If a house has four members, whose kundli should be used for its vastu?
Primarily the grih swami's, the person whose money is going into the house, because the house affects the owner and his next three generations; do not go by the name the property is registered in, such as a daughter. The facing and main door are chosen from the grih swami's chart, for example North if his Mercury is exalted or his North-side planets are strong. Where a child sleeps or studies is decided from the child's own chart: if the child's Mercury is weak, seat him in the South instead of the North, following the 180-degree rule for a weak planet.
Related questions
The 5th lord is Mars but the child studies in the West. Is that wrong?
See in which direction the 5th lord sits. If it sits in the West, that is excellent; if in the East, choose the East. The East and North are best for study anyway, and if the lord sits in the South, studying in the South also works.
Does being manglik stop a person from sleeping in the South-West?
Manglik has nothing to do with it. It is considered only at marriage matching, when Mars sits in the 1st, 4th, 7th or 12th house, and Mars is power, not inauspicious. For the SW check whether Rahu-Ketu are debilitated, and for the South whether Mars is bad.
Must the house be at 0 degrees for a sarvatobhadra dwar?
By definition it is at 0 degrees with doors in all four directions, but it can be made otherwise too: build the doors on whichever pads fall on Pushpdant, Grihakshat, Bhallat and E4.
My Jupiter is exalted and my door is on N6-N7, towards the North-East, yet we have had no problems. Why?
The Ishaan (North-East) corner belongs to Jupiter, so people whose Jupiter is exalted can keep their door there. This shows how the birth chart decides which door suits a person.
My house entry is in the SSW and my 10th house has Sun, Mercury, Moon and Ketu with an exalted Venus; can I move the entry to the South-East?
If you open the South, open it in the good South pada (S4 as heard), and if you open the West, in the good West pada (W4 as heard). The present South-West entry should be corrected with door power.
My new flat's entry is on N4, but Mercury sits in the 8th house of my Mesh lagna chart, though not debilitated. Will using this door activate the 8th house?
Such a case has to be judged from the birth chart, so it was taken up as a special case study in the evening astro-vastu class. For this analysis only the lagna chart is used, not the navamsha, so only the lagna chart needs to be sent.