Drain Mapping: How to Get a Plan of Your Property's Drains

To get a plan of your property's drains, start with what already exists: the water company's sewer records, any drainage plan in your deeds or building control file, and the drainage and water search from your purchase. If none of those shows your private drains, which is common in London, you need a drain mapping survey that traces each run on site and draws it to scale.

What is drain mapping?

Drain mapping is the process of locating every drain and chamber on a property and recording them on a scaled plan, with pipe sizes, depths and flow directions. The result is a drawing you can hand to an architect, a builder, building control or a buyer.

It overlaps with drain tracing but is not quite the same thing. Tracing answers a specific question, such as where one pipe goes or which chamber a gully connects to. Mapping records the whole system, so the plan can be used for design and future work, not just for one problem.

Where to look before paying for a survey

Check the free and existing records first, because occasionally they are enough. More often they show the public sewer but not the private drains on your side of the boundary.

• Water company sewer records. Thames Water holds maps of the public sewers across most of London and supplies extracts through an asset location search. These show the sewer in the road and sometimes shared runs, but rarely the pipes within a single property. Many private sewers transferred to the water company in 2011 are still missing from the maps.
• The drainage and water search from your purchase. The CON29DW search your conveyancer ordered states whether the property drains to a public sewer and whether a public sewer runs within the boundary. It includes a sewer map extract, not a plan of your own drains.
• Building control records. For houses extended or built in recent decades, the local authority's building control file may contain approved drainage drawings. Treat them as the design, not necessarily what was built.
• Your deeds and previous surveys. Older deeds sometimes include a drainage plan, and a previous owner's CCTV survey report may include a sketch of the runs.

If you do find a plan, check a couple of chambers against it before relying on it. Extensions, patios and later connections change drainage layouts more often than the paperwork does.

How a drain mapping survey works on site

The engineer lifts every chamber cover that can be found, measures what is in each one, and then follows each pipe between them. Where a run cannot be seen from the surface, its route is located from inside the pipe.

In each inspection chamber or manhole the survey records the cover size, the depth to the invert (the bottom of the channel), the diameter of each pipe entering and leaving, and which way the flow runs. Between chambers, a camera carrying a small transmitter called a sonde is pushed along the pipe while an operator above ground follows the signal with a receiver and marks the line. Gullies, rainwater pipes and soil stacks are checked with dye or water to confirm where they discharge.

The site measurements are then drawn up as a plan showing chambers, runs, directions of flow and connections, usually with a chamber schedule listing depths and sizes. If the plan needs to show levels relative to a datum, for example for an architect designing a new connection, that should be requested at the start, because it adds survey work.

Foul water, surface water and misconnections

A good drainage map separates foul drains, which carry toilet, kitchen and bathroom waste, from surface water drains, which carry rain. Many London properties have a combined system where both share one pipe, but many others have separate systems, and the map should show which you have.

The distinction matters when you build. Connecting a new bathroom into a surface water drain, or a new downpipe into a foul drain on a separate system, is a misconnection. Foul water in a surface water sewer ends up in rivers. Mapping is often the point at which old misconnections are found, and it is far cheaper to find them on a drawing than after a complaint.

When you actually need a drainage plan

You need one whenever someone has to design around your drains or take responsibility for them. The most common London triggers are building work, a sale, and managing a building with several owners.

• Extensions and basements, where the designer needs to know whether a drain runs under the new footprint and whether it is private or a public sewer.
• Selling or buying, where a plan and a condition survey answer questions that a standard search leaves open.
• Blocks of flats and commercial buildings, where a managing agent needs to know which runs serve which parts of the building before arranging maintenance.
• Repeat blockages or smells where no one can find the chamber that matters.

Mapping is often combined with a condition survey on the same visit, since the camera is already in the pipe. Our CCTV drain survey covers condition, and drain tracing covers route and connections.

What sets the cost of drain mapping

The cost depends mainly on how many chambers and runs there are, how easy they are to reach, and what the finished drawing has to show. A terraced house with two accessible chambers is a very different job from a block with buried covers and a pumped basement.

Covers that have been paved or tarmacked over, runs that cross into neighbouring land, heavy silting that has to be cleared before a camera can pass, and a requirement for levels to a datum all add time. Agree the scope in writing before the visit: which areas are included, whether levels are needed, and whether the output is a sketch or a scaled drawing. Our pricing page sets out how our survey work is charged.

If you only need to know where one pipe goes, say so. A short trace answers that question without paying for a full drawing of everything else.