THE SHORT ANSWER
Where was the first visible water?
Start with the first visible drip, not the puddle. A supply tail or valve, the waste outlet or trap, and water running down from the tap or worktop above are different routes. Note when water was already seen, keep any inspection dry and accessible, and get a repair assessed without guessing at a part.
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Where does the water first appear?
If the cupboard is dry enough to view safely, use what you have already noticed. Supply plumbing carries water to the tap; the waste outlet and trap carry water away from the sink. Water can also run down from the tap or worktop above. The wettest point is not always the source.
| What you already observed | What it may indicate | Useful next step |
|---|---|---|
| A visible drip at a tap supply tail, valve or connection while the sink is idle | This may be on the supply side. A drip elsewhere can travel along a surface and appear here. | Note the visible point and whether the sink was idle; do not touch or tighten it. |
| Water around the waste outlet, trap or waste pipe during or just after ordinary draining | This may be on the waste side. Timing can help describe it, but does not prove which joint is leaking. | Record what was already happening; do not run the sink to test it or dismantle the trap. |
| The cabinet underside is wet below the tap or worktop | Water may have run down from a higher point, including the tap/deck area. | Look directly above from a dry, safe position and report the first point you can see. |
| A visible appliance hose is wet | This is a separate location to report; it does not identify why the hose is wet. | Note the appliance/hose if known. Do not move the appliance or disconnect the hose. |
| Slow dampness or a small occasional drip, with no pooling or electrical exposure | It still needs attention if it returns, but dampness alone is not automatically an emergency. | Keep the area in view from a safe dry position and arrange prompt assessment. |
| Water is actively spreading, pooling or approaching electrical equipment | There may be property damage or an electrical hazard. | Keep clear and seek urgent help. Use the linked stop-tap guide only if its strict conditions apply. |
| You cannot tell where it started | The source may be hidden or water may have travelled. | Do not test it by draining the sink; report the uncertainty and arrange help. |
Water near electricity or spreading quickly
Keep people away from wet areas and electrical equipment. Do not touch a wet appliance, socket or exposed wire. Seek urgent plumbing help; affected electrical equipment needs assessment by a suitable electrician.
If damage is occurring
WaterSafe advises catching water and using a stop tap when a leak is causing damage. Use our stop-tap instructions only if the valve is positively identified as your private, safely accessible inside stop tap; they explain that stored hot water may continue and not to force a valve.
How do supply and waste leaks differ?
A supply tail or connection carries water towards the tap and may be under pressure even while the sink is idle. The waste outlet and trap carry used water away when the sink drains. A leak at a waste joint may therefore be noticed during or after drainage, while a supply leak may be visible at other times. These are clues only: an intermittent leak, splash, appliance discharge or water running along a surface can confuse the pattern.
Water to the tap
Supply-side examples
Visible examples include a tap tail, valve or connection; WaterSafe also lists worn washers, perished seals and loose fittings among possible leak causes. These are possibilities, not a diagnosis or instruction to tighten a joint.
Water away from the sink
Waste-side examples
The visible route may include the sink waste outlet, trap or waste pipe. WaterSafe lists a cracked or slipped plastic waste pipe as one possibility. Do not dismantle the trap to check.
Check above the puddle
Runoff from above
Water under the tap or worktop may have travelled down from above. WaterSafe recommends looking directly over a puddle; the hidden path may still leave the source uncertain.
What can you do from a dry, safe position?
Do not enter a cabinet with standing water or approach wet electrics. If the floor and access are dry and the pipework is visible without moving connected items, a torch can help you note the first visible wet point and what is directly above it. Do not touch the joint or deliberately run water to recreate the leak.
Name the visible route
Was the first drip at a supply tail/valve, waste outlet/trap, appliance hose, or an uncertain point? Say “not sure” if that is what you observed.
Note the ordinary timing
Was the sink idle, or had water already been going down the waste? If an appliance was already in use, note that too. Do not operate either one to test a theory.
Look above, if it is safely visible
Water may have run down from the tap or worktop. A photo from a dry position can show the location to a plumber or landlord.
Limit spread only if it is safe
If a visible drip can be reached from a dry position without touching fittings or approaching wet electrical equipment, place a container below it. Stop if water spreads or the access becomes wet.
Do not reach into a wet cabinet
If water is pooling, spreading or near an electrical item, stop inspecting and keep clear. For damage limitation, follow the known private stop-tap limits above only when access is dry and the valve is positively identified.
Should you tighten a pipe or take the trap apart?
A supply connection, waste joint and water running down from above can require different repairs. Avoid loosening or tightening a fitting before the source and connection type are identified. WaterSafe recommends using a qualified plumber for an under-sink leak.
Avoid a guessed repair
Do not tighten a flexible tail, valve or plastic compression nut, add sealant, disconnect an appliance hose, replace a hose or dismantle the trap based only on a wet patch. These guide steps do not establish the part or give a model-specific repair procedure.
If you rent, report the location and timing to your landlord or managing agent. If a tap, trap or appliance is under warranty, contact the supplier before dismantling it.
