Most refurbishment projects within the UK are undertaken using the JCT Design and Build Contract 2016.
The relevant insurance clause is 6.2, which states that the contractor is liable for and indemnifies the employer against loss or damage to property and (at least as a starting point) the existing structure.
In the case of co-insurance, JCT Option C will usually be applied.
In terms of the contractor's liability under clause 6.2, JCT Option C.1 means that the contractor's liability will not extend to loss or damage to the existing structure caused by the risks required to be insured under Option C.
If a waiver of subrogation can be procured in accordance with the lease or license to alter, JCT Option C.1 should be disapplied.
This means that the contractor's liability under clause 6.2 would extend to loss or damage to the existing structure of the building, but it would have the protection of the subrogation waiver, often subject to a cap.
The contractor must assume some liability for the existing structure, which they must insure under their public liability insurance.
Any liability to the existing structure due to the specified perils would then be capped to the amount of the public liability insurance.
The waiver of subrogation would then apply to claims over this cap.
This way, the building's insurer would cover the full loss for any damage to the existing structure, with the contractor's liability to it being limited to a level it can cover by its own insurance.
Any liability above the cap would be subject to the waiver.