Nice Question Engineer Mohammad, I would say that the planning phase in any construction project is very important. However, the most important is the execution phase. The reason i say so, you can start and finish the project without planning, yes you will pay more money, more time, and maybe less quality, but at the end of the day, you will have your project finished. In the other hand, no project can be finished without execution regardless of how accurate your planning is.
Conclusion: Execution stage is more important that planning stage.
Who agrees with me?