When should you call a plumber or landlord?
Prompt repair
Recurring drip or damp patch
Arrange prompt assessment, even if it is not spreading. Share the first visible point and whether it was seen while idle or after normal drainage.
Urgent help
Water actively spreading or pooling
Keep clear. If a safe, dry route and known private stop tap match the separate guide, it may limit incoming mains water; stored hot water can continue. Seek urgent plumbing help if water continues or the system is uncertain.
Electrical safety
Water near electrics
Do not touch wet electrical equipment or exposed wiring. Keep clear and seek urgent help; arrange suitable electrical assessment for affected equipment.
Tell the repairer whether the visible point was on the supply or waste side, what was happening during ordinary use, whether water appeared from above, and whether anything was spreading or near electrics. Give the make/model only if it is already visible or on paperwork.
How much does an under-sink leak cost to repair?
There is no directly matched current UK price for this leak. A supply connection, waste trap, damaged pipe or water running down from above can require different work, so a tap-drip or tap-base price would not be a fair estimate. Ask for a quote after the source is identified.
Itemised quote
Ask what the visit includes
Check the call-out or diagnostic fee, labour, any parts, VAT and whether a return visit or access work costs extra.
Repair scope
Ask what was found
Ask the plumber to show or explain the leak source and how the proposed work addresses it. A puddle location or timing clue alone does not prove which joint failed.
Common questions about a leak under the sink
How can I tell whether the supply pipe or waste trap is leaking?
Use only what you already observed: a supply-side drip may be visible while the sink is idle; a waste/trap leak may show during or after drainage. Neither is conclusive, and water can travel. Do not run the sink to test it.
Could the water be coming from above the cabinet?
Yes. Water can run down from the tap or worktop and collect below. Look above only if it is visible from a dry, safe position, and report where water first appeared if known.
What if an appliance hose looks wet?
Note which hose and appliance are visible and report it. Do not move the appliance or disconnect a hose to look for a leak.
Can I tighten the joint or take the trap apart?
This guide does not provide a generic repair procedure. The visible wet point may not identify the failed part, and pipe/fitting designs vary. WaterSafe recommends a qualified plumber.
Is a small damp patch an emergency?
Not automatically. Recurring dampness or a drip needs prompt attention, while active spreading, pooling or water near electrical equipment calls for urgent help.
Will the stop tap stop every leak?
The linked instructions apply only to a positively identified private inside stop tap that is safely accessible. It stops incoming mains supply only; water from stored hot-water systems may continue.
How much will the repair cost?
No directly matched price is available for this leak scope. Ask for an itemised quote after the source and repair are identified.
EVIDENCE & ACCOUNTABILITY
Sources and review
Prepared by Plumber.org.uk with AI assistance, using the sources listed below. These sources support the explanations; they do not endorse this website.
Source-checked guide with independent editorial review and AI-assisted research. No professional plumbing review is claimed. Initial research: 2026-09-28. Updated: 2026-09-30. This is general information, not an assessment of your installation. Prices can change; confirm compatibility and the full repair quote before buying or booking.
How we research and update guides. Spotted an error? Contact us with the page address and the detail to check. This form is for general enquiries and editorial feedback, not emergency assistance.
- Hansgrohe UK - Basin taps: more than just taps
Accessed 2026-09-28. FAQ “What should I do if my tap is dripping?”: a permanent off-state drip from a mixer may involve a faulty or limescale-coated cartridge. The next FAQ “The bottom of my tap is leaking” separately identifies water escaping at the basin base or lever cap and recommends a plumber assess the cause. Used as qualified manufacturer guidance, not a diagnosis of an unidentified fitting.
- Electrical Safety First - Electrical safety after a flood
Current page accessed 28 Sep 2026: keep clear of exposed wiring and water-damaged electrical equipment; have affected equipment checked by an electrician.
- WaterSafe Register - What to do with a leak under the sink
Accessed 2026-09-28. Sections “Is a leak under the sink an emergency?” and “How to find where the leak is coming from”: lines 13 - 28 recommend noting source, active dripping/flooding/damage, photos, using a stop tap if leak is causing damage and catching water in a bucket when safe. Lines 33 - 41 advise clearing/viewing the area with a torch, looking for drips/puddles and checking directly above; list worn washers, loose fittings/perished seals and cracked/slipped plastic waste pipes as possible causes. It advises not trying to fix it yourself and contacting a qualified plumber.
- Hightown Housing Association - Repairs Guide
Accessed 2026-09-28. PDF page 10, heading “Water leaks”, lines 492 - 505 asks tenants to distinguish supply from waste pipe and note hot/cold supply/electrical impact. Page 12, “Baths”, lines 554 - 573 gives a bath example: waste-pipe leakage may be noticed when emptied while supply-pipe leakage is described as constant. The same page lines 577 - 600 notes sinks/basins have traps and asks residents to report the leak location. Used only as a qualified timing/location clue; do not present the bath-specific wording as a conclusive sink test.