How Thames Water Affects Your Plumbing: London Homeowner Guide

The Responsibility Boundary

The most important concept for any London homeowner dealing with a water or drainage issue is understanding where Thames Water's responsibility ends and yours begins.

Thames Water is responsible for:

• The water mains running in public roads and pavements
• The connection between the water main and your property boundary (the communication pipe)
• The water meter (if fitted) up to and including the meter itself
• The public sewer system in the road
• Shared private sewers that were adopted in 2011 (see below)

You are responsible for:

• The supply pipe from your property boundary to your internal stop tap (rising main inside the property)
• All internal plumbing from the internal stop tap onward
• The private drain from your property to where it joins the public sewer or the adopted shared sewer at your property boundary
• Any drains that serve only your property

The property boundary is the legal demarcation. A burst on the street-side of your boundary is Thames Water's emergency; a burst on the house-side of the boundary is yours. If you are unsure where your property boundary is, the deeds of your property or Land Registry records will show it. The external stop tap (typically in a small chamber in the pavement or driveway, covered by a small metal or plastic lid) marks approximately the boundary point.

Minimum Pressure Guarantee

Thames Water is required by the Water Industry Act 1991 to maintain a minimum mains pressure of 0.7 bar at the point where the supply enters your property. This minimum is lower than most people expect — 0.7 bar is sufficient to supply water to ground-floor taps but is barely adequate for upper floors or for showers requiring higher pressure.

If you are consistently experiencing very low water pressure — not enough flow at upper-floor taps, showers that barely function — and your internal plumbing is not the cause, check with Thames Water. They can measure the pressure at the boundary and confirm whether it meets the statutory minimum. If the pressure is below 0.7 bar due to a supply network issue, Thames Water is obliged to investigate and remedy it.

Note that high pressure is the more common complaint in inner London — see the section on pressure reducing valves elsewhere on this site. Very low pressure is more typical in outer London areas where the supply infrastructure is older or where there is high demand in the distribution zone.

Water Quality and Hardness

Thames Water serves London from two main sources: the River Thames (surface water, treated at a series of water treatment works) and groundwater from the chalk aquifer beneath London (drawn via boreholes and treated). Both sources produce water that is safe to drink and meets the stringent requirements of the Water Supply (Water Quality) Regulations 2016.

London water is among the hardest in the UK. Water hardness is measured in milligrams of calcium carbonate per litre (mg/l) and is classified as:

• Soft: below 60 mg/l
• Moderate: 60–120 mg/l
• Hard: 120–180 mg/l
• Very hard: above 180 mg/l

London tap water typically measures 200–400 mg/l depending on location — substantially in the "very hard" category. This has practical consequences for plumbing:

• Limescale deposits build on heating elements, inside kettles, in showerheads, on tap aerators, and on the heat exchanger inside the boiler. This reduces efficiency and accelerates component wear.
• In central heating systems, limescale deposits on the boiler heat exchanger reduce heat transfer and increase fuel consumption.
• Shower enclosures, glass screens, and chrome fittings require more frequent cleaning to prevent scale buildup.

A water softener — connected before the branch point to the kitchen drinking tap (which the Water Supply Regulations require to remain unsoftened) — substantially reduces scale accumulation throughout the property's plumbing. This is a particularly worthwhile investment in London given the water hardness levels.

Shared Sewers: The 2011 Transfer

Before October 2011, private sewers and lateral drains shared between multiple properties were the collective responsibility of the properties they served — meaning that when a drain shared between several houses blocked or collapsed, it was the homeowners' problem to fix and fund jointly. This was a well-established source of neighbour disputes and uncertainty about responsibility.

In October 2011, the Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011 came into force. These regulations transferred most shared private sewers and lateral drains to the water and sewerage companies (Thames Water in London) for maintenance and repair. Since October 2011:

• Drains shared between two or more properties are the responsibility of Thames Water to maintain and repair
• If a shared sewer collapses or requires jetting, this is a Thames Water job, not a private plumber job at your expense
• The drains that serve only your individual property remain your private responsibility

The practical implication: if you have a drain blockage or collapse, establish first whether the drain serves only your property or is shared with neighbours. If shared, contact Thames Water rather than instructing a private drainage company at your own cost. Thames Water's emergency drainage number handles shared sewer problems.

Surface Water Drainage

Surface water — rainwater from roofs, driveways, and gardens — is handled by a separate drainage system from foul water (sewage). In London, responsibility for surface water drainage is split:

• Thames Water manages surface water sewers connected to properties
• The local council manages surface water drainage in roads and public spaces
• Transport for London manages drainage on TfL-controlled roads (the red route network)

If you are experiencing surface water flooding, the appropriate contact depends on where the water is coming from — a blocked surface water drain serving your property is a Thames Water call; flooding from an overwhelmed road gully is a council matter.

How to Report a Mains Burst Outside Your Property

If you see water streaming from the road surface, from a pavement, or from a visible mains pipe outside your property boundary, this is Thames Water's responsibility. Contact Thames Water directly on their emergency line: 0800 316 9800. Do not instruct a private plumber for a mains burst in the road — this is Thames Water infrastructure and they must attend it.

While waiting for Thames Water to attend a mains burst near your property, be aware that the burst may temporarily reduce pressure to your property. If your water supply is affected, report this to Thames Water when you call the burst in.

Metered vs. Unmetered Billing

Thames Water has progressively moved toward compulsory metering for most London properties. Water meters are now mandatory for:

• New connections
• Properties in designated water-scarce areas
• Properties where metering has been implemented as part of a Thames Water metering programme

If you are not currently on a meter, you can request a meter installation from Thames Water (it is free). For households with fewer occupants than bedrooms, moving to a meter typically reduces the bill — the average meter installation saving is cited by Thames Water as approximately 14% for smaller households. For large families in small properties, unmetered billing may be cheaper.

The Thames Water Leak Allowance

If you discover a leak on the supply pipe between your property boundary and your internal stop tap — the section of pipe that is your responsibility — and you arrange for a plumber to repair it promptly, Thames Water operates a leak allowance scheme. Under this scheme, Thames Water may agree to reduce your water bill for a period to reflect the water lost through the leak before detection and repair.

To apply for the leak allowance:

• Contact Thames Water after the leak has been repaired
• Provide evidence of the repair (plumber's invoice and confirmation the leak has been fixed)
• Thames Water will assess the claim and may offer a bill credit for the estimated water loss during the leak period

The allowance is at Thames Water's discretion and is not available indefinitely — it applies to leaks that were reported and repaired promptly. A leak that was known about for an extended period before repair is unlikely to attract the full allowance.