The “Scale Expansion Trap” in Cross-River Urbanization: Building Stock Vacancy and Carbon Lock-In for Nanchang, China

Understanding spatial characteristics of urban building systems is critical for unraveling urban building stock growth patterns, addressing housing vacancy challenges, and advancing urban carbon neutrality. However, existing research on built environment stocks and housing vacancy spatial distribution remains limited, particularly in underdeveloped cross-river cities—where rapid urbanization often prioritizes scale expansion over demand matching, leading to unresolved issues of resource waste and environmental pressure. This study integrated material stocks analysis (MSA) and geographical information system (GIS) to uncover the spatial patterns of urban building material stocks and housing vacancy at a high spatial resolution for Nanchang, China—a typical underdeveloped cross-river city facing the “scale expansion trap” in its urbanization across the Ganjiang River. Results show that (1) Nanchang’s building stock exhibits a “butterfly-shaped” spatial pattern centered on the Ganjiang River, with simultaneous horizontal expansion (40-fold urban area growth since 1949) and vertical growth (super high-rises in new west-bank districts), reflecting aggressive cross-river scale expansion; (2) the total building material stock reached 1034 Mt (204 t/cap) in 2021, with over 85% accumulated post-2000—coinciding with large-scale cross-river development. Vacant buildings locked in 405 Mt of materials (39.17%), which is a direct consequence of the “scale expansion trap” where construction outpaced actual demand; (3) total embodied carbon emissions from building materials amounted to 264 Mt, with 104 Mt (39.39%) attributed to vacant stocks. This “vacant carbon lock-in” stems from mismatched urban construction and actual demand in the process of cross-river scale expansion; (4) spatially, high-value clusters of material stocks and carbon emissions overlapped at two cores (old town and Honggutan CBD), while housing vacancy rates were significantly higher in the urban periphery and Ganjiang’s west bank—the primary areas of cross-river scale expansion—than in the old town and east bank. These findings empirically demonstrate how the “scale expansion trap” in cross-river urbanization drives building stock vacancy and carbon lock-in. These findings also offer data-driven strategies for optimizing urban resource allocation, reducing housing vacancy, and promoting low-carbon transitions, especially for other underdeveloped cross-river cities globally.