Before I write about Gruhapravesam, let me write about Bhoomi Pooja.
According to Sanatana System of Hindu rituals, the land is worshiped before taking up any construction in the land. If it is a construction of a house, initially a muhurat has to be taken up. According to Muhurat Sastra, the star of the day should be a Kshema Tara, Mitra Tara or Parammitra Tara. Sometimes Sampattara is also considered. The day should be preferably Panchami, Saptami, Dasami, or Ekadasi. The bright Trayodasi is also considered as best for the Bhomi Pooja. Chaitra, Vaisakha, Jyesta, Sravana, Aswayuj, Kartik, Margasira, Magha, Phalguna months are best suited. One has to remember that Jupiter or Venus shadowing should be avoided which is called popularly Guru Moodhami or Sukra Moodhami. Nija Kartari is also to be avoided which normally comes in the month of May every year.
Around one and half month prior to the aforementioned exercise, it is suggested to appoint a pundit for worshiping Mastya Yantra in the name of the owner of the building. When some one is interested to spend a crore or so on his house, one should not hesitate to spend a few thousands for the purpose of yantra.
At the time of Bhoomi Pooja, the following items are to be put into the pit that is specially dug in the north east corner of the site within the structural construction. 1) cow milk, 2) Nav Dhanya - nine varieties of grains, 3) Nag and Nagini, 4) five makara dhwajas, 5) hakik stones, 6) gold, silver, copper, coral, pearl - called as pancha dhatus, 7) the mastya Yantra. Four bricks are to be arranged as a square in the pit and a base is made up of cement mixture. Within the four bricks, first the yantra is to be put as per the time fixed in Muhurat, then all other items excepting No 1) and 2) above which are to be mixed and poured into the pit finally. Known near and dear are to be invited, and it is suggested that an elderly person in the family is given an opportunity of breaking a coconut initially and thereafterwards all other near and dear have to break coconuts. The broken coconuts are to be left there.
Please remember that before doing all the above, Lord Ganesh Pooja is a must where a coconut is broken and it is taken by all the owners of the building as a Prasad.