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Drain Mapping in Dorchester

Tracing and recording where the runs actually go, before anyone digs or builds.

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What is drain mapping?

Drain mapping is tracing the real route of a property's underground drains, their depth and the way they join up, then recording it as a plan you keep.

It answers questions a camera survey alone does not: which way a pipe goes after it leaves a chamber, which fittings feed which run, where your drainage joins a neighbour's, and where it meets the public sewer. The result is a drawing showing chambers, pipe routes, flow directions, connections and estimated depths. Mapping is about location. If you also need to know what condition the pipes are in, it is usually combined with a CCTV survey on the same visit, since the camera is often what does the tracing.

When do you need your drains mapped?

Drain mapping is worth doing before you build, dig or buy, and whenever there is a question about whose drain is whose.

  • Planning an extension, conservatory, garden room or outbuilding
  • Laying a new drive, patio or foundations for a wall
  • Applying to build over or near a public sewer
  • Adding a bathroom or kitchen and needing a connection point
  • Settling a dispute with a neighbour about a shared run
  • Chasing a repeat blockage or smell when nobody knows where the pipe goes
  • Finding a misconnection, such as a sink feeding the surface water drain

The common thread is that someone is about to make a decision, or spend money, based on where the drains are.

Why drain records are often wrong

Drain records are often wrong because extensions, conversions and re-plumbing change the drainage and the changes are seldom written down, so the paperwork and the pipes drift apart.

Original building plans show the drains as designed, not as laid, and not as changed since. A previous owner may have added a bathroom, diverted a run round an extension or connected a new gully without telling anyone. Water company records cover public sewers but rarely show private drains in any detail, and many of the shared sewers transferred to the water companies in 2011 were never accurately mapped. Over several decades of small changes, the gap between record and reality can be large.

How we map drains, step by step

We work out the layout from every access point we can open, prove the connections with running water, trace the routes from above, and draw up what we find.

  • We agree what needs mapping and the price before starting.
  • We locate and lift every chamber, gully and rodding point, noting depths and the direction pipes enter and leave.
  • We run taps, flush toilets and use coloured water where needed, to prove which fitting feeds which run.
  • Where a route cannot be seen from a chamber, a camera with a transmitter is pushed along the pipe and followed from above with a locator, marking its line and estimated depth on the ground.
  • We note where the drainage crosses the boundary or joins a neighbour's run.
  • We draw it up as a plan and send it to you.

How accurate is drain mapping?

Drain mapping is accurate enough to plan building work and decide where to dig, but depths traced from the surface are estimates, and where a route is uncertain the plan says so.

Chamber positions and depths measured directly are the most reliable parts. Routes traced with a locator are generally close to the true line, though depth readings can be thrown off by deep pipes, nearby metal, other buried services and reinforced concrete above. A run between two chambers is often assumed straight when it may not be; tracing it removes that assumption. Where a section could not be traced, it is shown as unconfirmed rather than guessed. If a dig depends on an exact position, a trial hole is the final check.

What's included in a drain mapping plan?

A drain mapping plan shows the position of each chamber and access point, the route of each pipe, the direction of flow, connections between runs and the estimated depths.

It also records:

  • Which runs carry foul water and which carry surface water
  • The fittings or areas each run serves
  • Where the drainage meets a neighbour's or crosses the boundary
  • The point where it joins the public sewer, where that can be established
  • Runs or sections that could not be traced, marked as unconfirmed
  • Any defects noticed along the way, where a camera has been used

The plan is yours. It is usually what an architect, builder or building control officer asks for when drainage affects a design.

Do I need drain mapping before building an extension?

Usually yes, if you do not already know exactly where the drains run, because building over or near a drain affects both approval and future access.

Under Building Regulations Approved Document H, any building over a public sewer, or close to one, needs the sewerage company's agreement. Around Dorchester that is Wessex Water, and the arrangement is called a build-over agreement. Since 2011 many runs shared by two or more homes are public sewers, so a pipe you thought was private may need one. Private drains under an extension still need protecting and access keeping. Knowing the route and depth before the foundations are set out is far cheaper than finding out with a digger.

Do I need my drains mapped when buying a house?

Mapping is not a legal requirement when buying, but it is worth having if you plan to extend, if the drainage looks shared, or if the search results and the visible covers do not match.

Your solicitor's drainage and water search draws on the water company's sewer records to say whether the house drains to the public sewer and whether any public sewer crosses the plot. It will not show private drains, and as those records can be incomplete, it may not show a shared run either. If you are buying with an extension in mind, mapping before exchange tells you where you can build. If you only need to know the drains are in good condition, a CCTV survey is the more useful check.

How much does drain mapping cost?

The cost of drain mapping depends on the size of the property, the number of runs and chambers, how much tracing from above is needed and what level of plan you need. As a guide, mapping the drains at a typical house costs £250 to £500, and larger or commercial sites from £500 to £1,000 or more. We agree a fixed price before we start.

The main factors:

  • The number of chambers, gullies and separate runs
  • Whether covers are buried, sealed or missing
  • How much of the route has to be traced with a camera and locator
  • Whether the drains need clearing before a camera can pass
  • Whether a condition survey is wanted at the same time

Combining mapping with a CCTV survey on one visit usually costs less than booking them separately. If we find the job is bigger than described, we stop and agree any change first.

Who is responsible for the drains on the map?

Pipework used by your household alone and lying within your boundary is yours to maintain; from the point it is shared or leaves your land, it normally belongs to the water company.

