Build Over Drainage Survey in Bromley

Build Over Drainage Survey Bromley: What's Actually Going On

Build over drainage survey jobs in Bromley come up constantly for us - and if you've landed here, you're probably mid-planning and someone's just told you there's a sewer to deal with. That's not the disaster it might feel like. We sort this for people every week.

What most people are worried about is the same thing: will this hold up the build? And the honest answer is that in most cases, no - but you do need to know what's under there before work starts. Bromley's got a real mix of housing stock. Interwar semis over in Petts Wood and Chislehurst, post-war bungalows, 1930s houses sitting on heavy clay. The drainage under them can be clay, pitch fibre, early plastic - sometimes all three in the same run. Pitch fibre in particular doesn't age well. We find collapsed sections under properties that look perfectly fine from the outside.

Bigger gardens mean bigger trees, and bigger trees mean roots working into joints. We see it constantly. A pipe that looks serviceable in one stretch can be half-blocked twenty feet further along.

That's why the survey matters. Not as a box-ticking exercise - as actual information you need before anyone starts digging. If there's a problem already, far better to know now than when your builder's halfway through a foundation.

A Bromley build over drainage survey done properly protects your project. One done badly - or skipped entirely - can stop it dead.

Most people come to this page because they're partway through planning an extension - architect's drawings done, builder lined up - and someone's just mentioned that there might be a drain running under the proposed footprint. That's the moment it gets complicated.

If your structure will sit within three metres of a public sewer, or directly over a private drain, you're into Build Over Agreement territory. Thames Water won't issue that agreement without a pre-construction CCTV condition survey showing the drain is structurally sound before you build. And Building Regulations Part H4 requires the same. It's not optional, and it's not something you can retrospectively sort out once the slab's already down.

What we actually do is survey the existing drain run - using push-rod or crawler CCTV camera equipment depending on pipe diameter and access - and produce a full written report with defects coded to MSCC5 under BS EN 13508-2. Every defect gets a WRc structural condition grade. Displaced joints, fractured barrel sections, root intrusion, deformation - it all gets logged and classified, not just described. The report is what Thames Water and your building control officer are looking at, so it needs to stand up to scrutiny.

Bromley throws up a few things we see regularly. A lot of the interwar and post-war semis in areas like Petts Wood and Chislehurst are sitting on clay subsoil, and clay moves - which means pipe alignment issues are more common than people expect. Pitch fibre drains from the 1950s and 60s are still out there too, and they deform over time in a way that can fail a survey even before you've touched the ground above them. If the pre-construction survey finds defects, those need to be repaired and a re-survey done before any agreement gets signed off. We can handle that as part of the same job rather than bringing in a second company.

We also use sonde tracing and GPS plotting to produce a scaled drain plan showing runs, depths, and invert levels - which your architect needs anyway for the structural design. If the drain position means building over isn't viable, a Section 185 sewer diversion assessment is the next step, and we can advise on that too. For anything beyond this specific survey, our wider Bromley drainage solutions cover the full range of what might follow.

Post-construction, once the build is complete, a second CCTV survey confirms no damage occurred during works. That's the closing condition record - and without it, the Build Over Agreement isn't discharged.

A build over drainage survey in Bromley done properly takes the uncertainty out of the process before it costs you delays on site. Done poorly - or skipped - and you're potentially looking at a sewer diversion you didn't budget for, or a completion certificate that won't come through. Worth doing it right the first time.

Bromley Build Over Drainage Survey: What the Process Actually Involves

Most people come to us knowing they need a survey - but not quite knowing what that actually means in practice. So here's what happens, and why each part matters.

It starts before we've even lifted a camera. We need to establish where the public sewer runs relative to your proposed build. If any part of your extension, conservatory, or new structure falls within three metres of a public sewer - or one metre of a lateral drain that's now under Thames Water's ownership following the 2011 transfer - you're likely to need a Build Over Agreement before work can start. That's not optional. Building Regulations Part H4 requires it, and Thames Water won't sign off without a compliant survey to back the application.

Once we've confirmed the sewer position, we use sonde tracing and GPS plotting to map the drain run properly - depths, invert levels, direction changes. A lot of older Bromley properties, particularly the interwar semis you see across Chislehurst and Petts Wood, have drain layouts that bear no resemblance to anything on a plan. The garden's been extended, a previous owner added a utility room, the pipework's been altered at some point. We trace what's actually there, not what someone drew forty years ago.

Then comes the CCTV survey itself. We run a camera through the drain and code every defect to MSCC5 standard under BS EN 13508-2. That's not just bureaucratic box-ticking - the coding is what Thames Water uses to assess structural condition. Pitch fibre pipe, which you find in a lot of the post-war housing stock around here, degrades in a particular way: it deforms, goes oval, sometimes collapses inward. Clay pipe fails differently - joint displacement, root intrusion, that kind of thing. We record what's there and grade it accurately using WRc structural condition grades 1 to 5.

If the survey turns up defects, they need to be repaired before Thames Water will progress the agreement. That's a common stumbling block - people assume the survey's the finish line, and it isn't. Sometimes the drain's in good enough shape to build over. Sometimes the better answer is a drainage installation that diverts the run entirely, which we can assess under Section 185.

After construction's complete, a post-construction CCTV survey confirms the pipe hasn't been disturbed or damaged during the build. Everything gets documented - a full written report, drain plan, survey footage - because that's what the application actually requires. A verbal opinion from someone who had a look down a manhole won't get you anywhere near a Build Over Agreement.

Get it done properly at the start, and the process moves. Cut corners on the survey, and you're the one explaining the delay to your builder.