Raft foundation are generally preferred to when the area required for individual footing is more than

Raft foundations are generally preferred when the combined area needed for individual footings would cover more than 50% (or roughly half) of the total building area, especially on soft soils, or when footings would overlap significantly, allowing the slab to spread the building's load efficiently over a large area. They are used when soil has low bearing capacity or when differential settlement is a concern.