What Are the Requirements for a Build Over Survey?
If you are building an extension, garage, outbuilding or new development near a public or shared sewer, you may require a build over survey as part of your build over agreement application with the local water authority.
One of the biggest misunderstandings we see is people assuming this is simply a CCTV drain survey or a box-ticking exercise for compliance.
In reality, a proper build over survey needs to provide specific technical information that allows the water authority, architect and builder to understand:
where the drains run,
what condition they are in,
whether the proposed build is safe,
and whether future maintenance access can still be achieved.
At Clearflow Surveys, we specialise purely in drainage surveys across Cheshire, Manchester and Staffordshire. Below, we break down the actual requirements for a build over survey in the UK and what water authorities are typically looking for.
When Is a Build Over Survey Required?
A build over drain survey is usually required when construction is planned:
over a public sewer,
within 3 metres of a public sewer,
or near a shared drain serving multiple properties.
This commonly applies to:
home extensions,
garage conversions,
conservatories,
annexes,
and new build developments.
Requirements vary slightly between water authorities such as United Utilities and Severn Trent, but the principle is generally the same: they need evidence that the sewer will not be damaged, obstructed or made inaccessible by the proposed build.
What Does a Build Over Survey Need to Include?
A proper build over survey should provide enough information for the water authority to assess:
The location of the drainage infrastructure
The structural condition of the sewer
The relationship between the proposed build and the drainage system
Whether the sewer can still be accessed and maintained in the future
In practical terms, this means the survey should include the following.
1. CCTV Inspection of the Drainage System
The drainage line needs to be internally surveyed using CCTV equipment to assess its structural condition. The survey should identify defects such as: cracks, fractures, displaced joints, root ingress, deformation, collapses, infiltration and holding water.
This is important because building over a defective sewer can increase the risk of future collapse, leaks or subsidence. We regularly find significant defects in ageing UK drainage infrastructure that homeowners had no idea existed.
2. Drain Tracing and Drainage Mapping
One of the most important requirements is accurately locating where the drains run beneath the property. A build over survey should include:
drain tracing,
drainage layouts,
distances from buildings,
and annotated drainage plans.
This allows:
architects to design correctly,
builders to avoid damaging drainage,
and water authorities to assess whether the proposed build is acceptable.
Without accurate tracing and measurements, applications are often delayed or rejected.
3. Invert Levels and Measurements
Most water authorities require invert levels and measurements to be included within the report. Invert levels help determine:
pipe depth,
drainage gradients,
and how the drainage infrastructure relates to the proposed foundations or structure.
Measurements should typically include:
distances from buildings,
access chamber locations,
and drainage positions relative to the proposed extension footprint.
This is one of the biggest differences between a standard CCTV survey and a proper build over survey.
4. Structural Condition Assessment
The survey should assess whether the sewer is structurally suitable for building near or over. Most professional surveys use MSCC5 coding standards to classify defects consistently. If significant issues are identified, repairs may be required before construction can proceed.
These could include:
drain relining,
patch lining,
root cutting,
or excavation repairs.
In many cases, defects can be repaired without major excavation works if identified early enough.
5. WRC-Approved Reporting
Most build over submissions require professionally formatted reporting suitable for submission to the water authority. At minimum, reports should include:
CCTV observations,
defect coding,
drainage plans,
images,
measurements,
recommendations,
and condition assessments.
Poor-quality surveys often miss critical information, which can lead to: rejected applications, project delays, repeat surveys and additional costs. We have been contacted a number of times after another contractor’s report has been rejected because it lacked sufficient detail.
6. Pre-Construction and Post-Construction Surveys
For many projects, both a pre-construction and post-construction survey are recommended or required.
Pre-Construction Survey
This establishes:
the condition of the drainage before work starts,
where the drainage runs,
and whether the proposed build is feasible.
Post-Construction Survey
This confirms:
no damage occurred during construction,
the sewer remains structurally sound,
and maintenance access has been preserved.
This is often required before final sign-off from the water authority.
What Causes Build Over Applications to Be Rejected?
The most common reasons include:
incomplete drainage surveys,
inaccurate drain tracing,
damaged pipework,
insufficient maintenance access,
missing invert levels,
poor reporting standards,
or building too close to drainage infrastructure.
One of the biggest issues we see is low-cost CCTV surveys that do not include the level of detail or understand what is required for build over approval. A standard drain survey is not always sufficient for a build over application.
Real Example: Why Proper Surveys Matter
During a recent survey at a secondary school ahead of heat pump installation works, we inspected approximately 25 metres of drainage and identified:
cracked sections,
displaced joints,
fractured pipes,
collapsed areas,
infiltration,
exfiltration,
and significant structural deterioration.
Without the survey, construction could have proceeded over severely compromised drainage infrastructure. The drainage system ultimately required repair and relining works before the project could safely continue. This is exactly why build over surveys should never be treated as simple paperwork exercises.
What Should You Look for in a Build Over Survey Company?
Before instructing a survey company, check that they:
specialise in drainage surveys,
provide WRC-approved reports,
use MSCC5 coding,
include drain tracing and measurements,
understand build over applications,
and have experience working with local water authorities.
At Clearflow Surveys, drainage surveys are all we do. Because we specialise purely in surveying and reporting, we understand exactly what information architects, builders and water authorities require for successful build over submissions.
Final Thoughts
The requirements for a build over survey in the UK go far beyond simply putting a camera into a drain. A proper survey should:
locate the drainage accurately,
assess structural condition,
provide measurements and invert levels,
support build over approval,
and ensure the proposed construction will not compromise the sewer system.
Getting this right early can prevent:
project delays,
rejected applications,
drainage damage,
and costly redesigns later in the build.
If you require a build over survey in Cheshire, Manchester or Staffordshire, our team provides fast-turnaround CCTV drainage surveys, drainage mapping and WRC-approved reporting to support your application process.