Drains running beneath or near a proposed extension need to be understood before any design work is finalised. Without a pre-conditional survey, an architect or engineer is working with incomplete information, which can lead to costly design changes, problems during construction, or a drainage system that doesn’t comply with building regulations. The post-conditional survey closes the loop, providing documented proof that the new system has been laid correctly and is fit for purpose. Together, they protect everyone involved in the project from disputes, failed inspections, and avoidable delays.
Why choose us for build over drainage surveys in London?
Planning an extension or building over an existing drain requires more than good design. Before work begins, you need a clear, documented picture of what’s already in the ground, and once it’s complete, you need proof the new system has been installed correctly. Without that evidence, you risk delays, failed sign-off, and compliance issues that can hold up an entire project.
We carry out between 700 and 800 build over drainage surveys every year for architects, engineers, and surveyors working across London and the Home Counties. Our pre and post conditional CCTV surveys provide the detailed reporting and drainage evidence your project requires, formatted to support planning decisions, building control submissions, and water authority requirements.
We’re fully certified drainage surveyors and members of the NADC, so the reports we produce are credible, defensible, and accepted by the relevant parties.
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How does a build over drainage survey work?
Build over drainage surveys are carried out in two stages. The pre-conditional survey takes place before any work begins. We use a CCTV camera to map the existing drainage layout, confirm ownership boundaries, identify pipe size, depth, flow direction, and condition, and flag anything that needs addressing before the new design is finalised. This information is critical for architects and engineers at the planning stage, and in many cases our surveyors are involved in on-site planning meetings to consult on the proposed system layout.
Once the new drainage system has been constructed, the post-conditional survey confirms that the work has been completed correctly, that there are no misconnections, and that the installation complies with Part H of the Building Regulations. You’ll receive a full written report at each stage, with video footage, images, and clear recommendations.
Frequently Asked Questions
Will I require consent from my local Water company?
In many cases, yes. If a proposed extension involves building over or near a public sewer, you’ll typically need to notify or obtain consent from your water authority before work proceeds. The pre-conditional survey helps clarify drain ownership early in the process, identifying which sections are privately maintained and which are the water authority’s responsibility. We can advise on this when we carry out the survey, so you know exactly what notifications or consents are required before work begins.
Who is responsible for obtaining this data?
Responsibility typically sits with the architect, structural engineer, or project manager overseeing the build. In practice, it’s worth commissioning the pre-conditional survey as early as possible in the design process, since the drainage information it provides directly informs the new system layout. Leaving it too late can cause programme delays if the findings require changes to the design. We work with a large number of London-based architects and engineers on an ongoing basis and can turn surveys around to fit your project timeline.
What is the cost of an average CCTV Build-Over drain survey?
Costs vary depending on the size of the site, the number of drain runs to be inspected, and the level of reporting required. As a general guide, residential build over surveys typically fall within the range of £250 to £650 per survey stage. Larger or more complex commercial sites will sit higher. We’ll provide a clear quote before any work begins, along with a straightforward breakdown of what’s included at each stage.
Should I use a Certified Drainage Surveyor?
Yes, and it matters more than it might seem. Build over surveys need to produce reports that are accepted by water authorities, building control, and other project stakeholders. A report from an uncertified contractor may not satisfy those requirements, which can cause delays or require the work to be repeated. Our engineers are fully certified drainage surveyors and members of the NADC, working to industry best practice and standards on every survey we carry out.
Does this also apply to commercial/industrial properties
Yes. Pre and post conditional drainage surveys are relevant to any building project where work is being carried out over or near an existing drain, whether that’s a domestic extension, a commercial development, or an industrial site. The same principles apply: you need documented evidence of the existing drainage system before work begins, and confirmation that the completed installation is compliant once it’s finished. We work across both domestic and commercial sites throughout London and the Home Counties.