Bobby,
For Griha Arambha and Griha pravesha the muhurtha is fixed taking your birth star. There is a procedure of calculation which is a seperate part known as Muhurtha Bhaga. The muhurtha bhaga deals with auspicious timings etc to perform certain events.
The astrologers will forecast the events taking into consideration the date of birth, time of birth and place of birth of the native.
Best of Luck