Geneva airport straddles two countries. It is mostly Swiss but there is also a French side to the airport. The story about how this came about is available on this Wikipedia page. The French side is very small and VERY easy to miss. However, when booking car hire on the internet it may be that hiring on the French side is cheaper than on the Swiss. If I'm asked for advice I will always suggest car hire from the Swiss side from a convenience point of view. I'm writing this piece to help people who have hired a car from the French side. Particularly those that are heading towards the the main ski areas of the Haute Savoie (places like Chamonix, St Gervais, La Clusaz, Les Carroz, Samoens, Morzine and Chatel)
For the average slightly lost tourist hiring a car from the French side can lead to a stressful end to a holiday. BUT for the accomplished and alert traveller it will not cause a problem.
The first issue is that from the French side you cannot guarantee the hire car will have a Swiss motorway vignette. If you need to drive on a Swiss motorway then you'll have to buy one for 40 CHF. You can avoid driving on a Swiss motorway, but to give you an idea of how ludicrous this is, the nearest motorway is only 200 metres from the hire centre!
If you are using a GPS the address for the French side is: Route Douanière, Grand-Saconnex, Switzerland or coordinates 46.233842, 6.111623
Leaving the French side of Geneva airport.
Actually, first you have to get there. All international or Swiss flights distribute luggage on the Swiss side. Only if you have arrived from France should you follow signs to France inside the airport BEFORE picking up your luggage. Unless of course you are travelling with hand luggage only, in which case you can follow the French sign straight away. There is more information on the GVA website.
Here is the Google route for heading to Morzine. You can see that the difference between taking the motorway or not is only a few minutes. Remember, I'm assuming you won't have a vignette and that you don't want to buy one! So all these routes avoid the 10km of Swiss motorway.
"Via D902" is the one.
From GVA to Chamonix you'll add on 10 minutes by taking the route through Geneva that misses the Swiss motorway.
For both Morzine and Chamonix once you leave P20 do the weird dog leg through the border (driving under the runway, round the border and then back under the runway!) then follow signs for Geneva centre. Once you hit the lake follow signs for Evian (if heading to Morzine or Chatel) or Annemasse (for Samoens or Chamonix), it can be a bit congested at times in Geneva but it usually moves. (Unless it's rush hour in which case this route can add 45 minutes)
Keep your eyes open on that weird dog leg, reversing it on the way home is the hard bit.
Returning to the French side of Geneva airport.
On the way you'll need to head through the centre of Geneva again. Then follow signs towards the airport. All the signs in Geneva point you to the Swiss side of the airport, and not to the French side. The trick when you get to the vicinity of the airport is to head for signs that say "La Faucille/Gex/Ferney"....whereas all logic would make you head towards Aeroprort/France...nicely illustrated here.
You need to be in the right hand lane and head into the tunnel (under the runway).
then get to the border, and take a hard left. See here. Above the sign that says Gex, it says "Aeroport secteur Francais", unfortunately google hasn't street viewed both sides of the road!
another little sign here
If you miss those signs then drive 100m to a roundabout and then head back around to the border. It's more obvious now.
You are aiming to get onto this fenced road, a weird piece of France in Switzerland. Obviously a deal the Swiss came to with the French when they swapped land for the runway.
Availability of all the car parks and prices here. For the French secteur it's P20.
Various versions of "how to get to the French side of the airport" are here.