In This Guide
Start here: the water meter test
Before working through the eight causes below, do the water meter test. It takes 30 minutes and immediately tells you whether the problem is a leak on the supply pipe or something else inside the property.
Switch off every tap, appliance and outdoor fitting. Read your external water meter and note or photograph the dial. Wait 30 minutes without using any water at all, including flushing a toilet. Read the meter again.
If the reading has moved, you have an active leak on the supply pipe or inside the property, and the underground leak section below explains what to do. If the reading has not moved, the cause is intermittent or is somewhere the test does not catch, and the causes below are where to look.
Our guide to how to check for a water leak at home covers the meter test in full detail, including how to find your meter and how to isolate whether a confirmed leak is inside the property or on the supply pipe.
The 8 most common causes of a high water bill
Cause 1: A running toilet cistern
A running toilet is the most common cause of an unexplained high water bill in London properties, and the easiest to overlook because it is often silent. A faulty flap valve inside the cistern allows water to trickle continuously from the cistern into the bowl. The toilet appears to function normally but water is being lost around the clock.
How much it wastes. Between 200 and 400 litres per day depending on how badly the valve is leaking. At Thames Water’s current metered rate that is roughly £100 to £200 a month added to the bill.
How to check. Add a few drops of food colouring to the cistern. Wait ten minutes without flushing. If colour appears in the bowl, the flap valve is leaking.
What to do. Replace the flap valve. A straightforward DIY repair costing well under £10 for the part, or a quick job for a plumber taking under 30 minutes.
Cause 2: A dripping tap
A dripping tap that appears trivial adds up significantly over a billing period. A tap dripping once per second wastes about 31 litres per day. Multiple dripping taps compound quickly, and the ones you cannot hear at the bath spout or the outside tap are often the worst offenders.
How to check. Feel the spout of every tap in the property when fully closed. Check both hot and cold at every basin, bath and sink. Even a slow drip that is not visibly forming a drop still produces measurable loss over time.
What to do. Replace the tap washer or ceramic cartridge, depending on the tap type. A straightforward repair for a plumber, typically under an hour.
Cause 3: A leaking appliance
Washing machines and dishwashers connect to the supply via fill hoses. A loose or deteriorating hose connection leaks slowly behind the appliance where it is rarely noticed until damp builds up in the flooring. The same applies to the fill valve inside a washing machine, which can stick open and allow water to run continuously into the drum.
How to check. Pull the appliance away from the wall and inspect the fill hose connections at the wall and at the machine for drips. Run a short cycle and watch for water on the floor beneath the machine.
What to do. Tighten or replace the fill hose connections. If the fill valve inside the machine is faulty, a washing machine engineer is required.
Cause 4: A Thames Water billing estimate
Thames Water issues estimated bills when a meter reader cannot access the meter. If the estimate was lower than actual consumption in previous periods, the correction in the next bill can produce an apparent spike that is not a reflection of current usage at all. It is a paperwork problem, not a leak.
How to check. Compare the meter reading on the bill to your actual meter reading. If the figures differ significantly, the bill was based on an estimate. Also check whether the bill is marked “estimated” or “actual” in the reading detail.
What to do. Submit your actual meter reading to Thames Water online or by phone. Future bills will be based on actual readings if Thames Water has been unable to access the meter. Consider requesting a smart meter if you do not already have one, since it removes the possibility of estimation entirely.
Cause 5: Seasonal usage increase
Garden watering, paddling pools, car washing and increased showering during warmer months all add to consumption in a way that is easy to underestimate. A hosepipe running for one hour uses approximately 540 litres, and irrigation systems left on a timer can add hundreds of litres a day without anyone thinking about it.
How to check. Compare the bill period to the same period in the previous year. If the increase aligns with warmer months and a visible increase in outdoor water use, seasonal consumption is the likely explanation.
What to do. Fit a water butt to collect rainwater for garden use. Check whether a garden tap or irrigation timer has been left running. Consider a water-efficient shower head if shower usage has increased.
