Cost to Add a Second Sink (Double Vanity) to a Bathroom in California
A single sink and two people getting ready at the same time is a daily bottleneck in a lot of households, which is why "can we just add a second sink" is one of the more common questions California plumbers get during a primary bathroom refresh. The answer is almost always yes — but the price depends entirely on one thing: whether there's already a way to drain and vent a second fixture nearby, or whether a plumber has to open a wall or floor to create one.
Typical cost to add a second sink
Converting a single vanity to a double, including the plumbing work and a basic double-sink top, typically runs:
- Straightforward conversion — existing rough-in already has room, drain and vent lines are accessible behind the same wall: roughly $800–$2,000.
- Standard job — new drain tie-in, extending the water supply lines, wet-venting the second lavatory off the existing vent: roughly $1,200–$3,000.
- Wall or floor opened — the new sink location has no nearby drain/vent access, requiring new piping run through a finished wall, slab, or crawlspace: roughly $2,000–$4,500+.
Those figures cover plumbing labor, the drain/vent/supply work, and a basic double-sink vanity top; they don't include cabinetry, countertop upgrades, or electrical (outlets, lighting) if the layout changes. For a fixture-only faucet swap without adding a drain, see our faucet and sink service page instead — that's a different, much cheaper job.
Why the drain and vent, not the sink, sets the price
The sink, faucet, and countertop cutout are the easy part. What actually determines cost is whether the second sink's drain can tie into an existing branch and share the existing vent stack, or whether a plumber has to run new pipe to get there. California follows the Uniform Plumbing Code's wet-venting rules, which allow two lavatories in the same bathroom group to share a single 2-inch vent as long as each sink keeps its own P-trap and each fixture drain connects independently into the shared horizontal wet vent — no trap-sharing, and no more than two lavatories draining as 1 DFU each (2 DFU combined) through a properly sized run. When the existing vanity's rough-in was built with a stub already capped for a future second sink (common in homes built with a double-vanity option in mind), the job is close to plug-and-play. When it wasn't, a plumber has to fish new drain and vent pipe to the new location, which is where the price jumps.
Water supply lines
Extending hot and cold supply lines to a second sink location is usually the least expensive part of the job — a few feet of PEX or copper branching off the existing lines, each with its own shutoff valve. The exception is a vanity relocated to the opposite wall or a different part of the room, which means running supply lines the same distance as the drain and vent, adding labor rather than materials cost. If the existing shutoff valves under the current sink are old multi-turn stops rather than quarter-turn ball valves, most plumbers will recommend replacing them at the same time since the wall is already open — a small add-on, not a separate trip.
Permit requirements
Adding a drain, vent, or new water supply line is plumbing work that requires a permit in virtually every California jurisdiction — a straight fixture swap (same drain, same supply) typically doesn't, but adding a second sink almost always does, because it adds a new drain/vent connection to the system. Skipping the permit on this kind of work is a common reason double-vanity conversions surface as problems during a pre-listing plumbing inspection years later, when an inspector finds unpermitted drain work behind the wall.
Space and layout constraints
Before pricing the plumbing, confirm the vanity footprint actually has room: a comfortable double-sink layout needs roughly 60–72 inches of counter width so the two basins and faucets aren't crowded against each other or the wall, and local code sets minimum clearances between a sink's centerline and any adjacent wall or fixture. In a smaller primary bathroom, this sometimes rules out a true double vanity and points toward a single elongated trough-style sink with two faucets instead — which uses one drain and one vent rather than two, and is meaningfully cheaper on the plumbing side even though it looks similar from the front. A plumber or bathroom designer can usually tell within a few minutes of measuring which option the room actually supports.
Is it worth it?
Beyond convenience, bathroom count and configuration are consistently one of the stronger drivers of home value in real estate pricing research. A 2021 hedonic price study of the Boulder, Colorado housing market by economist Mahdieh Yazdani, published via arXiv, modeled residential sale prices against structural characteristics and found bathroom-related features to be statistically significant, economically meaningful contributors to sale price across market segments (study on Google Scholar). That study looked at bathroom count broadly rather than double-sink vanities specifically, but it lines up with what real estate agents in California routinely tell sellers: a primary bathroom with two sinks is a feature buyers actively look for, particularly in family-oriented markets, and it tends to outperform a comparable single-sink layout at resale — one reason this upgrade is often bundled into a broader bathroom remodel rather than done as a standalone project.
Getting an accurate quote
Because the cost swings so much on what's hidden behind the wall, the only reliable way to get an accurate number is to have a plumber look at the existing rough-in in person — ask specifically whether the quote assumes the new drain/vent ties into the existing stack or requires new pipe run, since that single variable explains most of the price range above. If the vanity area is also getting new countertops or cabinetry, coordinate the plumbing rough-in first so the cabinet maker can cut openings to match the final drain and supply locations, not the other way around.
Request a free quote on Plumber Comparator to compare licensed local plumbers who handle double-vanity conversions across California, including plumbers serving San Diego and Sacramento.
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