Check that he/she is referring to DBI, as builders can take out a range of different insurance, such as public liability, contract works insurance etc.
To take out DBI, your builder must first be ‘eligible’ for the insurance. You can check the eligibility status of your builder here.
Eligible builders must then apply for an insurance policy to cover each individual building project. A Domestic Building Insurance Certificate will be issued, and a copy must be given to you.