done only when construction starts?
No. Do it at bhoomi poojan or shilanyas, when the first slab is laid, when the main door frame is installed and on griha pravesh; the start of construction and griha pravesh are essential. Also do it after a renovation in which walls are broken, for a marriage, the start of a job or business and the birth of a child, and once a year on a swayam siddh muhurta such as Akha Teej or Gudi Padwa for a year of happiness and peace.
Related questions
When do the Vastu Devtas have the greatest effect, and when should they be established?
Students answered griha pravesh and bhoomi poojan, which is only partly right: the greatest effect comes at the garbh vinyas or shilanyas, the day the land is taken over and the foundation is laid. Then the pishach, manushya, dev and brahma bhoomi are marked, the vastu pooja is done and every devta, 32 in the pishach bhoomi and the rest inside, is worshipped at its own place with its samagri, following the vidhi. A child cannot be born without a seed in the womb, and food works only through the mouth, not the ear or nose, so the biggest pooja, havan and sthapana belong to that day; building over the devtas afterwards is like tethering a cow first and then building its shed. Done later, remedies bring back only about 35% of that role, though even 5% is a lot.
At a bhoomi poojan, why is the first tilak given to the mason rather than the pandit?
Because the feelings (bhav) of the builder enter the building. Honouring the mason, contractor or Vastu consultant with tilak, good clothes and a coconut makes him work with joy, while a hurt craftsman's tears in the mortar bring repeated sorrow to the house.
Do the Vastu Devtas reside in the sky or on the ground?
On the ground. That is why they should be established at the shilanyas itself, and their bhog samagri is offered on the ground at each devta's own place.
Does the relationship with the Vastu Devtas end once the vastu pooja is done at the start of construction?
No. A farmer who sows a seed does not end his bond with it there: he protects, waters and nurtures it until it flowers, fruits and grows into a shady tree, even a kalpavriksha (wish-fulfilling tree). In the same way, Indian architecture holds that a structure of stone and lime is not lifeless; prana settles into the building and its soil and controls the feelings of the people who live there, so the devtas established in the land must keep being nurtured.
If many things are already kept at a devta's place, does that count as an offering?
No, goods lying there are not arpan or samarpan. A guest never picks sweets off the dining table himself; they are served on his plate and he is invited to start. In the same way, serve the devta its bhog in a leaf dona or on a paan leaf, which the devtas love, and request it to accept the offering and increase the energy of the place. If the pooja was not done during construction, offer it at griha pravesh: any dish made in desi cow ghee, such as gujiya.
If the North of a flat or house is polluted, for example by a toilet, should we advise removing it, and is a pooja needed afterwards?
Yes, advise removing it: the North is Kuber's place, and keeping filth at the place of wealth is like putting dirt in the mouth, so wealth cannot grow. After any demolition do a Vastu pooja; even a short one is enough, offering 45 small pieces of gud to the 45 devtas or invoking the nine lords with Bho Indra Vastu Dev Vastu Shanti Kuru Kuru and so on for each direction, with Brahma at the centre.