blogTL;DR: A build over agreement is written consent from your water company to build over, or within three metres of, a public sewer or lateral drain. In Cornwall, that means South West Water. You need one for most extensions, conservatories and garages that sit near buried public pipework. Skip it and building control can withhold your completion certificate, which causes serious problems when you sell.
Most homeowners find out they need a build over agreement at the worst possible moment: after the drawings are signed off, when building control asks a question nobody expected. There is a public sewer running under the back garden, and the extension foundations land right on top of it.
It happens more often than people expect. Since October 2011, thousands of pipes that homeowners had always assumed were private became the property of the water company. Shared drain runs behind Victorian terraces in Falmouth, Truro and Penzance are now public sewers, whether or not anyone told the residents.
Getting this wrong is expensive. Getting it right takes an application, a set of drawings, and in many cases a CCTV Drain Survey to prove the pipe is sound. This guide covers what a build over agreement is, when you need one, how to apply through South West Water, and what happens if you build without one.
What Is a Build Over Agreement?
A build over agreement is written consent from your water company to build over, or within three metres of, a public sewer or lateral drain. It confirms that your foundations, floor levels and access arrangements will not damage the pipe, and that the water company can still reach it to carry out repairs.
Two definitions matter before you go any further.
- Public sewer: a pipe that carries wastewater from more than one property. The water company owns it and maintains it, even where it runs beneath your garden.
- Lateral drain: the section of your own drain that continues past your boundary line. It serves your property alone, but the water company owns it once it leaves your land.
Before October 2011, most of these pipes were privately owned. The transfer of private sewers moved the majority of them into water company ownership across England and Wales. That single change is why so many homeowners now discover public pipework under a garden they have owned for thirty years.
The legal backing sits in two places. The Water Industry Act 1991 gives water companies the right to maintain, repair and renew public sewers. Requirement H4 of Approved Document H of the Building Regulations covers building over sewers and drains, and requires that work does not damage the pipe or remove access to it.
Put simply, the agreement protects the pipe from your building, and protects your building from the water company.
When Do You Need a Build Over Agreement?
You need a build over agreement when building work sits directly over a public sewer or lateral drain, or within three metres of one. The trigger is the position of the pipe, not the size of your project. A modest conservatory over a shared drain run needs consent. A large extension well clear of any public pipework does not.
Work That Typically Triggers an Agreement
- Rear, side and wraparound extensions
- Conservatories and orangeries
- Attached garages and car ports
- Basement excavations and lower ground works
- Underpinning and piled foundations
- Any new foundation within three metres of a public sewer or lateral drain
New foundations are the sticking point. Excavation next to a pipe removes the ground supporting it, and the weight of a new wall transfers load into the ground around it. Both can crack a clay pipe that has sat undisturbed since the 1930s.
Work That Usually Falls Outside It
Freestanding structures with no foundations, such as a timber shed on a slab, sit outside the requirement in most cases. Loft conversions rarely trigger it, because no new ground-bearing foundation is involved. Resurfacing a patio does not qualify either, provided you are not covering a manhole.
One exception catches people out. Even where the pipe on your land is genuinely private, some water companies still want an application if you are building within one metre of the point where it crosses your boundary and becomes a lateral drain. Check before you assume you are clear.
There is also a hard limit worth knowing early. You cannot enclose a manhole or inspection chamber inside a new building. If a chamber sits inside your proposed footprint, it has to be removed and rebuilt outside the extension, and that changes your drainage design before it changes anything else.
How to Find Out If a Public Sewer Runs Under Your Property
Public sewers are recorded on sewer maps held by the water company, and a drainage and water search during conveyancing will normally show one if it exists. Maps are not the whole story, though. Many post-2011 transferred pipes were never surveyed properly and do not appear on any record at all.
Start with the paperwork you already have. Your conveyancing pack from when you bought the house often contains a drainage and water search. If it shows a public sewer crossing the plot, you have your answer.
