I'm looking at changing my current car to an A8. But I'm particularly interested in the Adaptive Cruise Control. Is this always an optional extra or does it come standard in some models?

Welcome to the forum :) 

Good choice on next car by the way. The a8 is a lovely car! 

i suspect it will be standard on the a8L w12 or s8 but unsure on the other models. To be honest I'm surprised it isn't standard on the a8 anyway. 

Also if the acc does go wrong it can be very expensive to fix. The camera can also get dislodged by debris and at which point the car will need a 4 wheel alignment and camera recalibration which is a big job due to the complexity of the camera. 

Cheers

steve