This thesis addresses Hong Kong's subdivided unit (SDU) crisis by proposing a modular retrofit framework grounded in tenant needs, spatial equity, and scalable implementation. In a city where over 7.5 million people live on just 24.3 percent of built up land, and public housing wait times exceed seven years, SDUs have become a default housing form for low income residents excluded from formal systems. These units, private flats split into multiple micro dwellings, often lack ventilation, daylight, sanitation, and privacy, reflecting a deep mismatch between policy tools and lived domestic conditions.

Recent government responses - prefab pilots, site clearance, and regulatory guidelines released in 2024 - fail to engage the interior scale where overcrowding is most acute. This thesis reframes SDUs not as code violations, but as architectural sites for intervention. It develops a percentage based retrofit model, calibrated to the density of SDUs in each building, with three tiers: light, moderate, and full building retrofits. These range from layout improvements and ventilation fixes to the integration of shared kitchens, study nooks, and rooftop gathering spaces.

Using Chungking Mansions as a prototype, the proposal demonstrates how tenant centered design and modular spatial systems can convert overcrowded housing into resilient, community driven domestic infrastructure. By aligning policy, design, and lived experience, the project positions SDUs as an opportunity to redefine urban housing from within.