First, work out where the water starts
A leak under a sink can look worse than it is because even a small drip spreads across the cupboard base. Dry the area thoroughly, then watch each joint while the tap is running and again while the basin or sink empties. Fresh water appearing on a supply pipe points you towards a tap connector, isolation valve or compression joint. Water that only appears while the sink drains is more likely to come from the trap, waste pipe or the seal around the plughole. Condensation can also make cold pipes wet, especially in a warm kitchen, so look for a definite drip path rather than assuming every damp pipe has failed.
Common places a sink starts to leak
Flexible tap tails can loosen, plastic waste fittings can shift, and compression joints sometimes begin to seep after years without trouble. The trap itself may have been knocked when cleaning products were moved, leaving one threaded collar slightly out of line. On older metal fittings, corrosion can create a pinhole that is hard to spot until the pipe is dry. A leak from the base of the tap can also travel down the tap tails and make it seem as though the pipework is leaking. The exact starting point matters because tightening the wrong fitting can make the problem worse.
What you can check without dismantling anything
Remove anything stored beneath the sink and put a dry sheet of kitchen roll around each suspect joint. Run the cold tap, then the hot tap, then fill and empty the sink. The paper often shows the first wet spot much more clearly than looking into a dark cupboard. Check whether an isolation valve is wet around its spindle or compression nuts. If you can see water tracking down from above, inspect the tap base and the plughole seal as well. Avoid repeatedly tightening plastic fittings with tools; many are designed to be hand-tight and can crack if overdone.
When the leak needs quicker attention
A steady leak on the mains-water side of the plumbing deserves prompt action because it can continue even when the sink is not being used. If water is reaching electrical sockets, appliances, chipboard units or the floor below, shut off the affected supply if you know how to do so safely. Waste-side leaks are usually less urgent because they stop when the sink is not used, but they can still damage cabinets and flooring surprisingly quickly. A swollen cupboard base, musty smell or staining on the ceiling below suggests the leak has probably been present for longer than the visible drip suggests.
What a plumber is likely to look for
A plumber will normally separate the water-supply side from the waste side, then test the fittings while the sink is in use. They may replace a failed tap connector, reseal a plughole, remake a compression joint or change a distorted trap. If the pipework is old or badly positioned, a simple local repair may not be the neatest long-term answer. Ask the plumber to show you where the leak began before the area is dried and reassembled. That makes it easier to understand why a particular fitting needs changing rather than simply being tightened again.
Getting a useful quote for the repair
When asking for a price, say whether the leak is constant or only happens when the sink is used. Mention whether it is on a hot or cold supply, whether an isolation valve is fitted and whether the cupboard or flooring has already been damaged. A clear photograph of the pipework can save time because it shows the type of fittings and how much working space there is. For a small leak, the eventual cost is often driven more by access and the condition of the existing fittings than by the price of the replacement part itself.
Before you agree to the work
For a leak under a sink, ask whether the proposed repair deals with the failed fitting itself or only tightens what is already there. A joint that has moved because the pipework is under strain may start leaking again unless the alignment is corrected. If the cupboard base has swollen, ask whether the plumbing can be tested before anything is covered back up. It is also worth checking that the local isolation valves still work once the repair is finished. They are easy to ignore until the next fault occurs. A useful quotation should make clear whether the price covers just the leaking connection, replacement of a tap or trap if needed, and any awkward access behind drawers, appliances or fixed shelving.
This page is general guidance about leaking pipe under sink; the cause in an individual home still needs to be established from the actual installation.