Made ground — soil or fill placed by human activity rather than deposited naturally — can usually be built on, but it must be investigated first. Its composition and compaction are often variable and unknown, and it may contain contamination, so foundations are commonly taken through it or designed to bridge over it.
Made ground can be anything from clean imported fill placed and compacted under supervision to uncontrolled tipping of demolition rubble and household waste. The problem is that you cannot tell which you are dealing with from the surface, and its strength and settlement behaviour can vary sharply over short distances.
Investigation should establish the depth and nature of the fill, whether degradable material is present, and whether contamination or ground gas needs to be addressed. Where the fill is shallow, foundations are usually taken through it into natural ground; where it is deep, piling or a raft may be needed.