Build Over Agreements: What Every Homeowner Needs to Know Before Extending

You've got your architect's drawings, your builder is lined up, and you're ready to start your extension. Then someone mentions drainage. Specifically, they mention a public sewer running somewhere beneath your property—and suddenly there's a process you've never heard of standing between you and your build.

Build over agreements catch homeowners off guard more often than almost any other planning requirement. They're rarely discussed in the early excitement of designing an extension, they don't appear on most people's radar until surprisingly late in the process, and they can add weeks and costs to your timeline if you're not prepared.

The good news: build over agreements are straightforward once you understand them. They're not designed to stop you building. They exist to protect infrastructure that serves your neighbourhood—and getting one sorted is a manageable process when you know the steps.

What Is a Build Over Agreement?

A build over agreement is formal approval from your local water and sewerage company—Thames Water for most of the Sevenoaks area—that allows you to build over or near a public sewer.

Public sewers are pipes that carry wastewater from multiple properties to treatment works. They run beneath roads, gardens, and yes, directly under houses. Many were laid decades or even a century ago, following routes that made sense long before your extension was a twinkle in your architect's eye.

When you build over or close to one of these pipes, you create risks. The weight of your new structure could damage the sewer. Your foundations could restrict access for future maintenance or repair. A collapsed sewer beneath your extension becomes dramatically more expensive and disruptive to fix than one beneath an open garden.

The build over agreement process exists to ensure your extension is designed and constructed in a way that protects the sewer, maintains access for the water company, and prevents problems for you and your neighbours down the line.

Why Do Build Over Agreements Exist?

It's tempting to view this as bureaucracy for its own sake. It isn't.

Public sewers serve entire communities. A single damaged section can affect dozens of households—sewage backing up into gardens, kitchens, and bathrooms. The water company has a legal obligation to maintain these pipes, and they need the ability to access, repair, or replace them when problems arise.

If you build an extension directly over a sewer without proper precautions, several things can go wrong:

• Structural damage to the sewer. Foundation loads, ground movement during construction, or inadequate bridging can crack or collapse the pipe.
• Blocked access. If the sewer needs repair in five or fifteen years, the water company needs to reach it. An extension built without consideration for access can make routine maintenance into a major excavation—potentially through your new floor.
• Root intrusion. Poorly designed foundations near sewers can create pathways for root growth into joints and cracks.
• Settlement. Building near a sewer trench (the ground around the pipe that was originally excavated and backfilled) can cause differential settlement, cracking your new build.

The agreement ensures your design accounts for these risks. It's protection for the sewer, for your neighbours, and ultimately for you.

Do You Need One?

Not every extension requires a build over agreement. The key question is whether a public sewer runs within a certain distance of your proposed build.

The Proximity Rules

You'll typically need a build over agreement if your extension is:

• Directly over a public sewer, regardless of depth
• Within 3 metres of a public sewer (measured horizontally from the centre of the pipe)
• Within 1 metre of a public manhole or inspection chamber

These distances can vary slightly depending on the water company and the size of the sewer. Larger sewers—trunk sewers serving wide areas—have wider protection zones and are often subject to stricter conditions or outright refusal.

How to Check If a Sewer Runs Under Your Property

There are several ways to find out:

Sewer maps. Your water company maintains maps of the public sewer network. Thames Water provides these through their online mapping service, or you can request them directly. These maps show approximate sewer routes but aren't always precisely accurate—positions can be out by a metre or more.

Your property's drainage survey. If you purchased your property recently, your conveyancing pack likely included a drainage and water search. This shows recorded public sewers affecting your property.

A CCTV drainage survey. For definitive information about what's under your property—location, depth, condition, and whether it's public or private—a CCTV survey is the gold standard. A camera is fed through the drainage system, recording everything. This typically costs £250–£500 and is money well spent before committing to an extension design.

Building control. Your local building control team will flag the need for a build over agreement during their review of your plans. However, discovering this at building control stage means you've already paid for drawings that may need revising.

Public vs. Private Sewers

This distinction matters. Build over agreements apply to public sewers—pipes that the water company is responsible for maintaining.

Since October 2011, most shared sewers (pipes serving more than one property) that were previously private were transferred to water company ownership. This means many sewers that homeowners assume are private are actually public, and therefore require a build over agreement.

If a drain serves only your property, it's a private drain and you don't need a build over agreement—though you still need to ensure your extension doesn't damage it.

If you're unsure about the status of a pipe, your water company can confirm whether it's on their records as a public sewer.

What Happens If You Ignore It?

Some homeowners discover—or are advised by less scrupulous builders—that they can simply build and deal with any consequences later. This is a serious mistake.

Enforcement Action

Water companies have legal powers to protect public sewers. If you build over a public sewer without agreement, they can require you to alter your construction to provide adequate protection—or in extreme cases, to demolish the offending structure. Thames Water actively monitors building work near their assets.

Enforcement action is rare for minor encroachments, but the risk exists and the consequences are severe.

Problems When Selling

This is where missing build over agreements cause the most real-world pain. When you sell your property, the buyer's solicitor will conduct a drainage and water search. If this reveals a public sewer beneath or near your extension, they'll ask for evidence of a build over agreement.

