A doubled bill is not yet evidence of a leak
The number on the bill is volume multiplied by rate, over a period, plus fixed charges. Four things can move, and only one of them is a leak. Establish which one moved before you start opening floors.
First: was the previous bill estimated?
This is the single most common explanation and it costs nothing to check. If your last bill was based on an estimate that came in low, the next actual reading collects the shortfall as well as the current period. Nothing at the property changed. The bill is catching up with itself.
On the bill, find the two readings the charge is built from and look for a letter or a note marking each one as actual or estimated. Two estimates in a row followed by an actual read will produce a bill that looks alarming and is arithmetically correct.
Fix it by submitting a reading yourself and asking for the bill to be recalculated.
Second: is the period the same length?
Thames Water bills metered households every six months. A bill covering a longer stretch than the one you are comparing it with is not comparable. Read the "from" and "to" dates on both and, if they differ, divide each bill by the number of days it covers before you compare anything. A daily cost is the only honest comparison between two bills.
Third: did the rate change?
Water charges change on 1 April. If your comparison spans that date, part of the difference is the tariff, not you. Thames Water's published metered charges for 2026-27 are £2.7346 per cubic metre for clean water and £1.4721 per cubic metre for wastewater from 1 April 2026, on top of fixed charges of £66.87 for water and £128.13 for wastewater, or £80.43 with the surface water drainage rebate. Check the current figures on your own bill and your own supplier's charges page, because those are Thames Water's and they are set annually.
Two other rate changes catch people out. Losing a surface water drainage rebate raises the fixed part with no change in usage. Moving from rateable-value charging to a meter changes the basis entirely, and a household that uses a lot of water can pay more on a meter than it did before.
Fourth: did the volume change
If the bill is built on two actual readings, covers a comparable period, and the rate is unchanged, then the cubic metres genuinely went up. Now the question is a real one, and it has two answers: somebody used more water, or water escaped.
The way to tell them apart is that usage has a pattern and a leak does not. Usage rises when people are home, falls when they are away, stops overnight. A leak runs at three in the morning at the same rate as at noon. Telling a leak from a genuine change in usage sets out the discriminating tests.
Before assuming the worst, count the ordinary causes:
- More people in the house, including a lodger, a returning student or a new baby.
- A hot, dry summer with a hosepipe, a new lawn, or a pressure washer used repeatedly.
- A new appliance plumbed in, or an old one failing and running longer cycles.
- A filling swimming pool, hot tub or large aquarium.
- Building work on site, where someone has been running a tap or a mixer from your supply.
Any of those will double a bill honestly. None of them needs an engineer.
The test that settles it
Turn off every tap and appliance, take a full meter reading including the red digits, use no water for thirty minutes, and read again. Thames Water's own guidance is that identical readings mean no leak, and different readings mean a loss between the meter and the inside stop valve. Then close the inside stop valve and repeat, which separates a loss inside the house from one on the supply pipe outside it. The full two-reading method covers the arithmetic and the traps.
Run the test at night if the first attempt is inconclusive. Eight hours of genuine non-use registers a loss that thirty minutes may not.
If it is a leak, the bill is recoverable
Water companies are required by their licences to reimburse the cost of water lost through leakage as credits to the bill. That is the leakage allowance, it can apply to water supply charges and to wastewater charges, and it can only be claimed once the leak has been repaired. The order matters: repair first, claim second, and keep the evidence from the whole sequence. The evidence pack a claim needs lists what to keep and when.
An allowance covers the water. It does not cover a soaked floor, a stained ceiling or the cost of getting at the pipe. That side is a buildings insurance question, and trace and access cover is what pays for the opening up. The insurance claims hub covers what the loss adjuster expects a report to contain.
When to call us, and when not to
Do not call anyone while the answer is still an estimated reading, a longer billing period or a hosepipe. Do call when the meter moves with everything off and nothing visible accounts for it, when the meter still moves with the inside stop valve closed, or when a damp patch has appeared alongside the bill.
Detection is charged at £150 per hour, agreed before we attend, and if we do not find the leak you do not pay for the time we spent looking. We repair the pipe as well as finding it, and put the floor, wall or ceiling back afterwards, which is the part most detection firms leave to you. What the rate covers is set out in full.
