01 · Challenge
Groundwater and load-bearing capacity as the main challenge
The owners of a detached villa in Bloemendaal wanted to use part of the garden for a basement that connects to the existing ground floor. The sandy Bloemendaal soil is known for its relatively high groundwater level during the wetter seasons. This called for a watertight concrete construction that is pressure-resistant and does not load the existing foundations. At the same time, the municipality required a building permit for the excavation work and the structural intervention.
02 · Our approach
Waterproof excavation pit with steel construction
We designed a cast-in-situ concrete box with watertight connection joints to the existing foundations. A structural engineer calculated the steel sections supporting the existing floor slab. The construction drawings for the building permit included all retaining details and an installation plan for the sump pit. Because the design aligned well with the local soil conditions, the permit procedure proceeded without additional queries from the municipality.
03 · Result
35 m² dry basement space, building permit granted
The basement level is completely dry and offers 35 m² of extra usable floor area, fitted out as a wine cellar and hobby room. The permit was granted within 10 weeks of submission. The contractor could start immediately with the execution-ready details, without additional work for dimensional errors. The villa grounds in Bloemendaal look unchanged: the basement is invisible from the street.