Underground structures being brought into use as part of the living accommodation are likely to be subject to building regulation approval.
Waterproofing is a high risk area. Conversion works are costly and are entirely dependant upon the efficacy of the water proofing materials and installation work itself - if this fails the building works are at high risk and may require removal.
The waterproofing is nearly always covered up by new masonry walls or dry lining. Thus it is not readily accessible in order to establish ongoing efficacy. Thus the first an unsuspecting owner may know of failure of the waterproofing is sudden ingress of built up moisture or water from a defective seal in the materials used.
Surveys for this work should take into account soil type, strata and bed rock external and nearby drainage including taking into account that of any neighbouring property.
The latest British Standard ( revision of the standard BS8102:1990) – of November 2009 was brought out due to an uplift in demand for extension of living accommodation into basements and cellars. In 2010 Building Regulations require all such works to be Radon gas proof.