How to Plumb a Single Sink to a Double Sink (Step-by-Step Guide)

Upgrading from a single sink to a double sink can dramatically improve your kitchen’s functionality—especially if you cook often or share the space with family. But you might be wondering: Is it even possible to plumb a single sink to a double sink without calling a pro? The good news: yes, and with the right tools and guidance, you can do it yourself. In this guide, we’ll walk you through exactly how to plumb a single sink to a double sink, step by step, while keeping your plumbing code-compliant and leak-free.

Can You Convert a Single Sink to a Double Sink?

Absolutely—if your countertop, cabinetry, and plumbing layout allow it. Most standard kitchen cabinets (30–36 inches wide) can accommodate a double-basin sink, provided there’s enough space and structural support. However, the real challenge lies in the drain and supply lines.

According to the International Residential Code (IRC), kitchen sinks must have proper venting and trap configuration to prevent sewer gases from entering your home. Thankfully, converting from single to double doesn’t require a full replumb—just smart adaptation of your existing setup.

💡 Pro Tip: Before you start, shut off the water supply and place a bucket under the P-trap to catch residual water.

Tools & Materials You’ll Need

Gather these before you begin:

• Adjustable wrench
• Pipe wrench
• Basin wrench
• Teflon tape
• PVC or ABS pipes & fittings (1½” diameter for drains)
• Double sink tailpieces
• Continuous waste kit (includes Y-branch or T-fitting)
• Supply lines (braided stainless steel recommended)
• Plumber’s putty or silicone sealant
• Bucket & towels

Note: Always check local plumbing codes. Some municipalities require permits for sink modifications.

Step-by-Step: How to Plumb a Single Sink to a Double Sink

Step 1: Remove the Old Sink & Disconnect Plumbing

1. Turn off hot and cold water valves under the sink.
2. Open the faucet to release pressure and drain remaining water.
3. Place a bucket beneath the P-trap and unscrew it using a wrench.
4. Disconnect supply lines from the faucet.
5. Loosen mounting clips and lift out the old sink.

⚠️ Safety First: Wear gloves and eye protection—old pipes may be corroded or sharp.

Step 2: Prepare the Countertop for the New Sink

• If your countertop already has a double-sink cutout, great!
• If not, you’ll need to cut a new opening (use the sink template provided by the manufacturer).
• Apply plumber’s putty or silicone around the rim before setting the new sink.

Step 3: Install Faucet & Accessories

• Most double sinks use a single faucet centered between basins.
• Install it before connecting drains—this gives you more room to work.
• Tighten mounting nuts from below using a basin wrench.

Step 4: Connect the Drain System

This is the core of how to plumb a single sink to a double sink:

1. Attach tailpieces to each basin’s drain outlet.
2. Use a continuous waste kit to join both tailpieces into a single drain line.
   • This typically includes a Y-branch fitting that merges the two flows.
3. Connect the Y-branch to your existing P-trap.
4. Ensure the trap remains within 24 inches of the sink drain (per IRC P3201.7).

📏 Measurement Matters: The horizontal arm from the trap to the vent should slope ¼ inch per foot to ensure proper drainage.

Step 5: Reconnect Water Supply Lines

• Attach new braided stainless steel supply lines to the faucet.
• Connect them to the shutoff valves—hand-tighten first, then give a quarter-turn with a wrench.
• Do not overtighten—this can crack fittings or cause leaks.

Step 6: Test for Leaks & Proper Drainage

1. Turn water back on slowly.
2. Run both hot and cold water in each basin.
3. Check all connections for drips.
4. Fill each basin halfway, then drain simultaneously—watch for slow drainage or gurgling (signs of poor venting).

If everything flows smoothly and stays dry, you’re done!

Common Mistakes to Avoid

For more on plumbing standards, see the Wikipedia entry on plumbing .

Double Sink vs. Single Sink: Is It Worth It?

Pros of a Double Sink:

• Separate prep and cleanup zones
• Easier to wash large pots and pans
• Better for households with multiple cooks

Cons:

• Takes up more counter space
• May require minor cabinetry adjustments
• Slightly more complex plumbing

📊 A 2023 survey by HomeAdvisor found that 68% of homeowners who upgraded to a double sink reported higher kitchen satisfaction—especially in homes with 3+ occupants.

FAQ: Your Top Questions Answered

Q1: Do I need a new P-trap when switching to a double sink?

A: Not necessarily. If your existing P-trap is in good condition and properly sized (1½”), you can reuse it. Just ensure the new continuous waste kit connects securely to it.

Q2: Can I use the same faucet with a double sink?

A: Yes—most double sinks are designed for a single faucet mounted between basins. Just confirm your faucet’s reach covers both bowls.

Q3: How much does it cost to convert a single sink to a double sink?

A: DIY cost: $150–$400 (sink, fittings, tools). Hiring a plumber: $400–$900, depending on location and complexity.

Q4: What if my cabinet is too narrow?

A: Standard double sinks need at least 30 inches of cabinet width. If yours is smaller, consider an offset double sink or a single bowl with a built-in prep area.

Q5: Do I need a permit for this plumbing change?

A: In many U.S. cities, minor fixture replacements don’t require permits—but major drain rerouting might. When in doubt, call your local building department.

Q6: Can I do this without plumbing experience?

A: If you’re comfortable with basic tools and follow instructions carefully, yes. But if you see rusted pipes, outdated galvanized lines, or no shutoff valves, consult a licensed plumber.

Conclusion

Converting from a single sink to a double sink is a smart, affordable upgrade that boosts both function and resale value. With this guide on how to plumb a single sink to a double sink, you now have the confidence and know-how to tackle the project safely and correctly.

✅ You’ve learned:

• How to adapt your existing drain system
• What tools and parts you truly need
• Common pitfalls (and how to avoid them)
• Whether your kitchen is ready for the switch

If this saved you a service call or inspired your next DIY project, share it with a friend on Pinterest, Facebook, or Instagram! Got questions? Drop them in the comments—we’re happy to help.

Remember: Great kitchens start with smart plumbing. 💧🔧