How to Add a Second Bathroom Sink

Adding a second bathroom sink requires extending plumbing lines, installing a new vanity or countertop, and connecting drain and supply lines. Most installations take 4-6 hours and cost $300-800 depending on the sink style and plumbing complexity.

1. Measure Twice, Cut Zero. Measure your bathroom to determine where the second sink will fit. Most double vanities need at least 60 inches of wall space total, with 30 inches minimum per sink. Check local building codes for required spacing between fixtures and ensure you have adequate clearance for doors and movement. Verify that your existing electrical outlet can handle additional lighting if needed.
2. Clear the Slate. Turn off the main water supply and drain the existing sink completely. Remove the old vanity top, sink, and any cabinets that need modification. If you're keeping the existing vanity base, you may need to cut openings for the second sink. Clean the wall area and check for any plumbing or electrical issues behind the vanity.
3. Set the Foundation Right. Mount your new double vanity or modify your existing cabinet to accommodate two sinks. Secure the vanity to wall studs using appropriate screws. If using a stone or heavy countertop, ensure the vanity can support the additional weight. Level the vanity carefully since an uneven base will cause problems with the countertop installation.
4. Branch the Water In. Run new copper or PEX supply lines from your existing plumbing to the location of the second sink. Install shut-off valves for both hot and cold water lines at each sink location. Use pipe hangers to secure lines inside the vanity. Test connections with water turned on briefly to check for leaks before proceeding.
5. Route the Waste Out. Connect the new drain line to your existing waste line using appropriate fittings. Install a P-trap for the new sink, ensuring proper slope for drainage. The drain should slope toward the main line at 1/4 inch per foot. Use PVC primer and cement for all drain connections and test the system by running water through both drains.
6. Secure the Workspace. Install your countertop according to the manufacturer's instructions. For undermount sinks, apply silicone caulk around the rim before securing with clips from underneath. For drop-in sinks, place them in the pre-cut openings and secure with the provided clips. Ensure both sinks sit level and flush with the countertop surface.
7. Fill Both and Check. Install faucets on both sinks, connecting them to the supply lines with braided supply hoses. Hand-tighten connections first, then use a wrench to snug them without overtightening. Turn the water back on and test both sinks thoroughly, checking for leaks at all connections. Run both hot and cold water for several minutes to ensure proper flow and temperature.
8. Seal and Polish. Caulk around the sink edges, backsplash, and any gaps between the vanity and wall. Use a quality bathroom caulk that resists mold and mildew. Install any trim pieces, drawer pulls, or cabinet hardware. Clean up any construction debris and test all plumbing connections one final time before considering the job complete.