Tired of sharing a single sink with your partner or family? You’re not alone—over 68% of U.S. homeowners consider dual-sink vanities a top priority during bathroom renovations (National Association of Home Builders, 2024). If you’re wondering how to change bathroom plumbing from one sink to two, you’ve come to the right place. This guide walks you through the entire process—from planning and permits to final testing—with safety, efficiency, and building codes in mind. Whether you’re a confident DIYer or just scoping out the project before hiring a plumber, we’ve got you covered.
Why Add a Second Sink? Real Benefits Backed by Data
Before diving into pipes and fittings, it’s worth understanding why this upgrade matters:
Increased home value: Homes with dual vanities sell 7–10% faster and often at a premium (Realtor.com, 2025).
Reduced morning chaos: No more toothbrush traffic jams.
Better space utilization: Modern double vanities are designed for compact bathrooms, thanks to wall-mounted and corner models.
“Adding a second sink is one of the highest-ROI bathroom upgrades under $3,000,” says Maria Chen, certified master plumber and founder of AquaFlow Solutions.
Can You Actually Split One Sink’s Plumbing Into Two?
Yes—but only if your existing supply and drain lines can support the added demand.
Most standard bathroom sinks use:
½-inch water supply lines (hot and cold)
1½-inch P-trap and drainpipe
To add a second sink, you’ll typically:
Tee off the existing hot and cold supply lines
Branch the drain line using a sanitary tee or double fixture fitting
⚠️ Critical Check: Your main vent stack must accommodate the additional fixture. According to the International Plumbing Code (IPC), two lavatories within 6 feet can share a single vent—but beyond that, you may need a new vent or air admittance valve (AAV).
Double sanitary tee or double fixture fitting (1½” ABS or PVC), pipe cement (if using PVC)
Extras
Plumber’s putty or silicone sealant, Teflon tape, bucket, towels
💡 Pro Tip: Use PEX tubing instead of copper if allowed in your area—it’s faster to install, cheaper, and freeze-resistant.
Step-by-Step Guide: How to Change Bathroom Plumbing from One Sink to Two
Step 1: Turn Off Water & Drain Lines
Shut off the main water supply or dedicated bathroom shutoff valves.
Open both hot and cold faucets to relieve pressure and drain residual water.
Place a bucket under the P-trap to catch drips.
Step 2: Remove the Existing Sink (If Replacing)
Disconnect supply lines and remove the P-trap.
Carefully detach the sink from the countertop or wall.
Cap or plug open pipes to prevent sewer gas entry.
Step 3: Plan the New Layout
Measure the distance between sink centers—minimum 30 inches is ideal for comfort (NKBA recommends 36″).
Ensure both sinks are within 6 feet of each other to share a vent legally under most codes.
Step 4: Install New Supply Lines
Cut into the existing hot and cold supply lines.
Install ½-inch PEX or copper tees.
Run new lines to each sink location.
Install individual shut-off valves under each sink (required by code in most states).
✅ Use 12–16 inch flexible supply hoses—long enough for easy connection but not so long they kink.
Step 5: Modify the Drain System
Remove the old P-trap assembly.
Install a double fixture fitting (also called a “dual lavatory waste fitting”) directly below the floor or in the wall cavity.
Connect each sink’s tailpiece to the fitting using 1½-inch trap arms.
Ensure proper slope: ¼ inch per foot toward the main drain.
📏 Example: If sinks are 36″ apart, the center of the fitting should align with the midpoint, and each arm slopes downward at 0.25″/ft.
Step 6: Install New Sinks & Faucets
Set sinks in place (use silicone or clips as needed).
Attach faucets before mounting if possible—it’s easier.
Reconnect supply lines and P-traps.
Step 7: Test for Leaks & Proper Drainage
Slowly turn water back on.
Check all connections for leaks—especially under pressure.
Run water in both sinks simultaneously for 2–3 minutes.
Watch for slow drainage or gurgling—signs of venting issues.
Common Mistakes to Avoid
Mistake
Consequence
Fix
Skipping individual shut-off valves
Whole system must be shut down for repairs
Always install valves per sink
Ignoring vent requirements
Gurgling drains, sewer odors
Use AAV or tie into existing vent within 5 ft
Using mismatched pipe materials
Corrosion or leaks
Stick to one material type (e.g., all PEX or all copper)
Over-tightening compression fittings
Cracked nuts or stripped threads
Hand-tight + 1–2 turns with wrench only
Cost Breakdown: DIY vs. Hiring a Pro
Expense
DIY Cost
Professional Cost
Materials (fittings, valves, pipes)
$120–$250
Included in labor
New faucets & sinks (optional)
$200–$1,200+
$300–$2,000+
Labor
$0
$450–$1,200
Permits (if required)
$50–$150
Usually included
Total
$370–$1,600
$800–$3,500
💬 Note: In cities like Chicago or Seattle, plumbing permits are mandatory for fixture additions. Always check local codes.
FAQ Section
Q1: Do I need a permit to add a second sink?
A: In most U.S. municipalities, yes—especially if you’re modifying drain or vent lines. Minor supply-line changes may be exempt, but it’s safest to call your local building department. Unpermitted work can void insurance or complicate home sales.
Q2: Can two sinks share one P-trap?
A: Generally, no. The IPC requires each fixture to have its own trap to prevent sewer gas backup. However, a dual fixture fitting connects two traps to a single drain line—this is code-compliant and commonly used.
Q3: What if my bathroom has no access from below (e.g., slab foundation)?
A: You’ll need to open the wall behind the vanity or cut into the floor. Wall access is preferred. Use an inspection camera first to locate pipes and avoid electrical lines.
Q4: Will my water pressure drop with two sinks?
A: Only if your main supply line is undersized (e.g., ½-inch feeding an entire house). In modern homes with ¾-inch or larger mains, running two lavatories simultaneously causes negligible pressure loss.
Q5: Can I do this with only one water shutoff valve?
A: Technically yes, but it’s not recommended. Individual shut-offs let you service one sink without disrupting the whole bathroom—a small upgrade with big convenience.
Q6: How long does the project take?
A: A skilled DIYer can complete it in 6–10 hours over a weekend. Beginners may need 2 days. Professionals typically finish in 4–6 hours.
Conclusion
Upgrading from one sink to two isn’t just about luxury—it’s a smart, functional improvement that boosts daily convenience and home value. Now that you know how to change bathroom plumbing from one sink to two, you’re equipped to tackle this project safely and efficiently.
✅ Remember:
Follow local plumbing codes
Vent properly
Test thoroughly
If you found this guide helpful, share it on Pinterest, Facebook, or with a friend remodeling their bathroom! Got questions? Drop them in the comments—we love helping homeowners succeed.
Disclaimer: This guide is for informational purposes. When in doubt, consult a licensed plumber.
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