Does it cost more to add another driver to my car insurance policy?

This depends on the age of the driver.

Adding an additional driver over the age of 25, will not impact your premium on a comprehensive motor policy.

Adding a driver under the age of 25, may impact your premium. If you choose not to list them, they will incur a double age excess in the event of a claim.

If the most regular driver of your car changes, the policy will be rated on that driver and the premium will most likely change.