Earthing of Building with Basement

Wow. Carving a 6.5 meter deep basement out of solid granite must be expensive. Drainage must be awkward. At least you will not have to contend with lava flows or coffer dams to hold back the tide.

I wonder what the local regulations say about earth conductors. You'll probably have to run a separate cable into the lagoon for a sea ground.

Consult a qualified Electrical Engineer locally, who will design the earthing requirements based upon the appropriate national electrical codes: in the UK, for example, BS7671; in India IS3053; etc., etc.

First of all I agree with PWSlack. In USA NEC[for instance] states:

[NEC 2014 Art.250.52 Grounding Electrodes. (A) Electrodes Permitted for Grounding. (3) Concrete-Encased Electrode.]

"Metallic components shall be encased by at least 50 mm (2 in.) of concrete and shall be located horizontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth."

See also [for instance] NECA 331/2009 Ch. 3.4 Concrete-Encased Electrodes[Ufer Grounds]

National Electrical Contractors Association

Building and Service Entrance Grounding and Bonding

https://www.necanet.org/store/product/neca-331-2009-standard-for-building-and-service-entrance-grounding-and-bonding-(ansi)-Standard

A comprehensive soil resistance test must be performed to conclude the effectiveness of the proposed earthing system. It maybe as simple as can be. But consider how good of a grounding system is required. What is the building with a basement going to be used for? In the electrical industry, one should consider the use of current prior to breaking ground.

The placement of the Building's main electrical and mechanical grounding system should be physically deep enough to achieve and meet the minimum ground resistance required, as defined by the local electrical code.. Must also be Mechanically sound, strong to maintain its safe grounding integrity..