A builder's risk policy provides coverage to an entire structure, even if the builder is only providing an extension to a large existing structure.
When a contractor expands an existing structure, the contractor's insurance extends to the entire existing structure, such that an explosion caused by a contractor working on the expansion, that damages the existing the structure, is covered by the contractor's insurance.
Here is the case citation: Medicine Hat College v. Starks Plumbing & Heating Ltd.  A.J. No. 1337. Alberta Court of Queen's Bench. McDonald J. November 14, 2007.
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