If your foundations will sit within 3 metres of the centreline of a public sewer, or within 1 metre of a public lateral drain, you need a build-over agreement from your local water company before you start work. Planning permission and Building Regulations approval do not cover this. It is a separate legal step, and building control will not issue your completion certificate without it.
Do not break ground until you have written consent. Skipping this step risks enforcement action, retrospective costs, and problems when you come to sell.
Your first move today:
- Check your water company’s online sewer map or order a sewer plan for your property.
- Look for manhole covers on your plot. These often signal a sewer running closer to your build line than old title deeds suggest.
- If in doubt, treat any pipe on site as public until your water company confirms otherwise.
Key Takeaways
A build over agreement is a legal requirement whenever foundations fall within 3 metres of a public sewer or 1 metre of a public lateral drain, and it must be secured before construction begins.
| Point | Details |
|---|---|
| Check first, design after | Order a sewer map and check manhole positions before your architect finalises foundation drawings. |
| Match the route to the risk | Self-declaration suits low-risk cases; larger or deeper foundations need a formal application with CCTV. |
| Budget realistically | Expect from £102 for self-declaration up to £772 or more for formal applications, plus survey costs. |
| Keep every document | Store the signed agreement, CCTV reports and completion certificate together for future conveyancing. |
| Sequence CCTV before groundworks | Booking surveys early avoids the delays that catch out unplanned North West extension projects. |
If your extension or conversion plans touch a public sewer, get the legal groundwork sorted before the diggers arrive. Complete-Property-Solutions manages this coordination as part of a fully project-managed build across Warrington, Great Sankey, Culcheth, Lowton, Rainhill, Haydock and the wider North West, sequencing sewer surveys, building control inspections and water company approvals so your programme doesn’t stall halfway through. Explore our house extension service to see how we handle this from first design through to final sign-off.
Table of Contents
- When do you need a build over sewer agreement?
- How do you check the exact position of a sewer?
- Which application route applies: declaration, formal application or protection agreement?
- What does a build over agreement cost and how long does it take?
- What happens after the build over agreement is approved?
- What are the most common mistakes homeowners make?
- How does Complete-Property-Solutions manage this on North West projects?
- What legal framework actually governs this?
- Who does what: householder versus water company
- Does a build over agreement affect property value or a future sale?
- What clauses typically appear in a build over agreement?
- What if the water company refuses your application?
- An editorial view: what actually matters here
- Sources
When do you need a build over sewer agreement?
The 3 metre and 1 metre triggers come from Approved Document H, the national technical guidance that water companies use when they assess build over applications. If your foundations fall within 3 metres of the centreline of a public sewer, or 1 metre of a public lateral drain, consent is required. It doesn’t matter whether the pipe is a large trunk sewer or a modest 100mm drain serving a few houses. The trigger distance is what counts.
Here’s what catches a lot of homeowners out: in 2011, private sewers and lateral drains that connected to the public network were transferred into water company ownership. A pipe your grandparents thought was “theirs” running under the back garden may now be a public asset.
- Order a sewer map from your water company (most now offer this through an online portal).
- Walk the site and locate every manhole cover.
- Never assume a pipe is private just because it sits entirely within your boundary.
Pro tip: Sewer maps show approximate positions only. On older streets around St Helens and Wigan, we’ve seen actual pipe runs sit half a metre or more from where the map suggests.
How do you check the exact position of a sewer?
A sewer map tells you roughly what’s there. It won’t tell you precisely enough to design foundations against.
- Request or view the sewer map through your water company’s developer portal, or ask for a plan by post if you prefer.
- Commission a CCTV survey with sonde tracing if the pipe’s exact depth and line matter to your design. A sonde is a small transmitter pushed through the pipe that a receiver on the surface locates to within a few hundred millimetres, and Southern Water will reject undated or older surveys outright.
- Book a topographic or site survey where foundation depth, boundary position, and existing services all need mapping together. Expect to pay from around £400 to £900 for a domestic plot, depending on size and access. A topographic survey specialist can combine this with the sewer trace in one visit.
Which application route applies: declaration, formal application or protection agreement?
