Why registration is not optional
Cover applies to named drivers. If someone else is driving when damage occurs, the supplier can decline the claim and charge the full repair cost, regardless of the package you bought.
The same applies to accidents where the other party was at fault — an unnamed driver removes your protection either way.
What it costs and requires
The fee
Usually a daily charge, so it scales with rental length. Some tariffs include one additional driver at no cost — worth checking before paying separately.
The same conditions apply
Every named driver has to meet the age and experience requirements independently. A second driver below the threshold cannot be added at all.
Documents at handover
The additional driver must be present with their own licence, and often passport, when the car is collected.
When it is worth it
On long single-day drives, sharing the wheel is a real safety measure rather than a convenience — particularly when adapting to unfamiliar road rules or driving on the opposite side.
For a week of short town trips with one main driver, it is usually an avoidable cost.
Adding one mid-rental
Most suppliers allow it, but it has to be done through the office, with the new driver's licence presented and the contract amended. An informal arrangement provides no cover at all.
Expect the fee to be charged for the remaining days, and get the amended agreement in writing before anyone drives.