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Section 75 · Leak allowance · WaterSafe certificate

Thames Water Leak Notice — Response & Repair London

Locate and repair the leak on your private supply pipe within the 21 or 28-day compliance window. Signed WaterSafe engineer certificate to close the notice and unlock the Thames Water leak allowance credit on your bill.

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Received a Thames Water leak notice — here is what it means and what you need to do

Thames Water issues four different categories of leak notice, and the response requirements are different for each. The most serious is a section 75 notice under the Water Industry Act 1991 — a formal statutory instrument requiring the occupier to repair a leak on the private supply pipe within a specific compliance window (typically 21 or 28 days). Non-compliance can result in Thames Water arranging the repair themselves and recharging you, plus enforcement action under the Act. The other three categories — the high-consumption alert letter, the SMART meter warning SMS, and the manual survey letter to unmetered properties — are informational rather than statutory, but responding promptly is what triggers the Thames Water leak allowance scheme.

Almost every Thames Water leak notice concerns the private supply pipe — the section of pipework you own as the property occupier, running from the boundary stop-tap (usually a chamber under a small metal cover in the pavement at the property boundary) to the internal stop-tap inside your home. Thames Water owns and maintains everything on their side of the boundary stop, and they fix their own leaks without asking you. When they notify you, it means the leak is on your side. The good news is that this section of pipe is typically 3–10 metres long, buried at 40–70cm depth under the front garden, driveway, or path, and repairable via a keyhole excavation rather than a full trench.

The Thames Water leak allowance scheme is the reason a fast, properly-documented response matters commercially. Provided the repair is completed within a reasonable window and a WaterSafe engineer certificate is provided showing the leak location, pipe material, repair method, and post-repair flow test result, Thames Water will typically credit the excess water charge for up to two billing periods (roughly 12 months). On a supply-pipe leak that has been running unnoticed for 6–8 months before the alert, that credit can be several hundred pounds — enough to substantially offset the cost of the repair itself.

Every supply-pipe survey and repair we deliver is by a WaterSafe registered engineer (UK Certification Ltd certificate 136356 issued 8 September 2025, expiry 18 August 2030), which is the qualification level Thames Water expects on the repair certificate. HWSS G3 certification (certificate 136359, same date range) covers any adjacent hot-water cylinder work. Public liability £5,000,000 via SiriusPoint through Eaton Gate MGU, policy BE26ACTT000000018221, period 07/05/2026 to 06/05/2027.

The four types of Thames Water leak notice

Read the notice carefully — the compliance window, the reference number, and the action required are different for each category.

High consumption alert (metered supply)

Automated letter or email from Thames Water flagging that your household usage over a recent 4–8 week window is significantly higher than the historical average for the property. Typical trigger: continuous flow at the meter overnight when no fixture should be running. The letter usually gives you 21 days to investigate and resolve, with a follow-up bill adjustment available under the Thames Water leak allowance scheme if a private-side leak is confirmed and repaired.

Action: Book a supply-side leak survey. Locate the leak, repair, submit the engineer certificate to Thames Water to trigger the leak-allowance credit on the bill.

Section 75 leak notice (Water Industry Act 1991)

Formal statutory notice issued under section 75 of the Water Industry Act 1991 requiring the property occupier to repair a leak on the private supply pipe. Thames Water is empowered to serve the notice when their monitoring shows a continuous unaccounted flow that has persisted beyond a reasonable window. Non-compliance can lead to Thames Water arranging the repair themselves and recharging the cost, plus enforcement action.

Action: This is a legal notice, not just an alert. Book an urgent supply-pipe survey — pinpoint the leak, repair within the 21 or 28 day window on the notice, submit engineer certificate to Thames Water and retain a copy for your records.

Metered leak warning (SMART meter customers)

Thames Water SMART meter customers receive an in-app alert or SMS when the meter has recorded continuous night-time flow for consecutive 24-hour periods — the flow signature of a leak, not household use. Typical trigger threshold: 4 L/h continuous flow between 02:00 and 05:00. The alert normally arrives within 3–7 days of the leak starting, which is much earlier than the equivalent bill-based alert.

