Can a dentist put a veneer over an already capped tooth?

Yes: If the porcelain on the old crown (cap) is chipped assuming there is no recurrent decay, the porcelain can be prepared, an impression is taken, the veneer is fabricated and bonded to the old crown. However, if there is a metal substructure of the old crown it should not be exposed. The best bonding will occur between two porcelain surfaces.

Not a good idea: A veneer is not going to cost much less than a new crown and it is not a good idea to try to get a veneer to stick to a crown so just get the crown replaced and they can make it the shape and color you want.

Yes: I have done this for my patients, even over an implant crown. It is not the ideal, but can often be done. See a prosthodontist.

Yes, but...: Yes, but if you have an old crown, why wouldn't you just put a new crown on that tooth? Old dentistry doesn't last forever, it should be replaced periodically to keep everything healthy.

Define veneer: If a tooth has a full crown to begin with, then a full crown to replace it is necessary if a tooth has a partial veneer to start with, then it can be replaced with another veneer or a full crown. A good restorative dentist will guide you as to the best treatment for your particular situation!

You could, but why?: While it may be possible to shave down the front of your existing crown and bond on a porcelain veneer, why would you want to do this? For about the same cost you can get a new crown, which will both look better then a crown/veneer combination and be less prone to future failure.

Not recommended: Not sure what is the reason behind it check with your dentist.