Things you'll need:
- Adjustable wrenches
- 3" drywall screws
- Slip joint pliers
- Tape measure
- Carpenter's level
- Stud finder
- Shims
- Utility knife
- Hole saws
- Power drill
- Caulking gun and caulk
First Things First
The first thing your need to do is get rid of the sink or vanity that's already in your bathroom and get the space ready for your new vanity. Check under the vanity to see how it's attached - probably with some screws through a rail on the back (often called a "nailer"), or in the corners. While your head is under the countertop, look to see how your countertop and sink are attached to the base. Again there may be screws holding the top in place, or it may just be held in place with glue.
Removing your old vanity will be easier if you take off the top (it will be lighter). Finally use a utility knife and run the blade around the edge, cutting through any caulking holding the vanity edges to the walls or backsplash.
Taking Out the Old Vanity
As in any plumbing project, before you begin, turn off the water to your bathroom. Hopefully this means closing the shut off valves on the water feed lines to the vanity, but in some cases it might mean you need to shut off the water to the whole house. You'll need an adjustable wrench to disconnect the supply tubes from the faucet bases, and a large wrench or slip joint pliers to undo the drain. (It's a good idea to have a bucket handy when you open the drain to catch the water that's remaining in the trap.)
Take off the doors and remove the drawers from the vanity. If there is any molding around the base of the vanity, remove it as well. Now get down on your knees and take out the screws holding the vanity to the wall, and if you're going to take off the top, undo the screws holding it in place or use a pry bar to break it away from the base.
You should be able to slide the vanity out and away from the wall, but in some cases you may have to pry it away from the wall. If you do, use a thin scrap of wood to protect the wall from the pry bar. It's a good idea to put down a thin sheet of plywood or even an old blanket to prevent gouging your floor when sliding the vanity.
Once you've removed the old vanity, inspect the area for any damage. This is when you can repair any dings or water damage in the walls and ensure your floor is sound and level.
Preparing the Space and Plumbing
Carefully measure the height and width of your new vanity. Determine where you want to position it and mark the edge locations with vertical lines. Now, measure and mark up from the floor in three places the height of your vanity. Use the highest mark and draw a level line through it, joining the two vertical lines. You now have an exact outline for the placement of your vanity.
Carefully measure the locations of the water pipes and drain lines coming out of the wall and transfer those measurements onto the back of your vanity. Use a hole saw (at least 1/2" larger than the pipes) and drill holes for your water lines. Cut the cutout for the drain with a larger hole saw or a saber saw.
Verify where at least one stud is in the wall behind your new vanity, measure and mark its location on the vanity nailer, and drill a pilot hole for the 3" screw that will hold it in place.
Finally, Installing the New Vanity
Minimize the chance for damage (and make the vanity lighter and easier to move) by taking out any drawers and removing the doors.
Protect your floors and move the vanity into position, then slide it into place with the water and drain pipes coming through the holes you cut in the back. Check that the cabinet is level (both side to side and front to back), and use shims to level it if necessary. Once the vanity is level, attach it to the wall using 3" screws.
Installing the Top and Sink
If your new vanity has a built in or molded sink, you will need to install the faucet before you put the top in place. Start by putting the faucet through the precut holes and hand tighten them. Now turn the vanity top over and use a wrench to firm them up. Attach flexible water supply lines to the base of the faucet and the tail pipe to the sink drain. Finally spread a bead of silicone caulk all around the edge of the countertop, then turn it over and position it on the vanity with the backsplash tight against the wall.
If you're putting a separate sink into your new vanity, you will need to install the countertop first, then install the faucets to the sink, and finally put the sink into the countertop.
Final Steps
Attach the flexible feed lines to the water supply valves and connect the sink tailpiece to the trap and drain. Run a small bead of caulk around the base of the faucet and where the vanity top butts against the backsplash, and then install moldings around the base of the vanity. Finally, turn the water back on and check for any leaks.