Bathroom Upgrade: How To Add a Second Sink to a Bath Vanity

A double vanity is a high-demand add-on to your bathroom that can increase your property value and make your home more marketable. It also makes your bathrooms more functional. If you’re remodeling your bathroom into a space for you and your spouse, both of you can get ready for the day at the same time and organize the space around the sinks as you desire. Outside the master suite, a double sink could also simplify the morning routine for your kids.

The best part is you may not always need to completely replace your vanity to get that double sink you desire. This Old House plumbing and heating expert Richard Trethewey shows you how to convert an existing single vanity into a double vanity. 

Planning Your Double Sink Installation

Before you start, take some time to plan out your installation and gather the right tools.

Assessing Your Existing Vanity

Not every vanity can accommodate a second sink, so determine if yours is right for the job before you start buying materials. Consider these factors:

• Countertop material: You’ll need to cut into your vanity’s countertop to install the new sink. Some materials, like natural stone, may need professional cutting to avoid damage.
• Plumbing access: Different vanities have different storage configurations. Check the space underneath where your second sink will go to be sure that drawers and cabinets don’t get in the way of any necessary plumbing.
• Structural integrity: Sinks can be heavy, so your vanity’s structure should be sturdy enough to support the weight of an additional one.
• Vanity width: Your vanity should be wide enough to fit two sinks comfortably. Vanities that are 60 inches or longer typically fit the bill.

If your existing vanity isn’t the right fit for a second sink, replace it with a prefab vanity that already has a double sink built in.

Tools and Materials

• Adjustable wrench
• Copper pipes and fittings
• Drill/driver
• Hacksaw
• Hole saw bit (1 3/4-inch)
• Jigsaw
• Measuring tape
• New sink and faucet
• PVC pipes and fittings
• Pliers
• Plumber’s putty
• Propane torch
• Safety goggles
• Shut-off valves
• Silicone sealant
• Tubing cutter
• Work gloves

Preparing the Vanity Top for Second Sink Installation

If you’ve determined that your vanity can fit a second sink comfortably, here’s how to get started adding one yourself.

Marking the Sink Location

First, remove the new sink from its packaging and place it upside down on the vanity top. You’ll use it to make the first marks on your countertop.

The sink should be about 2 inches away from your backsplash and 4 1/2 inches away from the end of the countertop. Measure and mark the final position by tracing around the edge of the upside-down sink. Then, double-check your measurements. Make sure that there will still be enough workspace around each sink and that the two sinks are evenly spaced on your vanity.

Cutting the Countertops

Next, take the sink off your countertop. The markings you made will tell you where the sink goes, but you’ll actually use the manufacturer’s template as a guide for the cutting. “That’s the important dimension,” Trethewey says. Trace around the edges of the template so you have both an outer ring and an inner ring marked.

Some countertop materials need professional cutting, but Trethewey’s working with a basic laminate that’s easily cut. He drills four pilot holes around the inner ring, then inserts a jigsaw into the holes and cuts following the line.

Smooth the cut edges with sandpaper to get rid of any rough spots.

Modifying the Existing Plumbing to Accommodate Your New Sink

Next, you must modify your existing drain and water supply piping to accommodate the second sink.

Adapting the Drain System

Trethewey cuts into the existing PVC drainpipe with a hacksaw, then installs a PVC female adapter onto the drainpipe stub protruding from the wall.

With the adapter in place, dry-fit the necessary pipes and fittings to make a new drain configuration. Sometimes, you may need to bore through the undersink cabinets with a hole saw for the pipes to run through, but don’t do this until you know you’ll need to.

Once you’re satisfied with the layout, glue the PVC components together using both PVC primer and cement. Wear a respirator and open your windows for this step if possible, since PVC cement can give off strong fumes.

Updating the Water Supply Lines

The water supply lines will be a little more complicated to update, since you’ll both be routing new pipes and adding new valves. “I want to have shut-offs under each of the sinks for future service,” says Trethewey. He burns off the existing shutoff valves with a propane torch, then installs new copper T-fittings and shutoff valves onto each hot and cold supply pipe. Trethewey also screws bell hangers along the back of the vanity to secure the pipes in place.

Installing the New Sink and Faucet

With the plumbing modifications complete, you can now install the new sink and faucet.

Preparing the Sink

Trim out the sink by connecting the drainpipe underneath. Then, flip the sink over, apply plumber’s putty around the underside of the spout and faucets, and thread them through the right holes in the basin. Secure them from the underside with nuts and washers according to the manufacturer’s instructions. “So we don’t mangle the threads or the chrome, we’ve got a little shoulder on each side right here that allows you to put an adjustable open-end wrench on it,” Trethewey says.

Next, add the drain assembly with plumber’s putty for a tight seal, and adjust the stopper mechanism so it holds tight.

Securing the Sink

Apply a continuous bead of silicone caulk around the underside of the sink’s flange, then carefully lower it into the cut opening in the vanity top. Wipe away any excess with a rag, then allow it to cure overnight.

Connecting the Plumbing to Your New Sink

The final step is to connect all the plumbing components and give the sink a test run.

Connect the PVC drainpipes to both sinks, using plumber’s tape on the threaded fittings to prevent leaks. Next, apply a little pipe joint compound to the threads on the flexible water supply lines, and connect them to the right places on the faucet. Open the shutoff valves to get the water running.

Test the sink by running water through both faucets. Check for proper flow and drainage, and test the stoppers to make sure they can hold water when you need them to.