You might be talking about different types of insurance, normally bank wants house insured, usually a very limited policy as it is in the course of construction and no contents will be covered, that is an annual policy. However I'd say the OP was talking about a self build insurance policy which covers liability to workers etc, this can be for a longer time like the 18 months as it's giving you enough time to build the house and is not a policy you would normally be renewing.
Some self build policies will include sufficient cover to satisfy the bank so one policy will do for the lot. However while the bank will sometimes only require the house to be covered with an ordinary house insurance however that won't cover your building if you are doing a self build so you either need a policy that covers both or separate policies.
Is it a self build or is there an overall contractor? If contractor their insurance may cover the build, double check it though. Bank will still require cover on the building itself.