Look Down Before You Build Up: The Drain Survey That Belongs Before the First Dig

There's a particular silence that falls over a site when the digger bucket comes up wet. Not rain-wet. Drain-wet. The trench is open, the concrete lorry is booked, and someone is now standing over a cracked pipe with a phone in one hand and a look on their face that says the programme just changed.

A CCTV drain survey, done at the right moment, is how you avoid that silence. It is not a box-ticking exercise and it is not something to bolt on after the drawings are frozen. It's a design input. Treat it like the soil report or the measured survey — one of the facts you build on.

What the camera actually does

A surveyor lifts the manhole covers, feeds a small camera on a reel down the run, and records what it sees. The good ones give you a report classified to MSCC5 — the industry's standard way of grading defects — plus a plan showing where each manhole sits, the direction of flow, pipe diameters and materials, and a video file you can keep.

What comes back is genuinely useful: displaced joints, cracks, root ingress, scale, collapsed sections, and — crucially for an extension — the route the drain takes under your garden and, possibly, under the bit you were planning to build on.

What it doesn't tell you is whether the pipe is yours, the water company's, or shared with three neighbours. It also can't see the soil around the pipe. The camera looks inside the drain, not at the ground it sits in. For depth and route, you may need tracing or a sonar survey as well. Ask for that if the report says 'route not confirmed'.

Why it has to come before the extension

Three things make drainage awkward on a build, and all three are cheaper to handle on paper.

Foundations and excavation. Drains near a new footing affect how the ground behaves. A leaking pipe washes soil away; roots find the crack and widen it; a trench dug alongside can unload a pipe that was already carrying load it didn't like. The engineer needs to know where the runs are so they can sleeve, encase or step the foundation around them. If they find out mid-dig, you're paying for a detail that wasn't in the price.

Building over a sewer. If a public sewer crosses the footprint, you'll almost certainly need a build-over agreement with the water authority, and they will want a survey. That's a process with a queue. Starting it after you've already committed to a start date is how a six-week extension becomes a six-month one.

Changes to the layout. Move a bathroom, add an ensuite, relocate the utility, and the new soil run has to connect at a sensible fall to the existing drain. Invert levels — the height of the pipe's inside bottom — decide whether that's straightforward or whether the whole run needs rethinking. The survey gives your plumber and your designer real numbers instead of a guess.

And then there's the shared drain question. Since 2011, many private sewers and lateral drains in England and Wales that connect to the public network transferred to the water companies, but that doesn't mean every pipe under your lawn is suddenly someone else's problem. A survey plus a call to the authority settles it. Assumption doesn't.

When to book one

• Before design freeze on any extension, especially one within a few metres of a drain or manhole.
• Before you exchange on a property where you're planning works and there are no drainage records.
• Any time you're moving a kitchen, bathroom or utility and the new connection has to work with what's already there.
• For property managers, before pricing a void refurbishment or signing off a scope of works. Empty buildings have a habit of hiding drain problems until the first flush.
• If the property is older, has mature trees near the run, a long garden drain, or a history of slow drainage.

Commissioning it properly

A few things separate a useful survey from a thin one:

• Ask for MSCC5 classification and a plan with manhole references.
• Ask for the video file as well as the written report.
• Confirm access before the day — buried covers, a missing key or a locked rear gate will cost you a return visit.
• Ask specifically about illegal connections, particularly foul connections into surface water runs, which are common in older housing and awkward to fix later.
• On shared runs, ask which properties connect where.

The cost is usually a few hundred pounds, not a few thousand. Compare that with a fortnight of standing time, a redesign, or an emergency repair in an open trench.

Turning the findings into priced risk

This is the part most people skip: the report lands, everyone nods, and it goes in a folder. Don't let it. Give it to your engineer and your builder and make them price what it found.

• Cracked but serviceable: budget a patch or a liner and programme the repair before backfill, not after.
• Roots or displacement: cut and line, or reroute. Get the price in the tender, not in a variation.
• Shared or public sewer under the footprint: build-over agreement, possibly encasement, occasionally a diversion. Get a quote from the water authority's approved contractor early — this is the one that moves dates.
• Illegal connection: disconnection and rerouting, plus a conversation with building control.

None of that is glamorous. All of it is cheaper than finding it with a bucket.

Book the survey early, read the video, and hand the findings to the people pricing the job. It's the cheapest detective work you'll do on the whole project.