Rapid urbanization is destroying green spaces at a never-before-seen speed throughout the developing globe, which raises major concerns about public health, climatic resilience, and ecological sustainability. Declining green spaces in planned capitals reflects a weakened foundation of planning principles, not just a loss ofenvironment. The landscapes that were formerly characterized by open areas and green belts are changing due to the rapid urbanization in Islamabad, the capital city of Pakistan. Originally intended to be a "garden city" that balanced natural barriers with constructed infrastructure, Islamabad is currently under increasing strain from land conversion and construction, especially in Sector D-12, close to Margalla Hills. The loss of green space in Sector D-12 is investigated in this study using the theories of urban ecology and sustainable development. It views the industry as a networked urban ecosystem where natural systems are directly impacted by construction operations. Concerns regarding long-term environmental sustainability have been raised by the disruption of biodiversity, changes to natural drainage patterns, and escalation of local heat conditions caused by the replacement of plants with buildings. A qualitative case study and 15 semi-structure KIIs were conducted. Findings revealed that open spaces, green belts, and tree covers have significantly decreased, as well as deteriorating environmental comfort, soil erosion during rainfall, and rising temperatures. Although the Capital Development Authority (CDA) has started park development projects and plantation campaigns, these initiatives seem to be primarily reactionary. Conservation measures are hampered by inadequate zoning enforcement, little monitoring, and ongoing development pressures. Islamabad is deviating from its initial ecological objective, this study reveals. Ecological corridors, rooftop gardening projects, more community involvement in environmental monitoring, greater enforcement of regulations, and required green integration in private projects are all necessary for sustainable urban growth.