The additional driver, satnav and excess ... all these can add up to more than your original car hire quote.

According to research for The Observer, adding an extra driver, particularly one under 25, a car seat, a satnav and excess car hire insurance at the rental desk all bring extra cost - in some cases more than the price of the actual hire. The most expensive optional extra is adding a young driver, but choosing to hire a satnav can also make a sizeable difference to how much you pay.

Don’t assume, either, that you will pay the same price for identical extras with the same car hire company in different destinations. Hertz, for example, charges 57 for a car seat in Faro but 38 in Nice. A satnav with Avis in Nice will cost 97; but 50 in Faro.

The cost of fuel can be another hidden nasty if you use a car hire company that employs a “fuel empty” policy, where consumers are charged for a full tank of petrol when they pick up the car and told to return it empty with no refund for unused fuel.

“Our research has shown that these unavoidable fuel charges can triple the price of a rental, turning what seemed like a good deal to potentially a very expensive one,” said Which? Travel expert Rochelle Turner. “All charges should be shown up front at the time the booking is made so consumers can make the right choice and compare prices easily.”

In both cases, it is relatively easy to avoid car seat and satnav charges by taking your own. Airlines’ policies on car seats differ, but most will allow you to take one on board (but often only if it is pre-approved and/or is a rigid-framed five-point harness type), or will allow you to check it in with your hold luggage. You sometimes pay a fee, but this is typically cheaper than hiring a seat from the car company. If you already have a portable satnav, you can usually buy a card compatible with the country you are visiting to put in it.

Some of the extras you simply won’t want. However, excess waiver insurance is one that many drivers who have had their fingers burnt will be wary of dropping. This can add on as much as 35 a day, but if you don’t take it you could be liable to pay a typical 1,600 if the car gets damaged or stolen.

“Take out excess waiver insurance rather than with your car supplier,” says Bob Atkinson of travelsupermarket.com. “A standalone policy is significantly cheaper and gives you greater levels of cover and protection in case of accident or damage to your vehicle.”

Guardian News and Media 2013