I am remodeling my bath. My wife bought a new vanity cabinet that came pre-plumbed with sinks in the counter top, faucets, drains, traps and piping. The existing bath drain stubs from the wall are 18" from the floor. The new vanity has drain piping 24" from the floor (could be adjusted to 22" from the floor). Obviously the new will not line up with the old. I supposed I could add and extension to the sink drain pipe to lower the trap and drain pipe to 18". The only problem is the new vanity has a shelf ~19" from the floor. Other than cutting a hole for the trap and a slot for the drain pipe and adding length to the sink drain stem is there any other suggestions??

You have one option and just explained it to us quite well. That shelf must be cut unless you want to open the wall and try to cut and install a new drain lateral. Are you drains PVC? It might not be as bad as you think to raise the drain lateral.

Bob nailed it! You could attempt to raise the sanitary tee and stubout but that's a real hassle. Better to bite the bullet and cut the shelf. Extend the tailpiece using either a threaded tailpiece of a compression tailpiece extension .(see image)

I think you should notch the shelf. Many people do so. Some vanities have cut out in the shelf to accommodate drain installation. Or you can drill nice hole in the shelve and drop the drain pipe through it. Unless you have wall open, raising San T may be quite a messy work for you. But, it is an option, for sure... Let us know what you decided to do... Good Luck... Milo

Thanks to all that responded. I agree that the best option is to notch the shelf. I just wanted to make sure that I had not overlooked an obvious option. Thanks again to all.