The construction of an underground car park required an excavation in sand under the water table adjacent to the cathedral of Buen Pastor in San Sebastián, Spain. Because of the close proximity of the cathedral and other buildings, control of ground movements was a paramount consideration in the design. Construction involved the use of diaphragm walls propped by the floor slabs using the top-down technique. A monitoring system was installed to control the performance of the excavation throughout. For water control during the excavation, the diaphragm walls reached an underlying low permeability silt layer. This decision required the adoption of special measures to connect hydraulically the foundation ground of the cathedral with the water levels prevailing in the outside area. Excavation was completed with very limited ground movements that caused no observable damage in the cathedral and other nearby structures. Piezometer records also proved that the installed hydraulic connection performed satisfactorily.