A double vanity looks like a simple upgrade, but the plumbing behind it is where the planning happens. Each sink needs hot and cold supply, a drain and a vent. How you arrange those pipes decides whether both sinks drain quietly or gurgle when they run together.
This guide is for homeowners planning a remodel and for anyone reading a contractor's drawing. The rough-in is the plumbing installed in the open wall and floor before the finish work. After the inspection, walls close, and changes cost far more.
Spacing and layout
Standard double vanities put the two sinks at least thirty inches apart, center to center, with the drains in line. A common layout runs both drains into one shared horizontal drain in the wall, which then connects to the vent and the main drain. The shared drain makes the venting question important.
Check the cabinet maker's drawing against the rough-in dimensions. A vanity with a center drawer bank may need the drains offset. Mark the drain locations on the floor and the supply stubs on the wall before the drywall is hung, so the cabinet fits.
| Item | What to confirm |
|---|---|
| Drain size | Each sink drain connects to a pipe sized to code. |
| Vent | The shared drain has a vent within the allowed distance. |
| Supply lines | Hot and cold stubs for each sink, with shutoff valves. |
| Wall blocking | Solid backing for the cabinet and the faucets. |
| Slope | Horizontal drains must slope toward the main drain. |
| Access | A clean path to the main stack or branch line. |
Drain and vent basics
Drain pipes must slope so water and waste move on their own. Vent pipes bring air into the system so water does not suction the traps dry. A drain without a proper vent gurgles and can let sewer gas into the room.
A common approach for a double vanity is a wet vent or a common vent, where one vent serves both fixtures. Code limits the distance between a trap and its vent and sets minimum pipe sizes. The City of Saratoga building division or the Town of Los Gatos can tell you which code edition and rules apply to your address.
Supply lines and shutoff valves
Run hot and cold supply stubs to each sink, with a separate shutoff valve for each line. Hot is on the left when you face the fixture. Pressure should be balanced, so use the same pipe size for both. Leave a little slack in the stubs so the valves can be installed after tile.
If your house has older galvanized or copper lines, this is a good moment to look at the supply side. A remodel that opens the wall is the cheapest time to replace tired lines. See our repiping page for what that involves.
Trap arms, heights and cabinet fit
The trap arm is the pipe between each sink's trap and the vent. Code limits how long it can be and how steeply it can slope. If the drain stub sits too far from the vent, the trap can siphon dry. Installers work backward from the vent location to set the drain height, then check that the cabinet leaves room for both traps.
Vanity depth matters too. A deep cabinet hides the pipes but moves the faucet away from the front edge. A shallow one fits narrow rooms but leaves little space for the traps. Share the cabinet model with your plumber before the rough-in, not after.
What it does to the schedule
The rough-in usually takes a day or two for a vanity alone, and a longer window if the work includes a shower or a toilet move. Plan the inspection date before tile and drywall are ordered. A short delay at inspection can hold up everyone behind it.
Local factors for a Saratoga bathroom remodel
San Jose Water Company serves most Saratoga homes, and its groundwater reads roughly three times harder than Valley Water imported surface supply. Which source reaches your tap depends on your street.
Practical meaning: hard water leaves scale in faucet cartridges and aerators. When you choose fixtures for the new vanity, check that parts are easy to replace, and plan to clean the aerators every few months.
Hillside homes in Saratoga can have long drain runs to the sewer connection. West Valley Sanitation District serves part of Saratoga, according to the Santa Clara County LAFCO service review, so ask whether your lateral and connection are in the district before you add fixtures.
Sources: San Jose Water Company annual water quality report, Santa Clara County LAFCO service review.Permits and inspection
Adding a sink or moving drains in an existing bathroom generally needs a permit. The rough-in inspection comes before the walls close. The inspector checks pipe sizes, slopes, venting and pressure tests. A failed inspection delays the wall work, so it pays to have a licensed plumber do the rough-in and handle the inspection.
California plumbing work above the small-job limit requires a licensed contractor. You can verify a license on the CSLB website. Our plumbing installation page describes how we handle remodel projects, from rough-in to final fixtures.
Common mistakes
Setting the drain too high or too low for the vanity is a frequent one, so measure from the finished floor. Another is skipping the blocking, which leaves faucets and cabinets wobbling. A third is forgetting access to shutoff valves behind a deep drawer.
If you are also changing a toilet or a shower, plan them together. See toilet repair and installation for the toilet side. Los Gatos readers can see our Los Gatos page.
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Questions Saratoga Homeowners Ask
A common minimum is thirty inches center to center. Check your cabinet drawing and code.
Not always. A shared or wet vent can serve both if it meets code spacing. Your plumber confirms the layout.
Usually yes. Ask the City of Saratoga building division about your address.
If they are in good shape, yes. A remodel is a good time to replace old galvanized or corroded lines.
A missing or blocked vent is the usual cause. A plumber can trace the vent.