Design development post contract signing in New Zealand: Client's or contractor's cost?

By offering fixed-price contracts for designs supplied by clients, contractors legally warrant that they can build what has been designed and do so within their fixed price. Yet detailed drawings are often issued during construction in response to contractors' requests for information on the basis that they cannot otherwise build what has been designed. Claim-entitlement decisions are often made by construction professionals (architects, engineers and quantity surveyors) without legal training in contractual interpretation, potentially varying who pays for design development after contract signing, contractors or clients. Prior studies have addressed buildability obligations relating to ground conditions and foundations. This study applies key principles of contract law to consider who should pay for instructed drawing details post contract signing under the New Zealand standard NZS 3910:2013, 'Conditions of contract for building and civil engineering construction', in terms of (a) when a variation claim may be accepted; (b) the effect of contractor involvement on design development; and (c) the effect if claimed from a building subcontractor to a consultant manager (no head contractor). A claim-entitlement flow chart and a table comparing the head contractor's and consultant construction manager's obligations provide practical guides for contract administrators. Identifying terms prone to interpretation informs contract drafters towards reducing ambiguity for contract users and therefore the potential for dispute.