Water constitutes one of the most structurally decisive elements in the architecture of traditional Iranian domestic spaces, operating simultaneously across functional, climatic, symbolic, and spatial-organisational registers. In Qajar-period houses of Shiraz and Isfahan two climatically similar yet morphologically distinct historic cities of central and southern Iran water achieved its most architecturally refined expression through the institution of the howzkhane: a semi-subterranean, vaulted water chamber positioned at the intersection of the house's principal spatial axes. This study undertakes a systematic comparative architectural analysis of the howzkhane as a typologically and tectonically specific water space, examining its relationship to traditional water supply systems (qanats, wells, and internal hydraulic networks) and its role in mediating spatial organisation, bioclimatic performance, and cultural meaning in Qajar domestic architecture. The research adopts a qualitative, descriptive-analytical methodology, combining documentary and library-based data collection with field observation and spatial analysis of selected exemplary houses. Following a systematic review of theoretical and historical foundations, a comparative analytical framework of twenty candidate architectural dimensions was developed and progressively refined through expert consultation employing the Fuzzy Delphi Method (FDM) with a fourteen-member panel over four sequential rounds, applying escalating consensus thresholds (2.5, 3.0, 3.5, 4.0 on a five-point Likert scale) and reducing the candidate set from twenty to fifteen, twelve, ten, and confirming ten validated comparative dimensions. Expert consensus was confirmed by Kendall's coefficient of concordance, reaching 0.781 in the final round.