Per Dexter and Lippert: 3" inch MINIMUM clearance from the top of the tires to the wheelwell.
Also, bump stops are requiered to limit upward axle travel.
There are several ways to get more clearance. You can add a 2" square tube between the frame and the spring hangers... Lots of units are made that way from the factory. You can get longer spring hangers... Some units come from the factory with spring hangers that have several holes to adjust the height.
You can install lift blocks between the axles and springs. I have done this on two TTs.
You can install springs with more leaves.
All of these methods will work. My present TT has a combination of a thicker spring pack, and a small lift block installed, to get the needed height. I only use solid steel lift blocks, and never more than 2" thick. As always, doing it correctly is IMPORTANT... Any of this done wrong ..... Can be an issue.