Replacing the alternator belt on most cars is relatively easy, since they are usually somewhere pretty accessible. Thanks the tight packaging on the uniquely styled Audi TT, however, replacing the alternator belt on this car is a bit more challenging. By using the following procedure, it should be pretty straightforward, though, and can be done in a couple of hours. This applies to the first generation Audi TT, made from 1998 to 2006.
Disconnect the negative terminal of the battery.
Remove the plastic trim on the passenger's side of the engine bay. The alternator is located on the left side of the engine (as you are looking at it) between the bottom and top of the motor.
Remove the trim that covers the manifold. Loosen the clamp that holds the air intake pipe and remove the rubber hose portion for access. There is also a black canister above the alternator that needs to be moved. Disconnect the hose on top of the canister and unbolt it so it can be moved out of the way. There is also a small metal plate to the right of the canister held on with 5-mm allen bolts that needs to be loosened so that it can be moved a little out of the way.
Use a 15-mm box wrench and pull the belt tensioner toward the front of the car to slacken the alternator belt. Remove the belt, noting its orientation so that the new belt can be installed correctly.
Install the new belt and tighten the tensioner. Move the metal plate back into place and tighten bolts. Reinstall manifold trim and plastic engine trim.
An optional first step is to jack up the car and remove the belly pan, so that if anything falls down through the engine bay, it won't get lost.