Multiple Bathroom Sink Plumbing: Double Vanity Guide

Double vanity bathrooms — two sinks in one vanity cabinet — are standard in primary bathrooms. Adding a second sink to an existing single-sink bathroom is one of the most common plumbing requests in bathroom remodels. The plumbing isn’t complicated in concept, but the execution depends on what’s accessible below the floor and how the existing drain is configured. Here’s how multiple bathroom sink plumbing works, what the drain configurations look like, and what adding a second sink costs.

How Double Vanity Sink Drains Work

Standard double vanity drain configuration:

Each sink has its own drain strainer, drain tailpiece, and P-trap. The two P-trap outlets connect to a shared drain pipe — a tee or cross fitting that combines both drain flows into one run to the drain connection in the wall.

The P-trap requirement:
Each individual sink needs its own P-trap. The P-trap is the curved section of pipe that holds water — this water barrier is what prevents sewer gas from entering the bathroom. You cannot remove one P-trap from a two-sink arrangement and share one trap between two sinks.

Shared drain pipe:
After both P-traps, the drain lines combine and run to the common drain connection at the wall. This combined run can typically handle both sinks — the drain sizing is sufficient for two standard bathroom sinks sharing one drain run.

Venting:
Both sinks share the same vent (the existing vent connection in the wall). This is allowed under plumbing code when configured correctly — the shared vent serves both fixture drain connections.

Adding a Second Sink to an Existing Vanity Space

Scenario: Same vanity, second sink added where there was one sink

This is the most common double vanity addition — replacing a single-sink vanity with a double-sink vanity in the same location, using the same drain connection and supply locations.

What’s involved:
1. Remove existing single-sink vanity and plumbing
2. Install new double vanity cabinet (wider, with two sink openings)
3. Connect both sink drains to the existing drain connection
4. Run supply lines (hot and cold) to both faucet locations
5. Install faucets and drains on both sinks

Does this require a permit?
Typically no — if you’re using the existing drain connection and supply locations, and the new vanity occupies the same general footprint, this is replacement in kind. The drain rough-in in the wall doesn’t change.

Cost: $500–$1,500 for plumbing labor, not including the vanity, sinks, and faucets (which are separate fixtures costs).

Adding a Second Sink to a New Location

Scenario: Adding a sink where one didn’t exist, or adding a sink in a different part of the bathroom

This requires new drain and supply rough-in at the new sink location — permit-required work.

What’s involved:
1. New drain connection at the new sink location
2. Run drain from new location to existing drain in the wall (requires opening floor or wall)
3. Run supply lines (hot and cold) to the new location
4. Vent configuration for the new drain location

Does this require a permit?
Yes — new drain rough-in requires a permit. Inspection before walls are closed.

Cost: $1,500–$3,500+ for plumbing labor, depending on access and distance from existing connections.

Drain Sizing for Two Bathroom Sinks

Standard bathroom sink drain sizing:
– Each sink: 1.5-inch drain (standard)
– Shared drain run: 1.5 inches (sufficient for two bathroom sinks — 2-inch is preferred for any run over 8 feet)
– Connection to the stack or main drain: existing drain run sizing (typically 2–3 inch)

The UPC (Uniform Plumbing Code, Seattle’s code base) assigns drainage fixture units (DFU) to each fixture. A standard bathroom lavatory is 1 DFU. Two lavatories sharing a drain is 2 DFU — well within the capacity of a 1.5-inch drain run.

When larger drain sizing is needed:
If the drain run to the connection is long (over 8 feet), or if additional fixtures connect to the same run, sizing up to 2 inches is appropriate and provides better drain performance.

Venting Two Bathroom Sinks

Double vanity sinks sharing a drain can use a single vent connection.

Wet venting:
One common approved configuration for double vanity sinks is wet venting — the drain run from one or both sinks connects to the vent at an appropriate point, allowing one vent to serve both fixtures. This is standard practice for double vanity installations and is code-compliant when configured correctly.

Common vent:
Both sink drains connect to a common horizontal drain run that connects to the existing vent. The vent serves the shared drain run. This is the most typical configuration in practice.

What doesn’t work:
Two sinks sharing one P-trap (without individual traps) — this is a code violation. Each sink must have its own trap. The shared part is the drain run after the traps, not the traps themselves.

Can a Bathroom Sink Share a Drain with a Toilet?

No — the toilet has its own drain and drain connection.

The toilet drain (3 or 4 inches) connects directly to the drain stack or a large-diameter drain run. Sink drains (1.5 inches) cannot connect into the toilet’s drain connection.

Sinks and toilets in the same bathroom typically connect to the same drain stack — but through separate connections on that stack, not through each other.

The ventilation connection:
Sinks and toilets can share a common vent — the vent stack that serves the toilet can also serve nearby sink drains when the connections are within the code-required distances. This is different from sharing a drain pipe.

Slow Drain or Gurgling in Double Vanity Sinks

If one sink drains slow or gurgles when the other drains:

This indicates a problem with the shared drain configuration:
– Insufficient slope on the shared horizontal drain run
– The drain connection point is too far from the trap, causing the drain flow from one sink to siphon the other
– A partial blockage in the shared drain run affecting both sinks
– Incorrect vent configuration causing negative pressure

Diagnosis:
A plumber can assess whether the problem is a blockage (most likely in older homes) or a configuration issue. Hydrojetting the shared drain often resolves blockage-caused gurgling. Configuration problems may require rerouting.

Second Bathroom Sink Installation Cost in Seattle (2026)

Costs are for plumbing labor. Vanity, sinks, faucets, and any tile or carpentry work are separate.

FAQ

Q: Can two bathroom sinks share one drain?
A: Yes — the standard double vanity configuration has two sinks, each with their own P-trap, both connecting to a shared drain pipe that runs to the wall drain connection. This is code-compliant and standard practice.

Q: Do both sinks in a double vanity need their own P-trap?
A: Yes — each sink must have its own P-trap. The P-trap is what prevents sewer gas from entering the bathroom. You cannot share one P-trap between two sinks.

Q: Does adding a second sink require a permit in Seattle?
A: It depends on scope. Adding a second sink using the existing drain rough-in (in the same vanity location): typically no permit required. Adding a second sink at a new location requiring new drain rough-in: permit required.

Q: How much does a second bathroom sink installation cost?
A: $500–$1,500 for plumbing if the existing drain connection can serve both sinks (same vanity location). $1,500–$3,500+ if new drain rough-in is required at a new location. Vanity, sinks, and faucets are additional costs.

Q: Why is one sink in my double vanity slow or gurgling when the other drains?
A: Most commonly a blockage in the shared drain run that affects both sinks, or a vent problem causing negative pressure. A plumber can diagnose with a drain camera to determine whether it’s a blockage or a configuration/venting issue. Hydrojetting the shared drain resolves most blockage cases.