Population-building coupling is boundary-dependent across urban strata in the Greater Bay Area, China

Balancing land development with population dynamics is essential for sustainable urbanisation, yet conventional assessments based on two-dimensional land expansion and fixed urban boundaries overlook vertical built-space accumulation and the changing urban strata. This study develops a population-building framework to map annual urban core-suburban-rural (USR) strata in the Greater Bay Area, China (GBA), from 2015 to 2025. Population potential (Q), derived from Landsat imagery, was converted to building volume (V) at a 1-km grid resolution using conditional expectation and a scaling law. The Q-V relationship showed strong explanatory power (R2 = 0.802), supporting the reliability of building-volume surfaces. Building-volume density increased from 3.25 × 105 to 4.41 × 105 m3/km2 in the GBA. Suburban space operated as a dynamic transition pool: 24.6% of the 2025 suburban cells newly entered from rural areas, while 21.2% of the 2015 suburban cells moved into the urban core. Under dynamic strata, rural and suburban building-volume density declined or remained weak, whereas fixed 2015 rural and suburban boundaries showed clear growth. Dynamic urban-core population density decreased from 7,152 to 6,043 persons/km2, whereas the fixed cells slightly increased in population density. In contrast, per-capita building volume increased under both dynamic (401 ~ 642 m3/person) and fixed boundaries (401 ~ 538 m3/person). These findings demonstrated that built-space intensification, population redistribution, and boundary changes jointly reshape the population-building structures, with per-capita building volume providing a comparatively boundary-stable indicator.