Building In A Swamp: Which Is Better, Raft Or Pile

Pls my dear engeneers in the house, i salute you all. I have read some of your advices on building on waterlogged areas and i appreciate your comments, but mine is a little different and i'll need your candid advises. Please i bought a plot of land of 60 * 120 in iyana iba area, Lagos. its a waterlogged land (for sake of cost). with a layman experience i used a long narrow stick to see if i can determine how deep to the (Bedrock) solid ground beneath, it took between about 3ft (close to shore dept) to 6ft (centre dept) to pass the softtop(debris, decaying grasses, no mud, the soil is sandy, looks like sharpsand) to get to strong ground. i've filled it to some extent with filling sand, meanwhile some building are being erected in there but some didn't get their foundation well done, which is part of what i'm trying to avoid. Now, i'm ready to build, (atleast the FOUNDATION for now), pls like how much will it cost to lay a good durable but cost effective foundation of 2 storey building of single 4 bedroom flat, (1 flat at ground flour, 1 flat at 1st flour) (BQ sort of.) and which type of foundation will be best option, considering its various depts and the costs (cos I don't have much, just trying to escape from high lagos rents and planning for retirement.). What will it require' Also can the foundations be done in bits, (ie, casting now and german floor much later if raft or also in piles) or must everything be done at once' Please your help is highly needed for me to know what next to do. Many thanks from bigi.
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