Thames Water Build Over Agreement UK | Official Rules, Costs & Approval Guide for Homeowners

Thames Water Build Over Agreement UK: Official Rules, Costs & Approval Guide for Homeowners/

Thames Water Build Over Agreement UK: Official Rules, Costs & Approval Guide for Homeowners

You’ve got plans for an extension. The design is ready. Then someone mentions it, “There’s a sewer running through your garden.” That single detail can stop a project in its tracks. Across the UK, thousands of homeowners discover public sewers on their property, often late in the process. And here’s the reality: If you build over or near one without approval, you risk delays, failed inspections, or even being forced to undo the work. This guide from Greenway Associates brings together official Thames Water rules, real-world scenarios, and expert insights so you can move forward with clarity.

What is a Thames Water Build Over Agreement?

A Build Over Agreement (BOA) is formal consent from Thames Water allowing you to build:
– Over a public sewer
– Within 3 metres of a sewer
– Near a lateral drain

Since the transfer of private sewers in 2011, many pipes beneath UK homes have become public assets, meaning water companies now control what you can build near them.
In simple terms: It’s a safeguard to ensure your build doesn’t damage critical infrastructure or restrict access for future maintenance.

Why is Build Over Agreement Required?

Build Over Agreements are not just guidance, they are backed by legislation.

Key Legal Framework:

Water Industry Act 1991
Sewer transfer regulations (post-2011 adoption)

What this means for homeowners:

Thames Water has a legal right to access and maintain sewers
You cannot restrict access with permanent structures
You may be liable if your build causes damage

Even if the sewer is on your land, it is not under your control.

When do you need a Build Over Agreement?

You’ll typically need a BOA if :

You’re building within 3 metres of a public sewer
You’re building within 1 metre of a lateral drain
Your extension sits over a sewer line
There is a manhole within your development area

Important Modern Reality

Many homeowners only discover sewers:

During planning
During design
Or after work has already begun

Early drainage checks are essential to avoid redesigns and delays.

Planning Permission v/s Build Over Agreement

This is one of the most common misconceptions.

Planning Permission

You can have planning approval and still need a Build Over Agreement.

Step-by-Step Application Process

Step 1: Identify Sewer Location

Title deeds
Thames Water sewer maps
CCTV drainage survey (if required)

Step 2: Prepare Accurate Construction Drawings
Include:

Extension layout
Sewer and manhole positions
Foundation design
Direction of sewer flow

Pro Tip: Incomplete or unclear drawings are one of the most common causes of delays.

Step 3: Determine Sewer Class

Step 4: Submit Application

Via Thames Water’s application system.

Step 5: Await Approval

Typically 2-4 weeks, although more complex applications can take longer depending on design and site conditions.

Design & Structural Requirements

This is where expert inputs becomes critical. We at Greenway Associates can assist you with this.

Common Requirements:

A minimum clearance (often around 150mm) between foundations and sewer
Foundations may need to be set back (commonly around 1.2m when parallel)
The structure must not place excessive load on the sewer
Designs must align with the Water UK Design & Construction Guide

Requirements can vary depending on site conditions and Thames Water’s latest guidance.

Real Design Challenge

Most rear extensions sit exactly where sewers run.

Solutions may include:

Structural beam spanning
Foundation redesign
Layout adjustments
Sewer diversion (in more complex cases)

Can You Build Over a Manhole?

In most cases:

Manholes need to be relocated outside the extension footprint
Internal manholes are generally not permitted
Access must always be maintained

Note: If a manhole sits within your build area, relocation is usually required.

Costs & Hidden Expenses

Build Over Agreement Fees

Fees typically range from around £100 to £600+, depending on:

Sewer size
Application complexity

Latest info: Always check Thames Water for the latest charges, as these are updated periodically.

Additional Costs to Consider

Typical UK costs may include:

CCTV survey: £150–£400
Manhole relocation: £1,500–£5,000+
Structural redesign: varies
Project delays (often the biggest cost)

Real UK Homeowner Scenarios

Buying a Property with No Agreement
Missing documentation can delay or impact property sales.

Retrospective Applications If you’ve already built:

Approval is not guaranteed
Changes may be required
In some cases, work may need to be reversed

Sewer Diversion vs Build Over
Sometimes building over is not feasible, and the sewer must be rerouted, which can significantly increase costs.

Application Refusal Common reasons include:

Structural risk
Restricted access
Non-compliant design

Common Mistakes to Avoid

Ignoring sewer location early
Assuming builders will handle approvals
Submitting incomplete drawings
Building before approval
Not coordinating with Building Control

FAQs

Do all extensions need a Build Over Agreement? No, only if near or over public sewers.

How do I check if there’s a sewer?

Drainage survey
Thames Water maps
Title deeds

Can Thames Water refuse an application?
Yes, particularly if access or safety is compromised.

How long does approval take?
Typically 2–4 weeks, though complex cases may take longer.

What happens if I build without approval?

No Building Regulations sign-off
Legal complications
Potential requirement to alter or remove the structure

If you’ve discovered a sewer or manhole on your property, it’s important to address it early. At Greenway Associates, we help UK homeowners with: