There is no cost to make a claim with the NHBC.
But there is a minimum claim value, above which the cost of the works will be met by the NHBC below this £1,500 figure, the NHBC pay nothing.
As for the minimum claim value that's a moot point. I heard one story of a homeowner with defective lintels which the NHBC tried to make each one a separate claim so they all were below the £1500!
If they play that trick, you can always complain about the NHBC to the Financial Ombudsman.
Be sure to let the NHBC know you will do this and are serious if they play up!