One who is responsible for cause of leakage must bear the cost of plugging/stopping/preventing leakage. In case, the builder or the owner of the other apartment/flat is responsible, he should bear the cost of stopping and preventng leakage besides bearing the cost of damage, if any due to leakage. Ofcourse, first make a polite approach accompanied by say two neighbours/office bearers of welfare society. In absence of positive response, you can issue a legal notice to the deliquent party/culprit, if any.
Never tolerate injustice and infringements of your rights as a citizen but act after consulting wise and selfless relatives/friends/neighbours/colleages