If it shows nothing, look at the ground. Manhole covers in your garden, in a neighbour’s garden on the same line, or in the lane behind the property all point to a shared run. A drain that takes waste from the house next door before it reaches your boundary is a public sewer, regardless of what any map says.
Cornwall makes this harder than most counties. Terraced streets in Falmouth and Truro were built with shared clay drain runs threading behind the properties, and granite and shillet close to the surface pushed many of those pipes shallower than the standard depths water companies expect. Rural properties with septic systems add another layer, because a private drain to a Septic Tank Maintenance arrangement sits under different rules altogether.
When the records are unclear, the only way to know for certain is to look inside the pipe.
Why a CCTV Drain Survey Sits at the Centre of the Process
A CCTV Drain Survey uses a camera on a flexible rod to record the inside of a drain, confirming its route, depth, diameter, material and condition. Water companies rely on this evidence to decide whether a pipe can safely be built over, and to prove the pipe was sound before construction started.
The survey does three jobs in a build over application.
- It maps the pipe. You get the exact route and depth, which your architect needs to design foundations and floor levels around.
- It records the condition before work begins. Cracks, root ingress, displaced joints and partial collapses all show up on camera.
- It protects you afterwards. A second survey once the build is complete shows the pipe is in the same condition it was in before, which settles any argument about liability.
The condition report matters more than most homeowners realise. Water companies will not consent to a build over a defective pipe. If the survey finds a fracture or a collapsed section, that defect has to be repaired before consent is granted, and the cost of the repair sits with you.
This is where Drain Pipe Relining earns its place. A No-Dig Repair cures a resin liner inside the existing pipe, restoring structural integrity without excavating the garden you are about to build on. On a live extension project with a foundation trench already dug, that difference is measured in weeks. Every relining repair we carry out is covered by our 5-Year Guarantee.
Our engineers Schedule Your Expert CCTV Drain Survey across Cornwall and Devon, and provide the recorded footage and written report in the format water companies and building control expect.
How Do You Apply for a Build Over Agreement in Cornwall?
In Cornwall, applications go to South West Water. There are two routes: a Declaration of Works if your proposal meets the published consent criteria, and a full enquiry if it does not. Building control will also require a full plans application rather than a building notice for any work within three metres of a public sewer.
The process runs roughly like this.
- Establish the route, depth and diameter of the pipe, using sewer maps and a CCTV Drain Survey.
- Check your design against South West Water’s build over guidance notes, which set out the consent criteria in detail.
- If you meet the criteria, submit the Declaration of Works form. Consent is typically confirmed by letter.
- If you do not meet the criteria, submit the enquiry to build over or close to public drainage instead. South West Water will assess the proposal and may request a site visit or camera inspection.
- Repair any defects the survey identifies, then proceed to build to the agreed specification.
The consent criteria are technical, and they will shape your design. South West Water’s published guidance covers points such as the depth of the pipe from ground level to the invert, and the minimum headroom required between the underside of a new floor slab and the pipe. Your architect or structural engineer should read the criteria before finalising the foundation design, not after.
If the application is refused, that is not the end of the project. South West Water offers a small build sewer diversion process for homeowners whose build over application is unsuccessful, allowing the pipe to be moved rather than the extension redesigned. Details sit on their building near a public sewer page.
What Happens If You Build Without a Build Over Agreement?
Building over a public sewer without consent leaves you exposed on three fronts: building control can withhold your completion certificate, the water company retains the legal right to access the sewer even if that means removing your structure, and the missing paperwork will surface the moment you try to sell or remortgage.
Take each in turn.
The completion certificate. Building control will not sign off work that breaches Requirement H4. Without a completion certificate, your extension is not legally compliant, and a buyer’s solicitor will find that out.
Access rights. The Water Industry Act 1991 gives the water company the power to maintain and repair its sewers. That power does not expire because someone built a kitchen over the pipe. Where a structure obstructs access, the company can require its removal and pursue the cost.
The sale. This is where most people feel it. Conveyancing searches flag the public sewer, the buyer’s solicitor asks for the build over agreement, and there isn’t one. The chain stalls while everyone works out who pays for what.
