The NEC Form of Contract is now in wide use in construction projects. The first clause says that the parties "shall act as stated in this contract and in a spirit of mutual trust and co-operation", often referred to as "good faith". What this means in practice is a question of importance for the development industry, both in relation to this particular contract and to the numerous other standard form or bespoke arrangements where "good faith" obligations are included.

The extent of the good faith obligation was considered in a recent case, Costain v Tarmac. Here the Court was not prepared to allow a contractor to escape the effect of an express time bar clause by relying on the duty of good faith as imposing a positive obligation to point out its effect. The court commented that the express duty in this case said little more than was previously thought to be implied into all construction contracts in terms of a duty to co-operate. So, whilst a good faith obligation prohibits unreasonable conduct which is without regard to the interests of the other party, it does not, it would appear, extend to informing the other party about the adverse effect of a particular term of the contract of which it should already be aware.

Good faith clauses, therefore, do not prevent parties from relying on express terms of the contract. The effect is confined to a restraint on unreasonable conduct amounting to improper exploitation of the other party.