What is the most common cause of big projects over running on their estimated budget? For example, I recently learned that the HS2 train line in the UK will be nearly £75bn over the estimated budget! How was the estimate so wrong?

Question

What is the most common cause of big projects over running on their estimated budget? For example, I recently learned that the HS2 train line in the UK will be nearly £75bn over the estimated budget! How was the estimate so wrong?

In: Engineering

Answer 1

Changes in scope from the client and/or you don’t know what you don’t know. You try and plan for the knowns, the things you know tHt you don’t know, but the things you don’t know that you don’t know is sometimes much larger than anticipated.