Action: Fast response is easier here because the leak is caught early. Book a same-week supply-pipe survey — small leaks are cheaper to repair and rarely cause structural damage if caught fast.

Unmetered high-consumption manual check

Some unmetered properties receive a manual survey letter from Thames Water because the operational team has identified an unaccounted flow in the service run to the property from mains monitoring. This is rarer than metered alerts but occurs on older service pipes in central London. The letter typically requests that the occupier investigates and confirms whether a private-side leak is present.

Action: Book a supply-pipe survey. Even without a meter, Thames Water can identify supply-side leaks from mains-side pressure and flow monitoring — treat the letter seriously.

The six-step response — verify to certificate

The full end-to-end process from receiving the notice to closing it with Thames Water and unlocking the leak allowance.

01

Verify the notice and the compliance window

Read the notice carefully — Section 75 notices state a specific compliance date, typically 21 or 28 days from issue. High-consumption alerts and SMART meter warnings are informational rather than statutory but still trigger the leak allowance if actioned within the same window. Note the reference number — you will quote it back to Thames Water when submitting the repair certificate.

02

Isolation test — is the leak internal or private supply?

Close the internal stop-tap and check the meter (or the toilet cistern fill valve). If the meter continues to turn or the cistern refills spontaneously, the leak is downstream of the stop — inside the property. If both stop, the leak is upstream of the internal stop — on the private supply pipe between the boundary and the internal stop. Almost all Thames Water leak notices concern the private supply pipe.

03

Supply pipe survey — acoustic then tracer gas

AK acoustic survey along the supply-pipe route using ground microphone — copper pipes normally pinpoint within ±150mm. If the supply pipe is buried MDPE plastic (post-1975 installations) and acoustic returns no clear signal, we escalate to tracer gas (hydrogen 5/95). Typical survey duration 90–120 minutes on site.

04

Keyhole repair, not full excavation

Once the leak point is pinpointed within a workable margin, the repair is normally a keyhole excavation (300×300mm) rather than a full driveway or garden rip-out. Repair options depend on pipe material — copper: cut-and-solder or push-fit coupling, MDPE: fusion coupling or clamp, lead: mineralite compression sleeve (or scheduled full replacement if the whole run is failing).

05

Post-repair meter check and certificate

After the repair the internal stop is opened and the meter is monitored for 30–60 minutes with no draw-off — flow at the meter should be zero. If zero, the leak is resolved. The engineer issues a signed WaterSafe repair certificate documenting the leak location, the material, the repair method, and the post-repair flow test result. Submit this certificate to Thames Water to close the leak notice.

06

Submit for leak allowance

Thames Water's leak allowance scheme normally refunds the excess water charge for two billing periods (up to 12 months) provided (a) the leak was on the private supply pipe, (b) the customer took reasonable steps to repair once notified, and (c) a suitable repair certificate is provided. AK provides the certificate in the format Thames Water expects, with the leak location, WaterSafe engineer number, and post-repair flow test result included on a single-page document.

Cost — Thames Water leak notice response

Every job is quoted in writing before the engineer travels. Prices below indicative; exact cost depends on pipe material, depth, and access. The engineer certificate for the leak allowance is included at no extra charge.

ScopePrice (inc. VAT)Includes
Supply-pipe leak survey (acoustic)£300–£400Isolation test, acoustic sweep along supply pipe route, pinpoint marked on plan
Supply-pipe leak survey (tracer gas escalation)£450–£600Full drain-down, hydrogen 5/95 injection, semiconductor sweep
Keyhole excavation and repair (copper supply pipe)£450–£850Excavation to 60cm depth, cut-and-repair, backfill and surface reinstatement
Keyhole excavation and repair (MDPE supply pipe)£450–£800Fusion or clamp coupling, backfill and surface reinstatement
Lead supply pipe replacement (full run)£1,500–£3,500Full trench, 32mm MDPE replacement from boundary to internal stop, mole-plough or trench method
Engineer certificate for Thames Water leak allowanceIncludedWaterSafe engineer number, leak location, material, repair method, post-repair flow test result — the document Thames Water requires to trigger the allowance

Real Thames Water leak notice cases

Three recent notice responses, anonymised. Notice type, survey method, repair, and leak allowance outcome.