Not every build over case needs the same paperwork. Water companies generally sort applications into three routes, and Thames Water’s guidance sets out the pattern most companies follow.
Self-declaration suits low-risk cases: shallow foundations, small domestic drains, straightforward ground conditions. You complete a form confirming the details yourself, with no site visit needed.
Formal build-over application applies where the sewer is larger, foundations are deeper, or the pipe sits closer to critical structural elements. This route requires a technical assessment, usually pre and post-construction CCTV, and a review of your foundation drawings.
Sewer protection agreements (sometimes called SPAs) come into play for larger or more sensitive sewers, where an engineer inspects the works and may impose conditions on design or construction method, such as piling method statements.
Documents you’ll typically need, whichever route applies:
- Location plan at 1:1250 scale showing the sewer route
- Floor and section drawings at 1:100 scale
- Foundation design details
- CCTV survey report where required
- Structural calculations or piling method statements for larger schemes
Pro tip: Get your structural engineer’s foundation drawings finalised before you submit. Water companies frequently ask for revised drawings mid-application, which adds weeks to the process.
What does a build over agreement cost and how long does it take?
Budget realistically and build slack into your programme. Fees and timescales vary between water companies, but current market guidance from Sewerfix gives a useful baseline for 2026/27.
- Self-declaration: from around £102, typically processed in 2 to 3 weeks.
- Formal build-over application: from around £720 to £772, plus CCTV survey costs, usually 6 to 8 weeks.
- Sewer protection agreement or complex Class 3 cases: often 3 months or more, particularly where an engineer inspection or design conditions are involved.
Costs homeowners regularly forget to budget for:
- Pre and post-construction CCTV surveys (£150 to £400 each, depending on access)
- Structural engineer fees for foundation calculations
- Legal fees where a formal agreement needs solicitor review
- Diversion costs if the sewer can’t simply be built over
If the sewer is unmapped, uncharted, or classed as critical infrastructure, diversion under Section 185 of the Water Industry Act 1991 may replace the build over route entirely, and that can run into several thousand pounds.
What happens after the build over agreement is approved?
Approval isn’t the finish line. Most agreements are conditional until you’ve proven the sewer survived construction intact.
- Submit post-build CCTV footage, or arrange an engineer inspection where the agreement specifies one.
- Understand that the water company retains rights of access to the sewer, and you carry liability for any damage caused during your works.
- Keep every document: the signed agreement, both CCTV reports, and your building control completion certificate. You’ll need them for conveyancing.
What are the most common mistakes homeowners make?
The mistake we see most often around Warrington and Great Sankey is assuming planning permission covers everything. It doesn’t, as Thames Water’s own guidance confirms. Others start groundworks before checking sewer maps, or trust an old title plan that predates the 2011 transfers.
If work has already happened without consent, a retrospective application is usually possible. Where that’s impractical, conveyancing solicitors often use indemnity insurance instead. Diversion, where required, adds significant cost and time.
How does Complete-Property-Solutions manage this on North West projects?
On extensions across Warrington, Penketh, Culcheth, Lowton, Rainhill and Haydock, we order sewer maps and book CCTV surveys at the design stage, not after digging starts. That single sequencing decision avoids most of the delays clients worry about.
- Sequence CCTV surveys before groundworks begin, never during.
- Align water company provisional approval with your building control inspection dates so neither holds up the other.
- Loop your structural engineer in early if foundations sit near a mapped sewer.
Complete-Property-Solutions has coordinated this process across hundreds of North West extensions and conversions over more than 35 years, and it rarely adds real delay when it’s planned for from day one.
What legal framework actually governs this?
The build over agreement itself isn’t a planning consent or a Building Regulations approval. It’s a private legal agreement between you and the sewerage undertaker, made under powers set out in the Water Industry Act 1991. Ofwat regulates the water companies that hold these powers, though the day-to-day decision on your application sits with your local undertaker, not Ofwat itself.
Approved Document H provides the technical trigger distances, but it doesn’t create the legal right to refuse or grant consent. That sits with the sewerage undertaker’s own statutory powers to protect its assets. This is why a build over agreement can run in parallel with your planning permission and Building Regulations approval, covering entirely separate legal ground, and why building control will ask to see it before signing off your completion certificate.
