The concept of “sustainable building”, in its various interpretations, is becoming increasingly important for policymakers, planners and building sector practitioners. The ways actors work with building sustainability are influenced by requirements, guidelines, criteria for green procurement and certification, assessment tools, internal documents of their company, etc. The present study investigates how such artefacts mediate building design, focusing on how they support or define aspirations towards environmental sustainability. This investigation is based on case studies of public housing projects in Sweden and Cyprus. First of all, the cases indicate that artefacts set boundaries to the range of available sustainable design options. On one hand, regulations, certification systems and directives internal or external to the housing company codify and enforce minimum demands regarding sustainability. Several actors mentioned that a major driver of their work with sustainable design was the need to fulfil requirements set in regulations, plans, directives or procurement documents. On the other hand, cost calculations, procurement laws and requirements in development plans also restrict the range of possible design options. Artefacts also streamline and simplify the design process. The use of predefined or default options, checklists, standardised technical specifications and databases implies that some key design choices are not made within the project itself, but outside of it, when these guidelines and standards are developed. However, these various requirements, constraints and standardised options are often bent and adapted on a case-by-case basis, showing a complex and interdependent relationship between artefacts and actors’ agency. Significant differences were apparent between the Swedish and Cypriot cases. In the Swedish cases, the ways actors understand and operationalise the concept of “sustainable building” is strongly reified, codified and enforced through various interconnected artefacts. The widespread Miljöbyggnad certification is often a de facto definition of sustainability and a reference for actors to set practical sustainability criteria. Third party environmental databases for construction products also create black boxes where environmental performance criteria are unchallenged and not immediately visible to the user. This reification was less noticeable in the Cypriot cases. Cypriot designers’ work with sustainability seemed to depend to a larger extent on their motivation, experience, knowledge, skills and ability to convince their peers. The paper ends with a discussion of implications for the implementation of environmental performance criteria and decision support tools such as life cycle assessment (LCA).