Section 75 notice — Wandsworth Victorian terrace, buried MDPE

Homeowner received a section 75 notice citing 620 L/day unaccounted flow. Acoustic survey along the 8m front-garden supply run returned no clear signal (plastic MDPE). Escalated to tracer gas — pinpointed leak at 4.1m from boundary at a corroded MDPE-to-copper coupling at 55cm depth. Keyhole excavation, fusion coupling, thrust plate, backfill and turf reinstatement. Post-repair meter test zero. Engineer certificate submitted; Thames Water credited 8 months of excess consumption on the next bill. Total survey + repair £1,180.

SMART meter alert — Camden mews conversion, small early-catch leak

Customer received a SMART meter alert citing continuous 4.2 L/h night flow. Acoustic survey pinpointed leak at 2.3m from the internal stop-tap on a 15mm copper cold main under a concrete kitchen floor slab — a corroded compression joint from a 1990s rewiring. Small keyhole opened through the tile floor, joint remade with modern olive, tile relaid. Total survey + repair £680. Because the leak was caught within a week of starting, no structural damage occurred and the meter allowance credit was minimal — but the disaster was prevented.

High-consumption alert — Ealing 1960s semi, lead-to-MDPE transition

Household usage flagged at 480 L/day above the four-year average. Acoustic survey ruled out the internal plumbing. Ground microphone sweep on the front driveway identified a peak at 3.7m from the boundary — the transition point between the original 1960s lead main and a 1998 MDPE replacement, where the lead had corroded through at the compression union. Given the age of the lead run, customer opted for a full lead replacement rather than a spot repair. 6m of 32mm MDPE mole-ploughed from boundary to internal stop, new internal stop-tap fitted, meter test zero. Total £2,850. Thames Water allowance credited three billing periods.

Thames Water leak notice response across every London borough

Thames Water covers all 32 London boroughs plus fringe areas of Hertfordshire, Surrey and Essex. We respond across the whole Thames Water region. Click a borough for a page tailored to local supply-pipe conditions.

Barking and DagenhamBarnetBexleyBrentBromleyCamdenCroydonEalingEnfieldGreenwichHackneyHammersmith and FulhamHaringeyHarrowHaveringHillingdonHounslowIslingtonKensington and ChelseaKingston upon ThamesLambethLewishamMertonNewhamRedbridgeRichmond upon ThamesSouthwarkSuttonTower HamletsWaltham ForestWandsworthWestminster

