Objective: Examining the evolution of historical residential architecture not only contributes to the preservation of architectural heritage but also provides valuable insights for future design practices. In Arak, residential architecture underwent significant transformations in both spatial organization and physical form during the Qajar and Early Pahlavi periods. The main objective of this study is to analyze these architectural changes with a particular focus on the evolution of enclosed, semi-open, and open spaces. The research addresses two key questions: (1) How did the proportions of built and open areas evolve in the historical houses of Arak from the Qajar to the Early Pahlavi period? and (2) What was the trend in the transformation of spatial and connectivity patterns in residential architecture during this transition? To answer these questions, fourteen historically significant houses in Arak, including seven Qajar-era and seven Early Pahlavi-era houses, were purposively selected and examined.
Method: The study adopts a descriptive–analytical approach. To investigate spatial dimensional changes, the collected data were analyzed using SPSS software and statistical techniques, including the Mann–Whitney U test at a 95% confidence level.
Results: Statistical analyses revealed that during the Qajar period, approximately 65% of the total area was allocated to enclosed spaces, 26% to open spaces, and 7% to semi-open spaces. In the Early Pahlavi period, these proportions changed markedly to 37% enclosed space, 56% open space, and 7% semi-open space. The study also explored the possibility of mathematically modeling these spatial transformations.
Conclusions: The findings indicate a shift from the traditional spatial relationship pattern of “courtyard–iwan–room” to a simplified “courtyard–room” configuration. Together with the changing proportions of spatial elements, this transformation reflects broader cultural, social, and technological changes in Iranian lifestyles. Inferential analyses confirmed that the decrease in enclosed space (from 65% to 37%) and the increase in open space (from 26% to 56%) were statistically significant (Asymp. Sig. ≈ 0.003), whereas changes in the proportion of semi-open space were not significant and remained relatively stable (Asymp. Sig. ≈ 0.938). From an architectural perspective, these transformations reveal shallower floor plans, the weakening of nested spatial arrangements, the strengthening of the role of courtyards and façades/terraces, and the functional shift of iwans toward view-oriented and ventilation-related purposes. These findings provide a scientific basis for the development of contemporary design guidelines and conservation strategies.