It will really depend on which model of your current AirPort Extreme you have versus the latest generation (currently, the 5th) of the 802.11n AirPort Extreme Base Station (AEBSn). Apple has made a number of improvements with each generation, including redesign of the base station's antennae. You can read all the "gory" details from an excellent independant source here. What all this translates to is the the range has not necessarily increased, but the ability to provide a quality bandwidth for that range has.
You may find that by just replacing your current AirPort with a new one may be all you will need to reach the patio. However, if it doesn't then adding a 802.11n AirPort Express Base Station (AXn), and then, reconfiguring both for an extended network should do the trick.