Introduced in 1998, Audi's popular TT coupe/roadster might best be thought of as an upgraded version of its VW Golf and New Beetle cousins. Most of the same parts apply, as do upgrade procedures and tuning techniques. There are a lot of ways to go about lowering a TT, from complete coil-over suspension retrofits to cutting coils from the original springs. However, most people opt for the middle road and just replace the stock springs with shorter and stiffer units. Exact procedures and bolt sizes will vary by model year.
Raise the front of the vehicle and secure it so that both front wheels are off the ground. Remove the wheels.
Remove the ABS (anti-lock brake) wire grommet and cable from the strut and remove the lower "pinch bolt" that connects the strut to the steering knuckle.
Compress the spring with the spring compressor to relieve pressure from the suspension. Separate the strut from the steering knuckle (which might take some effort), lowering the lower control arm as needed. Remove the axle from the steering knuckle and lay it on the lower suspension.
Remove the windshield wipers and cover (later model Audi TTs) to access the upper strut bolt. Remove the single strut bolt while an assistant supports it from the bottom. Allowing the strut to simply fall without support may cause the spring clamp to jar loose, sending the highly pressurised spring flying across your garage.
Slowly remove the spring clamp from the strut assembly and lift the spring free of the strut. Replace the stock spring with one of the front springs from your kit; Audi TT springs aren't reversible, so make sure you put it on right-way-up.
Reverse the removal procedure to reinstall your strut and its new, lower spring. Repeat the entire removal/replacement procedure on the other side.
Raise the rear of the vehicle (allowing the suspension to fully droop), support it with jack stands and remove both tires. Remove the anti-lock brake sensor cable from the inner fenders (if so equipped). If replacing the shock absorber at this time, you'll need to partially remove the inner bumper lining to access its top bolt.
Put the spring compressor on the coil spring to relieve pressure from the suspension. Remove the shock-absorber-to-control arm bolt. Remove the compressed spring, then carefully remove the compressor from your spring.
Apply the compressor to your new spring and install it in place of the old. Depending on the amount of suspension drop, you may or may not need to use the spring compressor to install the new springs. If this is the case, simply slip the spring into place and use your floor jack to raise the rear suspension far enough to reinstall the shock absorber bolt. The remainder of installation is the reverse of removal. Take your now-lower Audi to a qualified shop and have the suspension realigned.
Don't make the mistake of thinking that this ends your lowering odyssey. The very engineering prowess of Audi works against you when it comes to modifying things like ride height. You may also need to change out the steering end link, sway bar links and install adjustable camber plates to the front in order to maintain factory alignment, handling/braking qualities and an acceptable level of tire wear.