During construction, plumbers install a home's basic plumbing system before fixtures are installed in a process called the plumbing rough-in. During the rough-in, plumbers install the home's drain and waste lines, the pipes that will deliver used water to the sewer or septic tank, and the supply lines, the pipes that will deliver fresh water to the home from the water supply. After the lines are roughed in, they're capped to await the installation of fixtures.
Because the rough-in is usually completed well before fixtures are installed, or perhaps even selected, the roughed-in lines are usually not installed with regard to the requirements of a particular fixture.
There are no universal standards for the height of drain lines and water supply lines in bathrooms, but most plumbers install the lines within a common range of heights to make them compatible with most fixtures.
Sink drain lines are typically between 18 and 20 inches above the finished floor. Water supply lines are usually just a couple of inches higher, between 20 and 22 inches from the floor, and installed one to each side of the drain line. These heights are compatible with most common vanity sinks.
The precise height of the drain and supply lines is usually not a concern during the installation of a vanity sink because the installation process allows for some variation in the position of the lines.
The drain line connects to the sink via a P-trap, a curved piece that prevents sewer gas from entering the bathroom, and a tailpiece, a straight piece that extends from the sink drain to the trap. In most cases, a standard tailpiece is the right length to reach from the sink to the trap, but in the case of a high drain line or an unusually low sink, the tailpiece can be cut to fit. In the case of a low drain line or high sink, an extra-long tailpiece will usually make up the difference.
Roughed-in water supply lines connect to the sink's faucet with flexible supply line connectors. These connectors are typically made from flexible plastic, vinyl or stainless steel, and they're available in a range of lengths. Because they're flexible, they can accommodate supply lines of varying heights.
A problem may arise if the drain line is extremely high or the sink is extremely low so that, after the sink is installed, the sink's drain is lower than the drain line. In this case, the sink will not drain properly because the wastewater from the sink would have to flow uphill.
In some cases, such as with pedestal sinks, the construction of the sink may limit the flexibility of drain or supply line connections. With most vanity sinks, however, the open space under the vanity allows plenty of room for adjustment.
In cases where the drain line emerges from the floor rather than the wall, it may be difficult to install the sink without using an S-shaped trap, a solution that is prohibited by most building codes. In this case, you should consult a plumber to sort out your options.
In extreme cases, the drain line may need to be moved to make the installation work, and that requires the services of a plumber.