This cost estimate is being calculated during the construction of the plant because the decommissioning plan should be one of the key documents for obtaining the license for operating the plant from the national regulatory body.
It depends on a country, law and regulatory practices how often it is necessary to re-calculate the cost estimate for the decommissioning. Usually this period varies between 3 to 5 years.
Each change in the construction of the plant and/or the inventory used in the plant should be mirrored in the cost estimate. The reason is very simple - to be able to regularly recalculate the cost estimates.
The decommissioning project management should be able to negotiate the best price for the sub-contracting activities. For this purpose the Class 1 estimate should be prepared.