This study examines and interprets the geometry and proportions of architecture in the facades of the historical neighborhoods of Birjand during the Qajar period, focusing on how spatial organization and facade design were formed. The architecture of this era was deeply influenced by Iranian-Islamic principles on the one hand, and by European styles on the other, resulting in an eclectic architectural style with its own distinctive characteristics, among which geometry and proportional relationships stand out as key features. Scholars such as Kamran Diba and Mohammad Ali Pirnia have repeatedly emphasized the importance of these aspects in the identity of Iranian architecture.This article analyzes the design strategies employed in utilizing geometry and proportion in shaping the visual appearance of the historical neighborhoods of Birjand. An examination of the existing buildings, historical maps, and documents reveals the high level of architects’ perception of human scale, climatic adaptation, and the cultural and traditional values reflected in the urban facades. The study further investigates the structural and geometric characteristics of these facades based on fundamental principles such as the golden ratio, human scale, and visual-spatial axes centered around the main historical pathway known as Khajeh Street.Methodologically, this research is descriptive-analytical in nature. Data were collected through archival studies, field surveys, and interviews with experts. To identify the most influential design parameters, six major criteria—geometry, scale, proportion, neighborhood image, climate, and culture—along with twelve sub-criteria were determined across 44 case samples. These indicators were then prioritized based on expert evaluation. Complementary data were gathered through Likert-scale questionnaires distributed via the snowball sampling method. The Analytic Hierarchy Process (AHP) and Thomas L. Saaty’s pairwise comparison matrices were used for weighting and ranking the indicators.The analysis of three key historical neighborhoods—Sardeh, Miyandeh, and Tehdeh—demonstrates that facade design during the Qajar period not only reflects a profound connection to Iranian cultural and architectural heritage but also mirrors the social, economic, and political interactions of the residents. Furthermore, these facades successfully address climatic and environmental challenges while maintaining cultural and aesthetic coherence.The findings indicate that the “scale” criterion holds the highest significance with a weight of 0.6, followed respectively by neighborhood image, geometry, proportion, climate, and culture. Overall, this research reinterprets the architectural language of Birjand’s historical neighborhoods to identify the fundamental parameters shaping their facades during the Qajar era. These results can serve as a theoretical and methodological foundation for future urban revitalization and heritage-based architectural design in historical contexts of Iran.