It is usually purchased before the commencement of a project work by either the contractor or the developer for the entire project period. The policy will also covers for the defect liability period after the completion of the project.
The sum insured of the policy is usually the project value. Other considerations may include the principal existing property, Removal of Debris, professional fees and machinery used.
The choice on the value of third party liability will depends on the complexity of the project, the neighboring properties and owners, Traffic situation and the requirement under the contract.