A geotechnical report is included with some house providers, but not with others. These are the subtle differences that illustrate that in the scope of services of the various offers, ancillary construction costs (to which a geotechnical report is attributable) may or may not be included in the house price. Providers of prefabricated houses - prefabricated houses in the sense of modular construction methods with prefabricated wall, ceiling elements, etc. - generally do not have a geotechnical report prepared, as with prefabricated houses almost exclusively the assembly from the base plate or basement is agreed upon. These companies are not specialized in deep or shell construction - unlike providers of turnkey solid houses, for whom a geotechnical report is more often part of the services, although it is only commissioned after contract signing.
A geotechnical report, also called a soil investigation, includes, in addition to statements on load-bearing capacity, a foundation recommendation as well as information on the groundwater level and an assessment of groundwater quality (e.g., chemically aggressive).
Anyone who wants to be on the safe side should have a geotechnical report prepared before contract signing and agree to use it as the basis for offers. If a construction company is already favored, it is advisable to coordinate this procedure in advance.
The foundation recommendations in the report are sometimes differentiated by options; therefore, it is impractical to agree on prices for multiple thicknesses of a base plate. If the load-bearing capacity is not given, it does not help to dimension the base plate "extra strong," other measures are required. As mentioned above, it is not only about load-bearing capacity; groundwater also plays a cost-relevant role. For example, which load case for waterproofing is assumed? Should the base plate be executed in WU quality? etc.
Due to the various options to respond to a subsoil situation that deviates from the ideal case, these measures can be assigned to deep or shell construction, which can then still be allocated to the construction companies. However, they can also be located in the scope of services for backfilling the excavation pit or earthworks of the outdoor facilities - e.g., drainage, which is usually commissioned additionally by the builder or carried out as self-performance. This is exactly where the difficulty lies in recognizing and assessing this. Especially in deep construction or earthworks, considerable additional costs can arise, especially since verifying the invoiced quantities in this area is difficult.
The elevation classification of the house also plays an important role here, especially with basements. The reduced excavation of the pit with elevation of the house above ground will be costly later when filling back in, especially if paths or terraces are to be constructed.
Recommendation: Obtain a geotechnical report early - ideally before contract signing and let it become the basis of the contract - and have it professionally assessed. Otherwise, it means planning a generous cost buffer.
