Veneers must always be supported by the underlying tooth structure, and since enamel (the outer layer of a tooth) is much stronger than the dentin (the inner substance of a tooth) it is always the dentist's desire to bond veneers to enamel. Bonded this way, they are much stronger and more durable. In addition, even though veneers can be quite thin — as thin as 0.4 mm, in places — there needs to be adequate room for the material so that the restored teeth do not appear bulky. Prepless veneers need to meet certain additional criteria if this technique is to be considered:
Teeth should ideally be slightly "retroclined" (located more toward the palate or tongue as opposed to the cheeks) to allow enough room for the veneer.
Discoloration should not be so severe the porcelain cannot cover it.
Teeth need to be shaped to allow enough space between them.
There must be enough room on the incisal or biting surfaces of the teeth for both bite strength and aesthetics.
For all these reasons, the majority of teeth need some minimal preparation in order to achieve a good aesthetic and functional result. It's also important to have a clear line of demarcation between the tooth and margin of the restorative material to make a smooth "finish line." This helps make it easier to keep your new smile clean and thus prevent additional staining and irritation to the gum.
It's always best to consult with your dentist to determine if you are a good candidate for veneers and what your options are. A beautiful smile feels great and can increase both your confidence and well-being.