TL;DR: You can move or divert a drain on your property, but permission depends on who owns the pipe. A private drain serving only your home can usually be diverted with Building Regulations approval. A lateral drain or public sewer belongs to South West Water and needs their consent under a formal diversion agreement. Get the drain surveyed and its ownership confirmed before you commit to any build.

You have planning permission, a builder booked, and footings marked out. Then the groundworker hits a clay pipe running straight through the middle of your new extension. Can you move or divert a drain on your property, or has the project just stalled? The answer depends almost entirely on one thing: whether that pipe belongs to you or to the water company. Most homeowners assume that anything buried in their garden is theirs to move. That assumption has cost people thousands. This guide explains who owns what, which permissions you need, how the diversion process actually works in Cornwall, and what to do when the answer comes back as no.

Can You Move Or Divert A Drain On Your Property?

Yes, a drain on your property can be moved or diverted, but only with the correct permissions. Private drains serving one household can be diverted under Building Regulations approval from your local authority. Lateral drains and public sewers are owned by the water company and require a formal diversion agreement before any work starts.

The distinction sounds academic. It isn’t. Diverting a private drain is a groundworks job that a competent drainage contractor can design, install and get signed off. Diverting a public sewer is a legal process involving South West Water, a signed agreement, an inspection regime, and a bill you pay in full.

Get the ownership question wrong and you can end up building over a public sewer without consent. That leaves you liable for the pipe, exposed at the point of sale, and potentially facing enforced removal of the structure above it.

So the first job is never digging. It’s identifying the pipe.

Who Owns The Drain? Private Drains, Lateral Drains And Public Sewers

A private drain is pipework that serves a single property and sits within that property’s boundary. A lateral drain is the section of pipe that runs from your boundary to the public sewer. A public sewer carries wastewater from two or more properties. Lateral drains and public sewers are owned and maintained by the water company, which in Cornwall is South West Water.

This split confuses people because it doesn’t follow the boundary line you’d expect. In 2011, a change in the law transferred most shared drains and lateral drains into water company ownership. Pipework that was private for a century became public overnight, and a lot of it was never mapped properly. Plenty of Cornwall homeowners are living above a public sewer without knowing it.

How To Tell Which Type You Have

If the pipe only ever carries waste from your house and stays inside your boundary, it’s a private drain.
If the pipe collects waste from your house and a neighbour’s, it’s a public sewer, even if it runs under your lawn.
If the pipe runs from your boundary out to the main sewer in the road, it’s a lateral drain and belongs to South West Water.
If you share a manhole with a neighbouring property, treat the pipe as public until proven otherwise.

A sewer map from South West Water is the starting point, but their records are indicative rather than exact, particularly for pipes adopted in 2011. The reliable answer comes from putting a camera down the line. A CCTV Drain Survey traces the run, plots connections, identifies shared pipework and gives you a report you can hand straight to building control or the water company.

Once you know who owns the pipe, you know which permission you’re chasing.

When Do You Need Permission To Move A Drain?

Permission is needed for almost every drain diversion. Altering underground drainage is controlled work under the Building Regulations 2010, so building control approval is required even on a purely private pipe. Any work affecting a lateral drain or public sewer additionally requires consent from South West Water.

Building Regulations Approval

Building control assesses the new run against Approved Document H of the Building Regulations, the government guidance covering drainage and waste disposal. It sets out pipe sizes, minimum gradients, protection of pipework, and where inspection chambers and access points must sit.

The practical consequence: a diverted drain has to keep enough fall to stay self-cleansing. Lengthen the run and you flatten the gradient. Flatten the gradient and solids settle. This is where a lot of DIY diversions fail, and why building control will want to see the drainage layout before you backfill the trench.

Skipping sign-off is a false economy. It surfaces at conveyancing, and the buyer’s solicitor will hold up the sale until it’s resolved.