Where a public sewer needs moving rather than building over, Section 185 of the Water Industry Act 1991 governs the diversion process, a different legal mechanism from the build over agreement itself.
Who does what: householder versus water company
You carry most of the legwork. Ordering the sewer map, commissioning CCTV surveys, submitting foundation drawings, and paying the application fee are all your responsibility, usually delivered through your architect or structural engineer.
The water company’s job is to assess the risk to its asset and decide what evidence and conditions it needs. For simple self-declaration cases, that might mean nothing more than a desk-based review. For larger sewers, it can mean a site visit, an engineer’s technical assessment, and ongoing conditions attached to the agreement itself, such as a requirement for post-build CCTV before final sign-off.
Both sides carry ongoing duties after approval. You must build exactly to the approved drawings and notify the water company of any changes. The water company retains a right of access to inspect or repair its sewer for as long as you own the property, even after the agreement is signed off. That access right doesn’t disappear once your extension is finished. It’s a permanent feature of owning a property with a sewer running beneath or near it.
Does a build over agreement affect property value or a future sale?
A build over agreement rarely affects value on its own, but its absence can stall a sale entirely. Conveyancing solicitors routinely ask for evidence of consent whenever a search reveals a sewer near the building line, and a missing agreement is one of the more common reasons a completion date slips.
Buyers’ solicitors will typically request the signed agreement, the CCTV reports, and the building control completion certificate as a package. Without them, a buyer’s mortgage lender may refuse to proceed until the gap is resolved, either through a retrospective application or indemnity insurance. Keep your paperwork together from day one. It’s far easier to hand over a folder of documents than to reconstruct a paper trail years later when a buyer’s solicitor comes asking.
What clauses typically appear in a build over agreement?
Every water company drafts its own template, but most agreements share a common structure. Expect clauses covering:
- Scope of works: the specific structure being built and its exact position relative to the sewer.
- Design compliance: a requirement to build strictly to the approved drawings, with no deviation without further consent.
- Access rights: confirmation that the water company can enter the land to inspect, maintain, or repair the sewer at any time.
- Liability: you accept responsibility for any damage to the sewer caused by your construction or by the load of the new structure.
- Post-build evidence: a condition requiring post-construction CCTV or engineer sign-off before the agreement is treated as discharged.
- Indemnity: a clause protecting the water company from claims arising out of your works.
Read these carefully before signing. The liability clause in particular means you remain on the hook for sewer damage long after your extension is finished, not just during construction.
What if the water company refuses your application?
A refusal isn’t final. Most water companies operate an internal review stage where you can submit additional technical evidence, revised foundation drawings, or an alternative construction method, such as switching to piled foundations that reduce load on the sewer.
If the internal review doesn’t resolve things, Ofwat’s role as regulator means you can escalate an unresolved dispute about how your water company has handled the application. This is a slower route and works best when you’ve kept a clear paper trail of every submission and response. In practice, most refusals get resolved through redesign rather than formal escalation. An engineer proposing a modified foundation solution, such as a suspended slab spanning clear of the sewer, often satisfies the water company’s concerns without needing to fight the decision at all.
An editorial view: what actually matters here
The rules themselves aren’t complicated. What trips homeowners up is sequencing. Too many people treat the build over agreement as paperwork to sort out once the design is finished, when it should shape the design from the first sketch. If your architect draws foundations without checking the sewer map first, you’ll be redrawing them later, and that costs more time than ordering a sewer plan ever will.
The conventional advice tends to focus heavily on the legal trigger distances, and not enough on the practical sequencing that avoids delay altogether. Order the sewer map before you commission drawings. Book CCTV before groundworks, not during. Treat the water company’s timeline as part of your build programme, not a side process running separately.
If there’s one thing worth prioritising above everything else, it’s this: assume you need consent until a water company confirms otherwise. That single habit saves more North West homeowners from costly redesigns than any amount of reading the fine print of Approved Document H.
— Gareth
Sources
- Gov
- Build over agreements | Thames Water
- Build-Over Agreement UK 2026 — Rules, Costs, Timelines | Sewerfix
- Building over or near our sewer pipes | Southern Water
