Architectural documentation is fundamental for determining the authenticity and integrity of heritage buildings, contributing significantly to establishing their value and significance. It also provides essential information on the condition of the structure, identifying decay caused by natural and human factors, and guiding preservation strategies. In Bangladesh, the current practice of architectural documentation is often incomplete due to insufficient equipment, knowledge, and expertise, typically limited to measured drawings and photographic catalogues. This inadequate documentation leads to improper conservation measures, particularly for religious structures like mosques and temples that are still in use, neglecting their heritage values and authenticity. This study aims to document the historic Mashjidkur Mosque located in Koyra, Khulna, Bangladesh, near the Sundarbans mangrove forest. Architectural features of the mosque suggest it was built during the Pre-Mughal Sultanate period, around the 15th century BCE, in the Khan-Jahan style specific to this region. The methodology integrates traditional measured drawing and photogrammetric data into digital drawings and 3D modeling. The photographic survey employs digital cameras, mobile phone cameras, and drone imagery, with data processed using photogrammetric software. In addition to architectural drawings, conditional drawings identify various forms of decay, such as critical cracks, salt crystallization, rainwater accumulation, biological decay, and human interventions. The field survey also includes stakeholder interviews. The study develops a matrix for heritage values and identifies character-defining elements through comparative analysis and literature reviews. It assesses potential risks from salinity, natural disasters, flooding, and sea-level rise, and recommends conservation interventions based on the gathered data. This comprehensive documentation approach aims to improve conservation efforts for the Mashjidkur Mosque and similar heritage structures.