You need contract works or all-risks cover for the partly built structure and materials, public liability cover, and employers liability where you engage labour directly. Your builder must carry their own public and employers liability policies.
A standard home insurance policy does not cover a building site. Self-build policies bundle contract works, public liability, employers liability and often legal expenses, and lenders normally require evidence of cover before releasing funds.
Where a main contractor is engaged, the contract should state who insures the works. Do not assume the builder's policy covers your materials on site or the structure itself.