It is possible to add this in via a Z clause for an NEC3 ECC, but we have doubts on the merits of not adjusting the prices as there would be little or no incentive on the contractor to suggest such proposals in the first place. While the time and cost implications of re-design are a factor, a blanket approach to not changing the prices does not seem logical. This gets even more complicated where a change in design liability is sought too.
There is now provision in NEC4 ECC for such proposals to be submitted by the contractor. These are found in clause 16 and the client determines the amount of value engineering percentage in the tender documents that is used in calculating the reduction in prices. The time and cost of re-design, whoever is to do this, must still be a factor in considering the proposals.