Voluntary sustainability standards, aimed at improving the environmental, social and economic aspects of agricultural production and trade, are becoming increasingly common. The coffee sector is a prime example, where sustainability certification could improve livelihoods for poor smallholders. However, as individual production volumes are low, smallholder farmers need to cooperate in certification as a group, which makes impact assessment more complicated. Previous empirical studies, reporting premia of up to 30%, have neglected the costs associated with group certification. We explore the issue using an agent‐based simulation of coffee producer organisations in Uganda, including the certification‐related costs for farmers. Our results suggest that certification can have a small positive impact on participating households. But the added value of certification is substantially lower than the price premium, because of certification costs. Increasing both the membership of the producer groups and their deliveries of certified coffee are necessary to improve the rewards of certification.