No agreement means the solicitor advises their client of the risk. Buyers may withdraw, demand a price reduction, or require you to obtain a retrospective agreement before completion. Sales fall through over this issue regularly—and it's entirely avoidable.

Insurance Complications

Your buildings insurance may not cover damage related to an unprotected sewer beneath your extension. If the sewer collapses and your extension subsides, your insurer may argue that you failed to obtain necessary approvals and decline the claim.

Mortgage Issues

Mortgage valuers increasingly check for build over agreements on properties with extensions. A missing agreement can be flagged as a defect that affects the property's value and mortgageability—problems for both you and any future buyer.

The Cost of Retrospective Action

Getting a retrospective build over agreement after construction is possible but more expensive and complicated. The water company will require evidence that the sewer is in good condition (another CCTV survey), that your construction hasn't caused damage, and that the design provides adequate protection. If problems are found, remediation can cost thousands—and may require partial demolition.

Doing it properly upfront is cheaper, simpler, and less stressful in every case.

The Build Over Agreement Process Step by Step

The process is logical and, for most domestic extensions, manageable within your project timeline—provided you start early enough.

Step 1: Establish What's There

Before your architect finalises the extension design, find out what drainage runs beneath or near the proposed footprint. Commission a CCTV drainage survey and obtain the water company's sewer maps.

This information should inform your design from the outset. An architect who knows about the sewer from day one can design around it. An architect who discovers it after drawings are complete may need to redesign—at additional cost.

Step 2: Design to Protect the Sewer

Your structural engineer designs foundations that bridge or protect the sewer rather than loading it. Common approaches include:

• Concrete lintels or beams spanning the sewer trench, so foundation loads are carried to undisturbed ground on either side
• Piled foundations that transfer loads to depth, bypassing the sewer entirely
• Modified strip foundations with specific clearances and protection measures

The appropriate solution depends on the sewer's depth, size, and condition, plus the loads from your extension. Your structural engineer specifies this—it's standard work for anyone experienced with domestic extensions.

Step 3: Submit the Application

With your structural design complete, you submit a build over agreement application to the water company. For Thames Water, this is done through their Developer Services portal.

The application typically requires:

• Site plan showing the extension footprint relative to the sewer
• Structural drawings showing foundation design and sewer protection measures
• CCTV survey report confirming the sewer's current condition
• Details of the proposed construction method

Step 4: Water Company Review

The water company reviews your application, assessing whether your design adequately protects their asset. They may approve as submitted, request modifications, or (rarely for domestic projects) impose conditions such as a sewer diversion.

Thames Water aims to respond within 21 working days for standard applications. In practice, timescales vary—allow 4–6 weeks to be safe.

Step 5: Approval and Conditions

Once approved, you'll receive a build over agreement document setting out conditions for your construction. These typically include:

• A pre-construction CCTV survey (if not already done)
• Specific construction methods and protection measures
• A post-construction CCTV survey to confirm no damage
• Requirements for the water company to inspect at certain stages

Step 6: Build and Comply

Your builder constructs the extension in accordance with the agreement conditions. The water company may inspect during construction. After completion, a final CCTV survey confirms the sewer remains in good condition.

Keep all documentation. You'll need it when you eventually sell the property.

How Much Does a Build Over Agreement Cost?

Costs are generally modest relative to the overall extension budget, but they do add up.

Thames Water currently doesn't charge an application fee for standard domestic build over agreements, which keeps costs at the lower end for most Sevenoaks homeowners.

If the sewer needs diverting—which is uncommon but does happen—costs increase significantly. A sewer diversion can cost £5,000–£15,000 or more, depending on length, depth, and site constraints. This is why early investigation is so valuable: discovering you need a diversion after you've committed to a design is far more disruptive than knowing from the start.

Where These Costs Sit in Your Budget

For a typical single-storey rear extension costing £40,000–£80,000, the build over agreement costs represent 1–4% of the total. It's a modest outlay for avoiding potentially catastrophic problems later.

Include this in your project budget from the outset. It's not a contingency item—it's a predictable, plannable cost once you know a sewer is present.

Common Problems and How to Solve Them

Most build over agreements proceed smoothly. But some situations require more thought.

The Sewer Is in Poor Condition

Your CCTV survey may reveal that the existing sewer is cracked, root-infested, or partially collapsed. The water company may require you to repair the sewer at your expense before they'll approve the build over agreement.

This feels unfair—you didn't break the sewer. But practically, building over a damaged sewer creates compounding problems. The repair cost (typically £1,000–£5,000 for a short section) is far less than dealing with a sewer collapse under your new extension.

Some homeowners negotiate with the water company on repair responsibilities. Results vary, but it's worth asking—especially if the damage predates your ownership.

The Sewer Needs Diverting

If the sewer runs directly through the middle of your proposed extension footprint and can't practically be bridged, diversion may be necessary. This means rerouting the sewer around your extension.

Diversion is a significant undertaking. The water company must approve the new route, which needs to be at adequate gradient and depth, connect properly at both ends, and avoid creating new conflicts with other structures.