None of this is theoretical. It is the single most common reason we get called to run a CCTV Drain Survey on a property that is already under offer.
Retrospective Agreements, Comfort Letters and Indemnity Insurance
Where work has already been built over a public sewer without consent, three routes are open: apply for a retrospective build over agreement, obtain a comfort letter from the water company confirming the sewer is in good condition, or take out indemnity insurance to cover the buyer against future loss.
A retrospective agreement gives the strongest protection, because it removes the water company’s right to demolish the structure. Water companies grant these by exception rather than as routine, and they will want to see the pipe’s condition before they agree to anything. That means a CCTV Drain Survey, and any defects found will need repairing first.
A comfort letter is a lighter option. You supply the survey footage, and if the water company is satisfied the sewer is sound, it confirms in writing that it has no current concerns. Buyers and lenders often accept this.
Indemnity insurance is the fastest route and the weakest one. It does not fix the pipe or change the water company’s rights. It simply pays out if things go wrong later, and insurers will not always offer a policy once the risk is known.
All three routes start with the same thing: knowing what condition the pipe is in.
Getting It Right Before You Build
Build over agreements are not a formality to deal with after the concrete is poured. They shape the foundation design, the floor level, and the position of every manhole on the plot.
- The trigger is three metres from a public sewer or lateral drain, not the scale of your build.
- Water companies will not consent to a build over a defective pipe, so the survey comes first.
- Building without consent puts your completion certificate, and your future sale, at risk.
If you are planning an extension anywhere in Cornwall and you suspect there is a public sewer under the footprint, find out now rather than in month three. Our engineers will survey the pipe, produce the recorded footage and written report your application needs, and reline any defects we find under our 5-Year Guarantee. There is No Call-Out Fee, you get a Fixed-Price Quote before any work starts, and we are on site anywhere in Cornwall within 1 to 2 hours.
Call our team on 01872 222555 or book your CCTV Drain Survey today.
Frequently Asked Questions
How long does a build over agreement take?
Timescales depend on which route your application takes. A Declaration of Works submitted against South West Water’s consent criteria is the faster path, with consent typically confirmed by letter. A full enquiry, where the proposal falls outside the criteria, takes longer because it may involve a site visit or camera inspection before a decision is made.
How much does a build over agreement cost?
Water companies set their own charges, and they are reviewed annually. The application fee is only part of the cost. Budget for the CCTV Drain Survey, any repairs the survey identifies, and any design changes needed to meet the consent criteria. Check South West Water’s current charges directly before you budget.
Can I build over a manhole or inspection chamber?
No. Water companies do not permit manholes or inspection chambers to be enclosed inside a building, because of the flooding and odour risk. If a chamber sits within your proposed footprint, it must be removed and a replacement constructed outside the new structure. Foundations also need to keep clear of the chamber walls.
Do I need a build over agreement for a private drain?
Not in most cases. A drain that serves only your property, entirely within your boundary, is private and falls outside the requirement. One exception applies: if you are building within one metre of the point where that drain crosses your boundary and becomes a lateral drain, some water companies still require an application. Confirm with South West Water before assuming you are clear.
What if the CCTV survey finds the sewer is damaged?
The defect has to be repaired before consent is granted, and the cost sits with the property owner. Drain Pipe Relining is often the practical answer, because a resin liner is cured inside the existing pipe without excavating the ground you are about to build on. Every relining repair Clear Stream carries out is covered by a 5-Year Guarantee.
Who is the water company for Cornwall?
South West Water is the sewerage undertaker for Cornwall and Devon. All build over applications, sewer diversion enquiries and public sewer records for properties in Falmouth, Truro, Newquay, St Austell, Penzance, Bodmin and the surrounding areas go through South West Water rather than any other provider.
Can I get a build over agreement after the work is finished?
Retrospective agreements are possible, but water companies grant them by exception rather than as standard. You will need a CCTV Drain Survey showing the sewer’s condition, and any defects found must be repaired first. If a retrospective agreement is not available, a comfort letter or indemnity insurance may satisfy a buyer’s solicitor instead.
