Bathroom vanities are attached to the walls and sometimes to the floor, too, making them essentially a permanent fixture. The materials include more than just caulk and adhesive; you’re looking at bolts and pipe attachments as well. To modify the height of the vanity permanently, you must uninstall it completely, and reinstall it at the new height. This is a little easier if you have a vanity that attaches to the wall only, although you still must make some major adjustments to other fixtures. Vanities that sit directly on the floor are more difficult to adjust, because, if you are raising them, you have to find something to set under them that is sturdy enough to handle their weight. If you want to lower them, you must chop off part of the vanity, and adjust or remove doors and drawers.
In addition to dealing with the height of the vanity itself, you have to adjust everything around the vanity, too. Many bathrooms have mirrors that are large panels placed right at the edge of the vanity; you must cut or change the mirror if you raise the vanity. Outlets pose a problem, too, because the new height of the vanity top could interfere with access to the circuitry. Lowering the vanity wouldn’t create issues with mirrors and outlets, other than repainting the newly exposed portions of the wall, but you would have to deal with shortening the sink faucet and drainpipes. Finally, if your toilet paper holder is bolted to the vanity, you don’t want to end up with it placed too high or too low.
If you need to raise the vanity so you are not bending down as much to reach items on the surface, but you do not want to do all the construction work needed to move it, you may prefer to add countertop shelves or plastic storage drawers. Install faucets that are slightly taller, too, so that you don’t have to bend down quite as much to wash your hands. Lowering the height because someone who is short is uncomfortable reaching for the faucets or items in back is much tougher. If the person does not want to or can’t stand on a stepstool, you might have to uninstall and move the vanity.
Do not move large mirrors if you can’t handle the weight, and do not attempt to modify pipes or move outlet wiring if you don’t have prior plumbing or electrical experience. Call a plumber and electrician for these jobs, because working on these yourself when you don’t have experience and adequate knowledge puts you at risk for flooding and electrocution.