Marine energy encompasses a rapidly advancing group of technologies designed to harness the renewable energy resource in the marine environment. Significant technology developments are ongoing in the general field of marine energy, but specific to this paper in the field of Tidal Energy Convertor (TEC) technology. In order to take advantage of this continuous, and at times stepped, improvement of the technology available it is beneficial in developing tidal energy sites to retain a degree of flexibility in the decision process for final device selection. Similarly, as the Environmental Impact Assessment (EIA) and (ES) are completed before a detailed technical assessment of the site has been undertaken, there remains a degree of uncertainty relating to installation techniques, foundation types, turbine size and specific technology viability. As a result, a “Design Envelope” has been defined. In addition to the TEC it is also useful to include with the Design Envelope elements of the balance of plant such as the offshore electrical system. The adoption of the Design Envelope approach allows a meaningful EIA to be undertaken by defining a set of “worstcase” parameters that decision makers can consider in determining the acceptability, or otherwise, of the environmental impacts of a project.