Most cost overruns do not come from material prices, but from things that were never checked at the start.
Many owners assume costs rise because materials get more expensive. In practice, overruns most often come from site conditions that only surface once work has begun.
Access is the first. Material trucks need a certain road width and turning radius. A site reached through a narrow lane forces everything to be carried by hand, and the labour cost of that becomes a line item nobody budgeted for.
Second, the ground itself. Soil bearing capacity determines the type of foundation. Fill ground or former rice fields usually call for deeper foundations or a denser cakar ayam raft. Establishing this early is far cheaper than adding structural work midway.
Third, boundaries and setbacks. The required distance from the plot edge, the road and any waterway is set by local rules. A design that breaches them has to be redrawn, and that pushes the schedule back.
Fourth, water and power. An adequate water source and electrical supply are often assumed to be there. In reality, upgrading the supply or drilling a well is a job of its own, with its own time and cost.
Fifth, orientation and slope. Where the sun sits determines which rooms stay hot all day. How the land falls determines where water goes in heavy rain. Both are rarely considered early, yet both strongly shape how comfortable the finished building is.
Checking these five before the drawings are locked usually takes a few days. Skipping them can cost weeks in the middle of construction.