The smell is not always coming from the drain below
Kitchen and bathroom sinks collect organic residue around the plughole, overflow and trap. Bacteria growing in that material can produce a strong drain-like smell even when the pipework is working normally. Start by cleaning the visible waste opening and overflow channel rather than immediately assuming there is a sewer fault. If the smell disappears after cleaning and stays away, the source was likely close to the sink. If it returns quickly, there may be more going on.
The water trap is there to block drain gases
A trap holds a small amount of water that forms a seal between the room and the waste system. If a little-used sink has been left dry for weeks, that seal can evaporate. Running water may restore it. A trap can also be siphoned if the waste arrangement is poorly vented, leaving the seal low after another fixture discharges. Gurgling at the same time as the smell is a useful clue. A loose trap joint can leak odour without necessarily leaking much water.
Kitchen residues behave differently
Grease and food deposits can coat the inside of a kitchen waste pipe and smell even before the sink blocks. Cleaning only the basket strainer will not reach build-up further along. If the sink is also draining slowly, the two symptoms are probably connected. Avoid pouring fats down the sink and do not mix aggressive chemicals in an attempt to mask the smell. Mechanical cleaning is more useful when a thick layer of residue has narrowed the pipe.
When the smell may point to a wider drainage issue
If several rooms smell at the same time, toilets gurgle or outside gullies hold standing waste, the problem may sit beyond the individual sink. A blocked or poorly ventilated drain can push odours back through weak seals. In that situation, cleaning the sink repeatedly will only give temporary relief. Check whether neighbouring fixtures behave normally and whether the smell is stronger after a bath, washing machine or toilet discharges. That pattern can help locate the affected branch.
What a plumber can investigate
A plumber can remove and inspect the trap, check the fall of the waste pipe and look for poorly sealed joints. They may also identify a missing or faulty air admittance valve where one is used. If the internal plumbing appears sound but the drainage beyond it is slow or blocked, a drainage contractor may be more appropriate. The aim is to restore a reliable water seal and free flow, not simply cover the smell with disinfectant.
Useful details before a visit
Say whether the smell is constant or only appears after the sink has been unused. Mention gurgling, slow drainage or smells from other fixtures. If the sink is in an extension or recently refitted kitchen, note that because waste arrangements are sometimes altered during building work. A photograph under the sink can show the trap type and whether unusual flexible waste pipes have been used. Those details help distinguish a hygiene issue from a plumbing-layout problem.
Do not mask a smell that has a plumbing cause
Air freshener and disinfectant can hide the symptom for a while, but a recurring drain smell is easier to deal with once the source is identified. If cleaning the overflow and trap removes the odour for only a day or two, note what happens just before it returns. A washing machine discharge, a bath emptying upstairs or a period when the sink is unused can each point in a different direction. Ask a plumber to check that the trap retains water and that the waste joints are actually gas-tight. If the smell is coming from an underground drain or defective soil system, the job may need a drainage specialist rather than more work on the sink itself. Persistent odour deserves a proper route back to its source.
This page is general guidance about bad smell from sink drain; the cause in an individual home still needs to be established from the actual installation.