If you're finishing a basement and building a bathroom, the vanity plumbing rough in is the step most DIYers get wrong. I've torn out three finished basements where the stub-outs were either too high or not vented at all. A proper vanity plumbing rough in takes about half a day if you plan it around the actual vanity you're installing. Here's how I do it.

What a Vanity Plumbing Rough In Includes

The rough-in is the set of pipes you install before drywall: the 1 1/2-inch drain stub-out, the hot and cold supply lines, and a vent that reaches the roof or ties into a wet vent. For a basement, that usually means cutting into the concrete slab to run the drain under the floor, or building a platform to hide a raised drain run. The goal of the rough-in is to have the pipes exit the wall exactly where the vanity will sit, at heights that let the p-trap and supply stops connect without fighting cabinet drawers.

The Heights That Matter

The most important number is the drain height. For a standard bathroom vanity, set the drain stub-out at 16 inches above the finished floor, measured to the center of the pipe. That works with almost every 21-inch-deep vanity cabinet sold at the big-box stores. A wall-hung sink or an unusually high vanity might need 18 inches, so mock up the cabinet first if you're not using a typical one.

The supply lines go at 20 to 22 inches above the finished floor, 12 inches apart center-to-center. That spacing aligns with standard faucet supply hoses and puts the shutoff valves below the sink bowl but above the drain line. I use drop-ear ells for both hot and cold and make sure they're perfectly level so the finished wall looks right.

Venting Rules You Can't Skip

The vanity drain needs a vent within 6 feet of the trap for 1 1/2-inch pipe under most versions of the International Residential Code. A dedicated 1 1/2-inch vent run up to the roof is the safest bet. You can use a wet vent in many jurisdictions, but if an inspector says it needs to tie into a stack a certain way, do it that way. Don't vent a basement vanity into a floor drain — I've seen it fail inspection more than once.

Step-by-Step Rough-In Order

Here is the order I use on every vanity plumbing rough in. First, measure the vanity location and mark the centerline on the wall and floor. Cut the slab with a wet saw, dig a trench 8 inches deep, and lay the drain pipe at 1/4 inch per foot slope toward the main drain. Dry-fit the whole run, check the 16-inch height, then glue and test with water. After the drain is solid, run the supply lines in PEX or copper to the drop-ear ells. Cap everything, seal the openings with foam, and pour the concrete back flush.

A common mistake is forgetting a cleanout near the drain. I add a cleanout adapter inside the stub-out so a plumber's snake can reach the line later. Another mistake is using a sanitary tee on its back for the vent connection. Use a proper wye fitting or a dedicated vent connection.

Common Mistakes and How to Avoid Them

The biggest mistake I see in finished basements is setting the drain too high. A 20-inch drain stub-out puts the p-trap above the sink tailpiece and forces the pipe to dive before it can enter the wall. Always verify the vanity's internal clearance before you glue.

Another issue is roughing in supplies before buying the vanity. The cheapest solution is to buy the vanity first, then set the pipes to its exact dimensions. If you're using a pedestal sink, the stub-out locations shift dramatically. Draw it out on the floor with chalk before you cut concrete.

The cost of a vanity plumbing rough in depends on how far the new pipes run from the main stack. In a typical basement bathroom, plan on $150 to $400 for pipe, fittings, and the drop-ear ells. If you're hiring it out, a plumber will charge $400 to $800 for the rough-in, plus the extra for a permit where your local code requires one.

Tools and Parts I Actually Use

My kit for this rough-in is short: a cordless PEX or copper cutter, a 1 1/2-inch PVC cutter, a torpedo level, and thread tape. For fittings, I reach for Oatey or Sioux Chief — brands I've installed in hundreds of bathrooms without a callback. If you're doing PEX, the Milwaukee M12 proPEX expansion tool costs about $350, but it's worth it if you're plumbing a full basement. For the stub-outs, a brass drop-ear ell keeps the pipe rigid behind the finished wall.

Bottom Line

A vanity plumbing rough in is a half-day job that saves you from cutting into a finished wall later. Set the drain at 16 inches, the supplies at 20, slope the drain 1/4 inch per foot, and vent it properly. Do that and the faucet connects in minutes. Plan the whole vanity before you cut the slab, and you'll never have to chase a leak. Above grade is for the real estate photos. Below grade is where you actually live.