Cause 6: A faulty water meter
A meter that is running fast records more consumption than is actually being used. This is less common than the other causes but worth investigating if all other explanations have been ruled out.
How to check. Do the 30-minute meter test with all water use stopped. If the meter moves with everything off, either there is a leak on the supply pipe or the meter is faulty. To distinguish between the two, close the internal stop tap and observe the meter. If the meter stops when the internal stop tap is closed, the leak is inside the property. If it keeps moving, either there is a supply pipe leak between the meter and the internal stop tap, or the meter itself is running fast.
What to do. Contact Thames Water and request a meter accuracy test. Thames Water will test the meter and replace it free of charge if it is found to be running fast.
Cause 7: A leaking garden tap or irrigation
An outdoor tap that drips at the wall connection, or an irrigation system with a faulty timer or leaking joint, can run for long periods unnoticed, particularly when the garden is not in regular use.
How to check. Inspect the outdoor tap connection at the wall for drips when the tap is both open and closed. Check all irrigation joints and the timer valve. Run the system through a cycle and observe for leaks at each connection.
What to do. Tighten or replace the outdoor tap connection. Repair or replace faulty irrigation joints. If the tap itself is leaking rather than the connection, a plumber can replace it in a single visit.
Cause 8: An underground supply pipe leak
An underground leak on the supply pipe between the external stop tap and the property is the only cause on this list with no visible sign inside the property. Water escapes into the surrounding soil rather than onto a floor or ceiling. In London’s clay soil it may never reach the surface at all.
How much it wastes. At a modest leak rate of two litres per minute, that is roughly 2,880 litres per day, adding around £150 a month at Thames Water’s current metered rate. Larger failures cost considerably more.
How to check. The 30-minute water meter test is the standard method. If the meter moves with all water use stopped and no internal cause can be identified, the leak is on the supply pipe.
What to do. Do not dig speculatively. The wet patch on the surface, if one is visible at all, is often metres from the actual failure point. Professional underground leak detection using acoustic listening equipment and tracer gas locates the failure to within centimetres before any excavation is recommended. Our guide to high water bill with no visible leak covers this scenario in full.
Quick reference: high water bill causes at a glance
Cause | Typical daily loss | Visible sign? | DIY fix? |
Running toilet cistern | 200-400 litres | No | Yes, replace flap valve |
Dripping tap | 10-50 litres per tap | Sometimes | Yes, replace washer |
Leaking appliance | Variable | Sometimes | Partial |
Thames Water billing estimate | n/a | n/a | Yes, submit meter reading |
Seasonal usage increase | Variable | n/a | Yes, reduce outdoor use |
Faulty water meter | n/a | No | No, contact Thames Water |
Leaking garden tap or irrigation | 20-100 litres | Sometimes | Yes |
Underground supply pipe leak | 1,000-5,000+ litres | No | No, professional detection required |
If the cause is an underground leak
An underground supply pipe leak requires professional detection before any access or repair work. Acoustic listening equipment identifies the area of the failure. Tracer gas, a safe mixture of 95 per cent nitrogen and 5 per cent hydrogen introduced into the isolated pipe, rises to the surface at the failure point where a handheld detector confirms the location to within centimetres.
Most UK buildings insurance policies include trace and access cover, which pays for the detection investigation, targeted excavation and reinstatement. If Thames Water has sent you an excessive usage letter, that letter can be used as supporting evidence for an insurance claim. For a full explanation of the cover, see our guide on trace and access cover. Separately, Thames Water offers a discretionary leak allowance that rebates the water charges once the leak is repaired, and our guide to the Thames Water leak allowance covers how to claim it.
How 0800 Homefix fixes an underground supply pipe leak
0800 Homefix locates underground supply pipe leaks across London and the South East using acoustic listening equipment and tracer gas detection, and confirms the failure point before recommending any excavation. Every investigation includes a written report documenting the detection methods used and the confirmed failure location, in the format insurers and Thames Water both accept.
