Many years back when I extended my first house in Setia Alam for an area of 18 feet X 11 feet using paya bakau. The extended area sunk and cracks as big as 2 fist. Demolished it and redo it the 2nd time and this time still with paya bakau with double the length. Still cracked at the end and gave up and sold back the unit to developer. I would recommend to spend some money on investigating the soil and maybe get a few contractors to give their quotes and approach to the implementation. For me, it's better to overdo rather than regret later as you would have spend more money in the interior later on.