When a builders risk is attached to the homeowner’s policy there is no function to add the builder as an additional named insured.
The issue is that when the house under construction has a property loss, the homeowner’s carrier will pay the homeowner and then go after the builder to get reimbursed under subrogation. All commercial liability policies have a care, custody and control exclusion (CCC) which leaves the builder bare without coverage.
The only proper way to cover a home (non-occupied) while under construction is to do a builders risk in the builders name as well as the homeowners name. This protects everyone.
Note: Other advantages are that losses paid during construction don’t affect future homeowner’s premiums.
Also, homeowner builder’s risks usually require fences and night security which h increases the overall cost.