A drip usually means a seal is no longer closing properly
A tap that drips after it has been turned off is normally failing to shut the water off completely. On an older compression tap, a worn washer or damaged valve seat is a common cause. Modern quarter-turn taps usually use ceramic cartridges rather than rubber washers, so changing a washer will not solve the problem. Mixer taps can have cartridges or small seals inside the body. Before buying parts, identify the tap type and, if possible, the manufacturer because apparently similar cartridges often have different dimensions.
The pattern of the drip can be useful
A tap that drips from the spout after both handles are closed has an internal shut-off problem. Water appearing around the base of a swivel spout suggests worn O-rings instead. A leak around the handle can point to a spindle seal or cartridge issue. If water only appears beneath the sink when the tap is running, the problem may be a flexible tail or connection rather than the tap mechanism itself. These distinctions matter because they determine whether the tap can be serviced from above or whether the connections underneath also need attention.
Before attempting a repair
Check that you can isolate the water to the tap. Many modern installations have small isolation valves on the pipework below the basin or sink, but older systems may require the main supply to be shut off. Protect the plughole before removing small screws or clips because they are easily lost. If the tap is heavily scaled, corroded or difficult to dismantle, forcing it can turn a minor drip into a broken fitting. Replacement cartridges can also be expensive enough that a complete new tap is sometimes the more sensible option, particularly on an older budget fitting.
Why a small drip is still worth fixing
A dripping tap wastes water and often becomes more persistent rather than curing itself. The constant wetting can also stain ceramic fittings and leave limescale around the spout. In a quiet house, the sound becomes irritating long before the leak is serious, but that does not mean the fault should be ignored. If the tap has become stiff, the handle is loose or the body moves on the sink, there may be more than one issue to deal with. Fixing the shut-off problem without securing a loose tap can leave the job only half done.
What a plumber will normally do
The plumber will isolate the water, identify the valve arrangement and dismantle only the part needed to reach the faulty seal or cartridge. On a traditional tap this may involve changing a washer and checking the seating surface. On a ceramic tap the usual repair is a replacement cartridge of the correct size and rotation. If parts are unavailable or the body is badly corroded, the plumber may recommend replacing the whole tap. A good repair should leave the tap operating smoothly without needing excessive force to stop the water.
How to get a clearer price
Tell the plumber whether it is a single tap, a mixer or separate hot and cold taps. A photograph of the whole fitting and, if possible, the pipe connections underneath is useful. If you know the make or model, include it because that may allow the correct cartridge to be brought on the first visit. Ask whether the price assumes a repair or a full replacement. For taps where spare parts are uncertain, it can be helpful to agree what happens if the plumber opens the fitting and finds that a compatible cartridge is no longer available.
Repair or replace the tap?
There is no automatic rule that a dripping tap should be replaced. A sound, good-quality fitting with an available cartridge or washer is often worth repairing. A cheap tap with corrosion, a loose body and hard-to-find parts may be poor value to dismantle. If replacement is suggested, ask whether the existing tap holes and pipe connections suit the new model without alteration. Mixer taps also vary in height and reach, so a technically compatible fitting may still be awkward over the sink or basin. If you buy the replacement yourself, check whether the plumber is happy to fit customer-supplied parts and what happens if the tap turns out to be faulty. Those details are easier to settle before the old fitting has been removed.
This page is general guidance about dripping tap; the cause in an individual home still needs to be established from the actual installation.