We intend to develop a computer-aided design (CAD) system that takes design problem definitions as input and presents a set of geometries that solves the problem. For such system, we perceive using a controlled natural language (CNL) as one method within a multimodal interface to capture some of the required input. To evaluate the feasibility of using a CNL for problem definitions, we conducted a user study with 18 participants. We found that using a CNL increases the quality of problem definition statements compared to statements written in natural language. While a CNL can limit the breadth of problem definitions, it can achieve a balance between enabling natural expression of problem definitions and capturing formal data necessary for computational design.