A 140sqm high-end basement extension in Cadogan Square forms part of a seven-storey mansion scheme within the Cadogan Estate. The design centres on a dramatic swimming pool with expansive grey tiling and a ceiling void that introduces natural light, creating a cave-like atmosphere. Wellness facilities include a gym, sauna, and steam room, forming a dedicated relaxation level. Carefully integrated with the existing structure, the basement reads as an original part of the house. Refined materials and contemporary glazing enhance light, space, and tranquillity, elevating the residence with a discreet yet luxurious subterranean addition.