Biodiversity offsets are commonly used to compensate for unavoidable development impacts on species or ecosystems by aiming to create an equivalent benefit for the same species or ecosystem elsewhere. To ensure an offset compensates for the impact of development, we need to be able to quantify how much benefit an offset action will provide for a species or ecosystem at the site level. For many poorly understood species and ecological communities, however, important knowledge gaps exist. This makes it hard to know what type and how much offset action is needed to offset a given impact. This project developed an approach for eliciting the knowledge of threatened species experts in a structured way, so as to guide estimates of both the benefits and the costs of alternative offset approaches. Here, we describe the approach and findings for three taxa of blackcockatoos in Western Australia: Baudin’s, Carnaby’s and forest red-tailed black-cockatoos.