HS2 Ltd confirmed that it was its decision to build the structure and not one required by Natural England. However, the Department and HS2 Ltd told us that, despite its cost, that they had concluded the ‘tunnel’ structure was the most efficient and appropriate way to protect the bats out of the 15 options considered.39 Other options were rejected because of either “excessive cost, technical infeasibility or ecological ineffectiveness”. Their assessment included “robust challenge” from HM Treasury, a review by Arup on whether the solution could be delivered more efficiently and work with the Department for Environment, Food and Rural Affairs and Natural England to test whether the structure would provide the legal compliance and whether there were any better solutions that also reduced costs.40