Frequently asked questions about Thames Water leak notices

What is a Thames Water leak notice and do I have to comply?
A leak notice from Thames Water is a formal or informal alert that the water usage at your property indicates a leak on the private supply pipe — the section of pipe between the boundary stop-tap (owned by Thames Water) and the internal stop-tap inside your property (owned by you as the occupier). A section 75 notice is a formal statutory instrument issued under the Water Industry Act 1991 and must be complied with, typically within 21 or 28 days. Non-compliance can result in Thames Water arranging the repair and recharging you the cost, plus enforcement action. High-consumption alerts and SMART meter warnings are informational — you are not legally compelled to act — but leaving the leak unrepaired means paying for the wasted water indefinitely and eventually receiving a section 75 notice anyway.
How does the Thames Water leak allowance work?
Thames Water operates a leak allowance scheme that refunds the excess water charge caused by a private-side leak, typically covering up to two billing periods (roughly 12 months) provided three conditions are met: (a) the leak was on the private supply pipe or in a hidden internal fitting the customer could not reasonably have detected sooner, (b) the customer took reasonable steps to repair the leak once notified, and (c) an engineer's certificate documenting the repair is provided. The certificate must show the leak location, the pipe material, the repair method, and the post-repair flow test result. Our engineer certificates are formatted specifically to Thames Water's allowance-scheme requirements.
Who owns the supply pipe — Thames Water or me?
From the mains in the road to the boundary stop-tap (usually a small chamber under a metal cover in the pavement at the property boundary), the pipe is owned and maintained by Thames Water. From the boundary stop-tap to the internal stop-tap inside your property, the pipe is your responsibility as occupier — this is what is meant by the "private supply pipe" or the "supply pipe". Almost all Thames Water leak notices concern this private-side section, because Thames Water fixes their own side themselves and only notifies you when the leak is on yours.
Do I need a WaterSafe registered engineer to do the repair?
For the engineer certificate to be valid for the Thames Water leak allowance, yes — the engineer must be Water Regulations 1999 competent, ideally WaterSafe registered. This is because the certificate is a formal declaration by the engineer of compliance with Water Regulations, and Thames Water reject certificates from unregistered plumbers. Our senior engineer is WaterSafe registered (UK Certification Ltd certificate 136356 issued 8 September 2025, expiry 18 August 2030). The certificate we issue on a leak repair is accepted by Thames Water without further checks.
What if the leak is on a lead supply pipe — do I have to replace the whole run?
Not always — a spot repair on a lead pipe using a mineralite compression sleeve is possible where the lead is otherwise sound and the leak is a single localised failure. But older lead runs (pre-1970s) that have leaked in one place have typically corroded to a similar thickness along the whole run, so a second leak within 1–3 years is common. On lead pipes over 30 years old we normally recommend a full replacement to 32mm MDPE using either a mole-plough (minimal excavation) or a trench method (where the ground conditions require it). This also removes the lead-in-drinking-water risk under the current Drinking Water Inspectorate guidance. The Thames Water leak allowance still applies to a full replacement, not just a spot repair.
How much water damage can happen while I wait to repair the leak?
A private-side supply-pipe leak upstream of the internal stop-tap is normally underground — under the front garden, under the driveway, in the front path. The leaking water percolates into the surrounding soil. Damage to the property structure is usually minimal because the water does not enter the building itself. But the ground around the leak becomes saturated over months, which can undermine driveway surfacing, path slabs, and — on Victorian terraces with London clay subsoil — can contribute to differential settlement of the front elevation. Fast repair is worth the money for the ground-condition reasons alone, quite apart from the water bill.
Can you do the survey and the repair on the same visit?
On a supply-pipe survey where the leak is pinpointed within a workable margin and the pipe is accessible for keyhole excavation, yes — most surveys convert to same-visit repair, subject to the depth and material. The customer authorises the repair after seeing the survey finding and getting a written repair quote on site. We carry MDPE fusion equipment, compression fittings, copper solder kit, and lead mineralite kit as standard on the survey van, so we do not need a return visit unless the repair requires a specialist plant hire (mole-plough for a full lead replacement).
How quickly can you attend for a Thames Water notice response?
Section 75 notice work is prioritised — supply-pipe survey within 24–48 hours of the quote being accepted. Same-day emergency response available for burst supply pipes (visible surface water). Written survey report and engineer certificate emailed within 24 hours of the repair. The engineer certificate is the document you submit to Thames Water to close the notice.
Do you carry public liability insurance?
Yes — £5,000,000 public liability via SiriusPoint International Insurance Corporation (UK Branch) acting through Eaton Gate MGU Ltd, policy number BE26ACTT000000018221, current period 07/05/2026 to 06/05/2027. The certificate is issued with every survey quote — required for supply-pipe excavation work in the highway or on shared driveways.
What if I do not have my property's stop-tap?
Many Victorian and Edwardian properties in London have never had a working internal stop-tap, or the stop is buried behind a boxed-in unit and inoperable. As part of the leak repair we routinely fit a new modern internal stop-tap at a sensible location (usually just inside the front elevation, in the void under a stairs or in an under-sink cupboard) with a full-bore valve rated for the mains pressure. This does not add much to the total cost and gives you a working isolation for future work.

Related plumbing services

Leak detection — main hub

Full explanation of every detection method — deployed on supply-pipe surveys.

Acoustic leak detection

The first-pass method on supply-pipe leaks.

Tracer gas leak detection

Escalation method for MDPE plastic supply pipes acoustic cannot resolve.

No find, no fee guarantee

Written guarantee applies to supply-pipe surveys.

Respond to a Thames Water leak notice

WaterSafe engineer certificate included with every repair. Response within 24–48 hours of quote accepted.

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