Objective: To develop a tool known as the Birth Unit Design Spatial Evaluation Tool (BUDSET), to assess the optimality of birth unit design. Background: The space provided for childbirth influences the physiology of women in labor. Optimal birth spaces are likely to enable women to have physiologically normal labor and birth. The measurement of an optimal birth space is currently impossible, because limited tools are available. Research into optimal birth unit design is also limited. Methods: The BUDSET was developed using a qualitative study. Data collection included an extensive literature review, interviews with key informants (architects, midwife clinicians, and researchers) and an expert panel. A Pattern Language format was used to synthesize the literature and data obtained from the key informants. Results: The BUDSET is based on 18 design principles and is divided into four domains (Fear Cascade; Facility; Aesthetics; Support) with three to eight assessable items in each. Conclusion: Birth units must be designed so that they facilitate and support the physiology of normal childbirth. The BUDSET may provide a way to assess the optimality of birth units and determine which domain areas may need to be improved.