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Does going from a single to a double sink in a bathroom vanity require plumbing revisions in the wall, or can the water supply and drain connections be doubled under the vanity?
While you can install T's on the water lines inside the vanity cabinet, the waste & vent are another story. A lot will depend upon local codes. Think of a double-bowl kitchen sink. Both bowls share a common waste & vent, but the bowls are in close proximity to each other. If you install 2 lavatory sinks into a single vanity which are also in close proximity to each other, your single waste line (drain) will function OK, though it may or may not be legal according to local code. If, on the other hand, you install 2 sinks at opposite ends of an 8-foot long vanity, the additional sink wouldn't be properly vented, causing it to gurgle when it drains, and possibly suck the water out of the trap, allowing sewer gasses to enter your bathroom through the drain. It also most certainly wouldn't be allowed per the building code.
You might be able to use a studer valve rather than run a new vent line. But again you will need to check with you local building dept to see if you can use one and what requirements there aRE FOR THEM.
Chris, the Studor [air admittance valve] usually works, but sometimes results in a slower flow of the drain line for the fixture, usually not a problem. But, in your case, it would not be appropriate. The purpose of a vent is to protect the trap from being siphoned dry by the negative pressure created downstream of the trap when other drains are flowing. In your case, the old sink already has its vent, so providing an air admittance valve [AAV] for your new sink would serve no purpose. If the AAV were to be installed upstream of the existing trap and downstream of the new one, you would create a double trap situation which most codes do not allow. If you installed the AAV downstream of the new trap and connected its drain downstream of the existing trap [but outside the wall], you would be double venting, but this would not be a problem with most codes. The easiest solution, if the 2 sinks are, say no more than 4 ft. apart [check your local code for max separation distance allowed], would be to not put a trap on the new sink but slope the drain using an elbow directly to and connected to the tailpiece of the existing drain [using a drain tee], and using the existing sink trap to trap both sinks.