Question to the Ministry of Defence:

To ask the Secretary of State for Defence, what steps his Department has taken to ensure that calculation of cost estimates for the new generation of Successor Trident submarines has taken into account optimism bias in compliance with the HM Treasury Green Book and supplementary guidance.

Successor cost estimates take into account optimism bias in a way consistent with Green Book guidance.
As announced in the Strategic Defence and Security Review 2015, the level of contingency represents around 35% of the submarine cost to completion. This is a prudent estimate based on past experience of large, complex projects, such as the 2012 Olympics.
The cost estimates have been subjected to, and will continue to be the exposed to, rigorous cross-Government scrutiny.