That division dates from the October 2011 transfer of shared private sewers and lateral drains to the water companies. A map often shows it plainly for the first time: the point where your pipe joins a neighbour's is usually where your responsibility ends. That matters when a neighbour's blockage starts affecting you, or when someone asks who should pay. In rented property, maintaining the drains normally rests with the landlord, who will want the plan.

What you can find out yourself first

You can learn a fair amount by lifting the light covers on your property and watching which chamber the water arrives in when you run each tap and flush each toilet.

Use gloves, leave heavy or rusted covers where they are, and keep small children and dogs well away from an open chamber. Note where each cover is and roughly how deep the water channel sits. That rough sketch helps us and can shorten the job. Do not dig to find a pipe, and do not drive stakes or posts into the ground where a drain might run. Drain mapping is rarely an emergency, but if a digger has struck a pipe or sewage is surfacing in the garden, call 0330 133 1625 straight away. Keeping your plan with the house deeds saves the next owner paying for the same work.

Covering Dorchester & the local area

We provide drain mapping throughout Charminster, Martinstown, Crossways, Puddletown, Broadmayne, Poundbury and the surrounding area. Call 0330 133 1625 for a free, no-obligation quote.

How it works

Simple, transparent process

  1. 1

    Tell us what’s happening

    Call and describe what you’re seeing — a slow sink, a gurgle, a smell or standing water. It usually tells us a lot before we arrive.

  2. 2

    We find the cause

    We clear enough to get flow back, then work out why it blocked — because a drain that blocks twice has a reason.

  3. 3

    A price before the work

    If it needs more than a clear — a repair, a lining, a survey — you get the price and the reason before we start it.

  4. 4

    You see the evidence

    Where we survey, you get the footage. Where we repair, you see the before and after.

Don't let it get worse

Get expert help right now

Tell us what the drain is doing and we will explain what is likely causing it and what the work involves.

Call 24/70330 133 1625

Other services

Emergency Unblocking

A drain that has stopped completely, water backing up or sewage escaping — the jobs that cannot wait until Monday.

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Blocked Sinks

Kitchen sinks, basins, baths and showers draining slowly or not at all.

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Drain Cleaning

Clearing build-up before it becomes a blockage — for properties that keep having the same problem.

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Drain Mapping: common questions

How long does drain mapping take?
For a typical house with a handful of chambers, mapping is usually finished in one visit of a few hours. It takes longer where covers are buried or missing, where runs have to be traced from above over long distances, or where the drains need clearing before a camera can get through. The written plan follows the visit. We can give you a better idea once we know the property.
Can drain mapping find drains that aren't on any plans?
Yes. Finding unrecorded drains is one of the main reasons people book it. Running water through each fitting shows which runs exist, and tracing a camera from above follows pipes that no drawing mentions. It often turns up old connections from earlier extensions, runs diverted around later building work, and redundant pipes that were never capped. Those are exactly the things that cause trouble when someone starts digging.
How do you trace a drain you can't see from the surface?
A small transmitter, usually built into the head of a drain camera, sends out a signal as it travels along the pipe. A handheld locator picks up that signal from above, which lets us follow the route across the ground and estimate how deep the pipe is at each point. Where a camera cannot be used, running coloured water between chambers still proves which ones are connected.
Can I find out which drains I share with my neighbours?
Yes. Mapping shows where your drain joins another property's, which is usually also the point where responsibility passes to the water company. We need access to your own chambers. Following the run onto a neighbour's land needs their permission, though a shared run can often be traced from your side. Knowing this helps if a blockage on their side backs up into yours, or the reverse.
Is drain mapping the same as a CCTV drain survey?
No, though they overlap. Mapping produces a plan of where the drains go; a CCTV survey produces footage and findings on what state they are in. Someone planning an extension mostly needs the plan, while someone chasing repeat blockages mostly needs the footage. Because the camera is often the tool that traces the route, the two are frequently done together, and the plan then shows defects as well as positions.
Can you map a drain that is blocked?
Partly. Chamber positions and depths can still be recorded, but a camera cannot travel through a blocked pipe, and water tests will not show a connection that is not flowing. In most cases the blockage is cleared first, then the run is traced. If we find a blockage on the day that was not expected, we explain what clearing it involves and agree the cost before doing it.
Will you need to dig to map my drains?
Usually not. Mapping works from existing access points and from tracing above ground, so the garden, drive and floors stay as they are. Digging is only needed if a chamber has been buried and must be uncovered, or if a section cannot be traced any other way. If either applies, we explain why and agree it with you before lifting anything.
Can mapping find a misconnected pipe?
Yes. A misconnection is a foul pipe, such as a toilet, sink or washing machine, wrongly connected to the surface water drain, which carries rainwater straight to a stream or river untreated. Dye testing during mapping shows where each fitting really goes. Misconnections are common after extensions and appliance fitting, and the property owner is responsible for putting them right once they are found.
Where can I see the official sewer map for my street?
The water company keeps records of public sewers and can provide extracts; for Dorchester that is Wessex Water. A drainage and water search during a property purchase draws on the same records. These maps usually do not show private drains, and many former private sewers that transferred in 2011 are missing or shown only roughly, which is why mapping on site is often still needed.
Can I give the drain plan to my architect or builder?
Yes, that is often the point of it. The plan shows where runs, chambers and connections are, so the design can avoid them, divert them or allow for a build-over application. It is yours to share with anyone involved in the project. If the design changes the drainage, update the plan once the work is done, so the next person to dig knows what is down there.