Water Company Consent

Where a public sewer or lateral drain is involved, you’re into a separate process. South West Water’s guidance on building near a public sewer covers both building over the pipe and diverting it. Their published consent criteria for a domestic build over include the pipe being 225mm diameter or less, an invert depth of less than 2 metres, strip or trench fill foundations, and at least 300mm of headroom between the underside of the new floor slab and the pipe.

Miss any of those and consent to build over is unlikely. At that point, diversion becomes the route forward.

How The Sewer Diversion Process Works In Cornwall

Diverting a public sewer is done under Section 185 of the Water Industry Act 1991, which places a duty on the water company to move its apparatus where a landowner needs it moved to improve their land. The applicant pays the full cost. In Cornwall, applications go to South West Water.

South West Water’s diversion process allows some of the work to be carried out by your own contractor rather than by the water company, which is usually faster and cheaper. For homeowners on smaller projects, they also operate a Small Build Sewer Diversion Process, available where a build over application has been unsuccessful.

The Typical Sequence

Survey the drain and confirm ownership. A CCTV survey and a South West Water sewer map together tell you what you’re dealing with.
Apply for build over consent first. If the pipe can stay where it is, that’s the cheaper answer. Diversion is what happens when this fails.
Submit the diversion application with drawings, levels, pipe details and your proposed method.
Site visit and technical review. South West Water inspect, take measurements, and agree the approach with you.
Sign the agreement and pay the fee. No work on the sewer can begin until this is in place.
Construct the new run under inspection, with the water company attending at key stages.
Abandon the old pipe properly, then submit as-built drawings.

Timescales are the thing homeowners underestimate. Technical review alone runs to several weeks, and that’s before construction. Build the diversion into your programme at design stage, not when the digger is already on site and the builder is charging you for standing time.

For a plain private drain with no water company involvement, the process is far shorter: survey, design to Part H, notify building control, install, get it inspected.

What Affects The Cost Of Moving A Drain?

The cost of moving a drain is driven by ownership, depth, pipe diameter and ground conditions. A short private drain diversion in soft ground is a straightforward groundworks job. A deep public sewer diversion under a Section 185 agreement carries application fees, inspection costs and formal drawings on top of the excavation itself.

The variables that move the number most:

Whether the pipe is private or public. Public means fees, agreements, inspections and a longer programme.
Depth. Deeper pipes mean more excavation, more spoil, and shoring where trenches get deep.
Available fall. If there isn’t enough gradient for the new route, you may need a deeper connection point or a pumped solution, which changes the job entirely.
Ground conditions. Granite and shillet in Cornwall are slow and expensive to break out compared with soft soil.
Access. Narrow lanes, terraced cottages and restricted rear gardens limit what plant can reach the site.
Reinstatement. Driveways, patios and hard landscaping all have to go back.

We quote drain diversions on a fixed-price basis after a survey, so you know the figure before anyone lifts a shovel. There’s no call-out fee for us to come and look.

Alternatives To Moving The Drain

Diverting a drain is not always the best answer. Three alternatives are worth assessing before you commit to excavation, and any competent drainage engineer should raise them at survey stage rather than after the trench is open.

Build Over Or Build Close To The Pipe

If the pipe meets South West Water’s consent criteria, you may be able to leave it where it is and build over it. Their build over guidance notes set out exactly what they will and won’t accept. One common condition catches people out: the pipe must be in sound condition. If the CCTV survey shows cracks, root ingress or displaced joints, those defects need repairing before consent is granted.

Repair Rather Than Replace

Where the pipe is in poor condition but in an acceptable position, Drain Pipe Relining can bring it back to standard without excavation. A resin liner is cured in place inside the existing pipe, forming a new pipe within the old one. For a homeowner facing a build over application with a defective drain, this is often the fastest route to consent, and it’s covered by our 5-year guarantee.

Redesign The Build

Sometimes the cheapest fix is moving the extension, not the pipe. Shifting a wall a metre to clear the 3 metre zone around a public sewer costs nothing at design stage and can save a five-figure diversion. Worth raising with your architect before drawings are finalised.

