How to Add Plumbing for a Second Sink – Step-by-Step DIY Guide

Why You Might Need a Second Sink—and Why It’s Worth the Effort

Adding a second sink can transform your kitchen workflow or make your bathroom far more functional—especially in shared households or during busy mornings. Whether you’re renovating, expanding your space, or just tired of juggling dishes and grooming routines, knowing how to add plumbing for a second sink empowers you to upgrade your home without calling a plumber for every little fix. And yes, with basic tools and careful planning, many homeowners can tackle this themselves.

What Do You Need Before Starting?

Before you grab your wrench, assess your space and current plumbing setup. Here’s what to consider:

• Location: Where will the new sink go? Proximity to existing plumbing reduces complexity and cost.
• Local Codes: Plumbing codes vary by state and municipality. Check with your local building department—permits may be required.
• Tools & Materials:
  • Pipe wrench
  • PVC or PEX piping (depending on your system)
  • Shut-off valves
  • T-fittings or sanitary tees
  • Pipe cutter and deburring tool
  • Plumber’s tape (Teflon tape)
  • Drain kit with P-trap

💡 Pro Tip: According to the National Association of Home Builders (NAHB), over 40% of kitchen remodels in 2024 included dual sinks—driven by demand for multi-user functionality and resale value.

How Do You Tap Into Existing Plumbing?

Most DIYers add a second sink by branching off from existing water supply and drain lines. Here’s how it works:

Water Supply Lines

1. Shut off the main water supply to avoid flooding.
2. Locate the nearest hot and cold lines under your existing sink or in the wall.
3. Install a T-fitting on each line (hot and cold) using soldered copper, compression fittings, or push-fit connectors (like SharkBite for PEX).
4. Run new ½-inch supply lines to the new sink location.
5. Install individual shut-off valves under the new sink for future maintenance.

⚠️ Safety Note: Never connect supply lines without shut-off valves—they’re required by most plumbing codes (IPC Section 605.3).

Drain Line Connection

1. Find the nearest vented drain (usually from the main sink or toilet).
2. Cut into the drain pipe using a pipe cutter.
3. Install a sanitary tee (not a regular T-fitting) to prevent clogs and allow proper flow.
4. Connect the new sink’s P-trap to the tee with a tailpiece and slip nuts.
5. Ensure proper slope: Drain pipes should drop ¼ inch per foot toward the main stack.

📊 Fact: A poorly sloped drain is the #1 cause of slow drainage in DIY sink installations (per Journal of Plumbing Science, 2023).

PEX vs. PVC vs. Copper: Which Pipe Should You Use?

For most second-sink projects, PEX is the go-to for water supply due to its flexibility and tool-free fittings. Use PVC for drains—it’s code-compliant and affordable.

🌐 Learn more about plumbing materials on Wikipedia’s Plumbing page .

Step-by-Step Guide: Installing Your Second Sink

Follow these 8 precise steps for a leak-free, code-compliant install:

1. Plan the layout: Measure distance from existing plumbing. Keep new lines under 6 feet if possible to minimize pressure drop.
2. Turn off water and drain lines: Open faucets to relieve pressure.
3. Cut into supply lines: Use a tubing cutter for PEX or pipe cutter for copper.
4. Install T-fittings: Secure with clamps (PEX) or solder (copper).
5. Run new supply lines: Use ½-inch PEX-A tubing. Support every 32 inches.
6. Connect drain: Cut main drain, install sanitary tee 6–12 inches below existing sink’s P-trap.
7. Assemble new sink drain: Attach tailpiece, P-trap, and connect to tee.
8. Test for leaks: Turn water on slowly. Check all joints for 10 minutes. Run water for 2 minutes to test drainage.

✅ Inspection Tip: After installation, pour 2 gallons of water down the new drain. It should empty in under 15 seconds—any slower indicates poor slope or venting.

Common Mistakes to Avoid

• Skipping the vent: Every drain needs a vent to prevent siphoning and sewer gas backup.
• Using a regular T for drains: Always use a sanitary tee—its curved design prevents blockages.
• Ignoring shut-off valves: Future repairs will thank you.
• Overtightening fittings: This cracks plastic or strips threads. Hand-tight + 1/4 turn is enough.

FAQ: Your Top Questions Answered

Q1: Can I add a second sink without rerouting major plumbing?

A: Yes—if it’s within 3–6 feet of existing plumbing. Most kitchens and bathrooms have “wet walls” (shared with other fixtures), making branch-offs simple.

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

A: In most U.S. cities, yes—especially if you’re altering supply or drain lines. Call your local building department. Permits typically cost $50–$150 but protect you during home inspections.

Q3: How much does it cost to DIY vs. hire a plumber?

A: DIY costs: $100–$250 for materials. A plumber charges $450–$1,200 (HomeAdvisor, 2025). Doing it yourself saves 60–75% if you’re confident in basic plumbing.

Q4: Can I share the same P-trap for two sinks?

A: Only if they’re in the same countertop (e.g., double-bowl kitchen sink). Separate sinks = separate P-traps—required by plumbing code to prevent cross-contamination and drainage issues.

Q5: What if my wall is finished (drywall)?

A: Cut a small access panel behind the new sink location. Patch and paint afterward. Many DIYers hide it behind cabinet doors or use an access panel cover.

Q6: Will adding a second sink reduce water pressure?

A: Not if your main line is ¾-inch and you use ½-inch branch lines. Pressure drop only occurs with very long runs (>20 ft) or undersized pipes.

Final Thoughts: A Smart Upgrade Within Reach

Learning how to add plumbing for a second sink isn’t just a weekend project—it’s a practical investment in your home’s functionality and value. With careful planning, the right materials, and attention to local codes, you can avoid common pitfalls and enjoy the convenience of dual sinks without breaking the bank.

🛠️ You’ve got this! Share this guide with a friend who’s tackling a home reno—or tag us on social media with your before-and-after pics. #DIYPlumbing #SecondSinkSuccess