Based on my past contracts in the private sector between private land owners and contractors.
When you issue tender drawings for pricing, you do not allow each builder onto the site to carry out their own site assessment, you as the land owner get a site assessment carried out prior to us (The Engineers) designing the foundations and any associated sub-structure works.
The OP is building a house, so a PPP/Public Works contract would not apply here I would imagine.
From the public works short form of contract, which of course is not your contract, but it is as close as there is to standard practice in the industry,
"4.4 In this Contract, Compensation Event means any of the following:........
the Contractor encounters on the Site unforeseeable ground conditions or unforeseeable man-made obstructions in the ground.........
In the above definition, something is unforeseeable if an experienced Contractor tendering for the Works could not have reasonably foreseen it, having inspected the Site and taking into account all the information provided by the Employer. "
To my mind this means the builder should have been foreseen the rock, how hard is it to stick a pole in the ground to test the soil.
Sorry to resurrect this, undertaking my own extension soon enough so trawling here for any tips.
A little bit off topic, but on the above, your interpretation is incorrect and wouldn't stand up if the disputed claimed came before a conciliator. Current public works contracts are priced from the contract drawings, with whatever GI reports completed included as supporting information. If the Client / Employer doesn't know rock is present, what hope has the Contractor? You also assume the rock is found at a shallow depth.
Regardless, if the Contractor carried the risk for such, the Client would still carry the cost as the risk would be accounted for in the completed rates in the Pricing Document.