For whole-house customization, which is more cost-effective: projected area or developed area?

Neither is absolutely more cost-effective; it depends on how you compare them. Projected area is calculated as width × height, which is simple and intuitive, but it can work against you when the internal layout of the cabinet is complex. Developed area is calculated by breaking down each board separately; it is transparent but laborious to price. Our wardrobes are priced by projected area. The conversion between developed area and projected area varies with the cabinet body layout and must be calculated according to the drawings. So don't just focus on the unit price. First have the vendor quote both bases using the same set of drawings, and then compare the total prices; that is reliable.