We charge the first hour’s labour in advance, with any further investigation or repair work quoted and agreed with you before it begins. Current rates are on our rates page. Our labour is guaranteed for 12 months, and any manufacturer part we fit carries the manufacturer’s own 12-month guarantee. We work seven days a week, 365 days a year. You can read more on our underground water leak detection service page.
Helpful resources
Thames Water’s leaks at home pages explain their responsibility split with homeowners and the leak allowance process. WaterSafe maintains the national register of approved plumbers for work on water supply pipes.
Frequently Asked Questions
Why has my water bill doubled?
A bill that has doubled usually indicates either a significant leak or a corrected Thames Water billing estimate catching up with previous underestimates. Start by comparing the meter reading on the bill to your actual meter reading. If they match and the bill is based on an actual reading, do the internal checks and the 30-minute meter test. A running toilet cistern, which can waste 200 to 400 litres per day, is the most common single cause of a sudden significant bill increase.
Can a leaking toilet really make my water bill that high?
Yes. A running toilet cistern with a faulty flap valve wastes between 200 and 400 litres per day. Over a quarterly billing period that is 18,000 to 36,000 litres of wasted water, adding roughly £300 to £600 to a quarterly bill at Thames Water’s current metered rate. It is the most common cause of an unexpectedly high bill and is easy to confirm using the food colouring test.
How do I know if my high water bill is due to a leak?
Do the 30-minute water meter test. Switch off every tap and appliance, read the external meter, wait 30 minutes without using any water, then read it again. If the reading has increased, you have an active leak. The internal stop tap isolation test then tells you whether the leak is inside the property or on the supply pipe. If all internal causes have been ruled out and the meter keeps moving, the leak is underground.
Will Thames Water reduce my bill if I have a leak?
Yes, sometimes. Thames Water offers a discretionary leak allowance for metered customers whose supply pipe leak has been confirmed and repaired. The allowance rebates the water charges only, not the detection or repair cost, and requires the repair within four weeks of the leak being found plus a claim within 90 days of the repair. It does not usually apply to visible internal leaks like a running toilet, since the negligence exclusion catches leaks you knew or should have known about.
How do I check my water meter?
Your external water meter is usually in a small rectangular chamber set into the pavement or path just outside your property boundary. The lid is marked “water” and lifts with a flat-head screwdriver or meter key. Read every digit on the dial including any moving decimal figures. Compare the reading to the figure on your bill. If they differ significantly, the bill was based on an estimate and you should submit the actual reading to Thames Water.
Is an underground water leak my responsibility?
The supply pipe from the external stop tap at your property boundary to your internal stop tap is the homeowner’s responsibility to maintain and repair. Thames Water is responsible for the main in the street and the pipework up to and including the external stop tap. If the meter test confirms a leak on the supply pipe within your boundary, the repair cost is yours, though it is often covered under buildings insurance trace and access cover.
How much does it cost to fix an underground water leak?
Detection using acoustic and tracer gas methods typically takes two to four hours. The targeted excavation and repair cost depends on the ground conditions and whether the pipe runs under a garden, path or driveway. Most UK buildings insurance policies include trace and access cover, which pays for the detection, excavation and reinstatement. Full pricing for 0800 Homefix is on our rates page, with any specific quote confirmed with you before work begins.
Received a Big Bill?
If your water meter test has confirmed a leak, or you have worked through the internal causes and cannot find the source, book an 0800 Homefix underground leak investigation. We locate the failure using acoustic and tracer gas methods before any excavation, and produce a written report your insurer and Thames Water can both use. Seven days a week across London and the South East. Book online or through our underground leak detection page.
Spotted Any of These Warning Signs?
Our leak detection specialists cover London and the South East using acoustic detection, thermal imaging and moisture meters — no unnecessary damage, full written report, available 7 days a week.
Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