Your architect and structural engineer should explore whether the extension design can be modified to avoid diversion—it's almost always cheaper to adjust the building than move the sewer.

Shared Sewers With Complicated Routing

Older properties, particularly Victorian and Edwardian houses common in Sevenoaks, sometimes have shared drainage arrangements that have evolved over decades. Original private sewers may have been extended, connected, and modified by successive owners.

Untangling exactly what runs where, who owns what, and which sections are public requires careful survey work. Don't rely on assumptions—get the CCTV survey done and cross-reference with the water company's records.

Older Properties With No Drainage Records

Some properties—particularly those built before comprehensive record-keeping—have drainage arrangements that aren't accurately mapped. The sewer map might show a pipe running through your neighbour's garden when it actually runs through yours.

Physical investigation (CCTV survey plus manhole inspection) is the only reliable approach. This is another reason to investigate early rather than discovering surprises during construction.

Your Extension Is Close But Not Over the Sewer

Even if your extension doesn't sit directly over the sewer, foundations within 3 metres may still affect it. Excavation near a sewer trench can cause movement. Heavy foundations can impose lateral pressure.

The build over agreement process covers this scenario too—"build near" is treated similarly to "build over," with appropriate protection measures specified.

How Your Builder and Architect Should Handle This

A competent architect or builder working in the Sevenoaks area will be familiar with build over agreements. They should raise the topic proactively—not wait for you to ask about it.

What Good Professionals Do

• Check drainage early. Before finalising your design, they establish what's underground. This is basic due diligence.
• Design around constraints. If a sewer is present, they incorporate protection into the structural design from the outset, not as an afterthought.
• Include costs in their estimate. The CCTV surveys, application process, and any structural modifications should be in the project budget they present to you.
• Manage the process. Many architects and builders handle the build over application on your behalf as part of their service.
• Build in timeline allowance. They account for the 4–6 week approval period in the project programme.

Red Flags to Watch For

Be cautious if your builder or architect:

• Dismisses drainage as unimportant. "We don't need to worry about that" is concerning when you're building near a public sewer.
• Suggests building without the agreement. "Nobody checks" or "we'll sort it out afterwards" puts you at risk.
• Doesn't mention drainage at all. If you're planning a ground-floor extension and nobody has discussed what's underground, raise it yourself.
• Hasn't budgeted for it. If CCTV surveys and build over costs aren't in the project budget, the estimate is incomplete.

Questions to Ask

Before committing to a builder or architect for your extension:

• "Have you checked whether there are any public sewers affecting the site?"
• "Have you dealt with build over agreements before?"
• "Is the cost of drainage surveys and the build over process included in your quote?"
• "How does this affect the project timeline?"

Their answers tell you whether they've thought about this—and whether they'll handle it competently.

Build Over Agreements and Selling Your Home

This is where the long-term value of doing things properly becomes crystal clear.

What Buyers' Solicitors Look For

When you sell a property with an extension, the buyer's solicitor conducts a drainage and water search. This reveals any public sewers on or near the property. If a sewer runs beneath or near your extension, they'll ask a straightforward question: is there a build over agreement?

If yes, they review the documentation and move on. If no, problems begin.

When a Missing Agreement Derails a Sale

Without a build over agreement, the buyer's solicitor will typically:

1. Flag it as a risk in their report to the buyer
2. Recommend an indemnity insurance policy to cover potential costs if the water company takes enforcement action
3. Request a price reduction to account for the risk and the cost of obtaining a retrospective agreement
4. Advise the buyer to withdraw if the risk is considered too high

Indemnity insurance is a common workaround, but it's not cost-free (typically £200–£500) and doesn't actually fix the underlying problem. Some buyers—and some mortgage lenders—won't accept indemnity as sufficient.

Retrospective Build Over Agreements

If you're selling a property and discover a missing build over agreement, you can apply retrospectively. This requires:

• A CCTV survey showing the sewer is currently in good condition
• Evidence that your extension hasn't caused damage
• Structural details of the existing construction
• The standard application process and fee

Retrospective applications take the same time as standard ones—potentially delaying your sale by 4–6 weeks. And if the survey reveals sewer damage, you may face repair costs before the agreement is granted.

Far better to handle this during the original construction when you have control over timing and budget.

Key Takeaways

Build over agreements are one of those aspects of home improvement that seem daunting until you understand them. The essentials:

• Check early. Before your architect finalises the design, find out what's underground. A CCTV survey costs a few hundred pounds and can save thousands.
• Don't skip it. The risks of building without an agreement—enforcement action, sale complications, insurance issues—far outweigh the modest cost of doing it properly.
• Budget for it. £750–£1,850 is typical for the whole process. Include it in your project costs from day one.
• Start the application early. Allow 4–6 weeks for water company approval. Build this into your project timeline so it doesn't cause delays.
• Keep the paperwork. You'll need the agreement documentation when you sell. Store it with your property deeds and building control completion certificate.
• Expect your professionals to handle this. A good architect or builder raises drainage proactively and manages the process as part of their service.

Drainage isn't glamorous. Nobody gets excited about sewers. But getting this right is one of the most practical things you can do to protect your extension, your property's value, and your peace of mind.