Cornwall-Specific Complications Worth Knowing About

Drain diversions in Cornwall run into ground conditions and property types that don’t feature in generic UK guidance. Granite bedrock, high coastal water tables, and Victorian clay pipework all change what’s practical, and they change the cost.

Granite and shillet sit close to the surface across much of the county. Achieving the depth and fall a new drain run needs can mean breaking rock rather than digging soil, and that dictates both plant and price. In low-lying coastal areas around Falmouth, Truro and the estuaries, a high water table means trenches fill as fast as you dig them, so dewatering becomes part of the job.

Older Cornish housing stock brings its own issues. Salt-glazed clay pipework is common in properties built before the 1960s, and it’s brittle. Excavating alongside it frequently damages it. Terraced cottages and narrow lanes limit access for excavators, which pushes more of the work towards hand digging or specialist small plant.

Then there’s the shared drainage problem. Rows of Victorian and Edwardian terraces across Cornwall were built with a single drain run serving multiple houses. Under the 2011 transfer, most of those became public sewers. If you’re in a terrace and planning an extension, assume the pipe is public until a survey proves otherwise. On properties with no mains connection at all, the question shifts to your Septic Tank Maintenance arrangements and where the discharge point sits, which comes with a separate set of rules.

Get The Drain Surveyed Before You Dig

Moving or diverting a drain is achievable. The projects that go wrong are the ones where nobody established who owned the pipe before the groundworks started.

Three things to take away:

Ownership decides everything. Private means building control. Public means South West Water and a Section 185 agreement.
A CCTV survey is the cheapest step in the whole process and the one that prevents the expensive mistakes.
Build over consent, relining, or redesigning the extension may all beat diversion on cost. Assess them first.

Our engineers survey drains across Cornwall and Devon, identify ownership, produce the reports building control and South West Water want to see, and carry out the diversion work itself. No call-out fee, a fixed-price quote before we start, and a 5-year guarantee on every repair. If you have a blocked or damaged drain holding up a build, call us today on 01872 222555 and we’ll be on site within 1 to 2 hours anywhere in Cornwall.

Frequently Asked Questions

Do I Need Planning Permission To Move A Drain?

Planning permission is not normally required to move a drain on its own. Building Regulations approval is required, because altering underground drainage is controlled work. If the diversion forms part of a larger project such as an extension, planning permission may be needed for that project, and the drainage will be assessed by building control as part of it.

Can I Move A Manhole Cover In My Garden?

A manhole on a private drain can be relocated with building control approval, provided access for maintenance is retained. A manhole on a public sewer or lateral drain cannot be moved without consent from South West Water, because the water company needs continuous access to its network for repairs and maintenance. Building over an existing public manhole is generally refused.

How Long Does A Sewer Diversion Take?

A private drain diversion can often be surveyed, approved and installed within a few weeks. A public sewer diversion under Section 185 takes considerably longer, because South West Water carry out a technical review, agree the design, and require a signed legal agreement before any work on the sewer starts. Factor the water company process into your build programme at design stage rather than treating it as a formality.

What Happens If I Move A Drain Without Permission?

Unauthorised work on a public sewer can result in the water company requiring the work to be corrected or removed at your expense, and you may become liable for the pipe. Unapproved work under the Building Regulations leaves you without a completion certificate, which typically stalls the sale of the property until it is regularised through building control.

Who Pays To Divert A Public Sewer?

The person requesting the diversion pays. Under Section 185 of the Water Industry Act 1991, the water company has a duty to divert its apparatus where reasonable, but it is entitled to recover the costs it reasonably incurs. That covers application fees, technical review, inspections and any work carried out by the water company itself.

How Do I Find Out Where My Drains Run?

Request a sewer map from South West Water for an indicative layout, then confirm it with a CCTV drain survey. Water company records are guidance rather than a precise plan, particularly for pipework adopted in 2011, which was often never mapped in the first place. A camera survey traces the actual run, records depths and connections, and produces a report you can submit with a build over or diversion application.

Can You Move or Divert a Drain on Your Property?