Is the soil survey included as a basis in your contract, or does it state the standard phrase that everything up to 30 cm is included? In the latter case, additional effort will have to be compensated.
One should always have the contract and construction specifications reviewed by someone knowledgeable, but also largely understand the contract and construction specifications oneself. Often what is important is not what is included, but what is not listed and thus will result in additional costs.
Even with a fixed price, additional costs almost always occur. For example, the upgrade in the electrical trade. The standard often only includes minimal equipment, which is no longer up to date. Additionally, a TAE socket is usually included, but networking is completely missing in the standard.