How to Plumb a Double Vanity from a Single Supply

Upgrading to a double vanity can transform your bathroom—boosting both functionality and resale value. But what if your current setup only has one water supply line and a single drain? Don’t worry. How to plumb a double vanity from a single source is a common challenge, and with the right approach, it’s entirely doable—even for confident DIYers. In this guide, you’ll get a clear, code-compliant roadmap to split your existing plumbing safely and efficiently.

Why Would You Need to Plumb a Double Vanity from a Single Supply?

Many older homes or compact bathrooms were originally built with just one sink. As families grow or lifestyles change, homeowners often seek the convenience of dual sinks. According to the National Association of Home Builders (NAHB), over 78% of new single-family homes now include double vanities in the primary bathroom—a clear sign of rising demand.

But retrofitting isn’t always straightforward. You might face:

• Only one hot and cold water line
• A single P-trap under the sink
• Limited space in wall cavities

The good news? With modern plumbing fittings and smart planning, you can expand your setup without tearing down walls.

Is It Legal and Safe to Split a Single Supply for Two Sinks?

Yes—as long as you follow local plumbing codes (typically based on the International Plumbing Code or IPC). The IPC allows supply lines to be branched using approved fittings, provided:

• Pipe sizing maintains adequate water pressure
• Each fixture has its own shut-off valve
• Drain lines are properly vented

💡 Pro Tip: Always check with your local building department. Some municipalities require permits for plumbing modifications, especially if walls are opened.

For more on plumbing standards, refer to the International Plumbing Code overview on Wikipedia .

Tools and Materials You’ll Need

Before you start, gather these essentials:

Tools:

• Pipe cutter or hacksaw
• Adjustable wrench
• Basin wrench
• Teflon tape
• Bucket & towels (for spills)
• Level

Materials:

• ½” PEX or copper supply lines (10–15 ft)
• Dual shutoff valves (angle stops)
• T-fittings (brass or PEX-compatible)
• Two new P-traps (1¼” diameter)
• Tailpieces and extension kits
• Plumber’s putty or silicone sealant

⚠️ Note: PEX tubing is easier for DIYers—it’s flexible, requires fewer fittings, and resists corrosion. Copper is durable but needs soldering skills.

Step-by-Step: How to Plumb a Double Vanity from a Single Supply

Follow these 7 precise steps to ensure a leak-free, code-compliant install.

Step 1: Turn Off Water & Drain Lines

• Shut off the main water supply.
• Open the existing faucet to relieve pressure and drain residual water.
• Place a bucket under the shutoff valves to catch drips.

Step 2: Remove the Old Vanity & Disconnect Plumbing

• Carefully detach the sink and disconnect supply lines and drain.
• Cap the open pipes temporarily to prevent debris entry.

Step 3: Install a New Supply Line Split

• From the existing hot and cold shutoffs, install ½” T-fittings.
• Run new ½” lines to each side of the double vanity location.
• Use individual angle stops for each faucet—this allows independent shutoffs for maintenance.

✅ Best Practice: Keep supply lines as short and direct as possible to maintain water pressure. Avoid excessive bends.

Step 4: Set Up the Drain System

Here’s where many DIYers stumble. You cannot share one P-trap between two sinks—it violates plumbing code and causes siphoning or sewer gas leaks.

Correct approach:

• Install two separate P-traps, one for each sink.
• Connect both tailpieces to a dual-outlet waste tee (also called a “double fixture fitting”) that feeds into a single drain line.
• Ensure the tee is installed below the overflow level of both basins.

📏 Measurement Tip: The horizontal distance between sink drains should not exceed 30 inches for proper drainage slope (¼” per foot).

Step 5: Mount the New Vanity & Sinks

• Level the cabinet and secure it to wall studs.
• Install faucets before placing sinks to avoid awkward angles.
• Apply plumber’s putty or silicone under sink rims to seal against the countertop.

Step 6: Reconnect All Lines & Test for Leaks

• Attach supply lines to faucets and angle stops.
• Connect drain tailpieces to P-traps.
• Slowly turn water back on.
• Check every connection with a dry paper towel—moisture = leak.

Step 7: Verify Venting

Each P-trap must be vented within 5 feet (per IPC) to prevent airlock and odor backup. In most double-vanity setups, the existing vent serves both if properly tied in. If unsure, consult a plumber.

Double Vanity Plumbing: Common Mistakes to Avoid

Cost Comparison: DIY vs. Hiring a Plumber

📊 According to HomeAdvisor (2024), the national average cost to install a double vanity is $850, including labor and materials. DIY can save 50–70%—but only if done correctly.

FAQ Section

Q1: Can I use one P-trap for two sinks?

No. Plumbing codes (IPC Section 1002.3) prohibit double trapping or shared traps between fixtures. Each sink must have its own P-trap to maintain the water seal that blocks sewer gases.

Q2: Do I need a permit to add a second sink?

It depends on your location. Most cities require permits for new fixture installations, especially if you’re modifying drain or vent lines. Check with your local building department—permits typically cost $50–$150 but protect you during home inspections.

Q3: Will water pressure drop with two sinks?

Not if you use proper ½” supply lines and avoid long, coiled runs. Modern faucets use 1.2–1.5 GPM (gallons per minute), so a standard residential line (40–60 PSI) can easily handle two running simultaneously.

Q4: Can I plumb a double vanity if my wall only has one set of supply lines?

Yes! Use T-fittings to split hot and cold lines just after the shutoff valves. Run new lines to each faucet location—no need to open the wall if you route lines through the vanity cabinet.

Q5: What if my drain is off-center?

Use an offset waste tee or flexible tailpiece extensions to align both sinks with the existing drain. Just ensure the final slope meets code (¼” per foot toward the main stack).

Q6: Should I use PEX or copper for the new lines?

PEX is recommended for DIYers—it’s cheaper, easier to install, and doesn’t require soldering. Copper is more rigid and durable but needs torch skills. Both are code-approved.

Conclusion

Now you know exactly how to plumb a double vanity from a single supply—safely, legally, and affordably. Not only will this upgrade make your mornings smoother, but it can also increase your home’s value by up to 5%, according to Remodeling Magazine’s 2025 Cost vs. Value Report.

If this guide saved you time or confusion, share it with a friend tackling a bathroom reno! Got questions? Drop them in the comments below—we’re here to help.

🔧 Remember: When in doubt, consult a licensed plumber. A small investment in expertise can prevent costly water damage down the line.