thanks all for the help and advises. my contractor had advised against it as he said that there are some cases where the neigbours below had complaint about water leakage. and these houses happened to have their water drainage embedded. while there's no direct link that the leakage is due to the embedded drainage, but it may be one of the factor. so i guess, there's always the risk. anyway, with the risk and the rest of the considerations pointed out, we have decided not to do it. once again, thanks yar!