The construction industry is under pressure to work much more sustainably. The government is the largest client and has a lot of potential to contribute to sustainability. However, little is known about the effectiveness of sustainability requirements in public procurement for construction projects. This study examines three Rijkswaterstaat road construction projects to understand the role of sustainability requirements. One striking finding is that contractors often adopt additional sustainability measures during construction. This research shows that agreements can evolve flexibly, which highlights the importance of contractual flexibility. This flexibility is essential for effective collaboration and achieving sustainability performance. Contractual adjustments can lead to improved environmental performance by allowing contractors to incorporate innovative and sustainable solutions as the project progresses. It is therefore important that public contracts allow for such adjustments to promote sustainability in the construction sector. Contractual flexibility appears to play a key role in the successful implementation of sustainable practices in construction projects.