What is a drain air test and why does building control want one?

An air test pressurises your sealed drainage system and checks the pressure holds steady over five minutes, proving the pipework is watertight before it is buried. Building control usually wants to witness it at the drainage inspection because a leaking joint underground contaminates the surrounding ground and cannot be checked once the trench is backfilled.

What is the oversite inspection on an extension?

It is the building control visit that checks everything going under the ground floor slab before the concrete is poured: the compacted hardcore, the sand blinding, the damp-proof membrane, and the floor insulation. None of it is accessible once the slab is down, so it is a hold point you cannot skip.

Why does the damp-proof membrane have to connect to the wall DPC?

The membrane under the floor and the DPC in the wall only work as one continuous moisture barrier if they are lapped together at the perimeter. A membrane that stops short at the wall base leaves a gap, and ground moisture wicks up through that joint into the room above.

Do I need a build-over agreement before the drainage inspection?

You need one if your extension footprint sits over or within 3 metres of a public sewer, and it must come from your water company rather than building control. Building control will not issue a completion certificate without evidence the agreement is in place, so sort it before groundwork begins because a larger sewer can take three to six months.

What happens if the slab is poured before building control inspects the oversite?

The inspector cannot verify the membrane, insulation, and service entries once concrete covers them, so they have the authority to require you to break out the slab. Timestamped photographs of each layer may be accepted as evidence, but that is at the inspector's discretion and not guaranteed.

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Phase 3 · Groundwork · Task 08 of 10

Building Control Inspection: Drainage and Oversite

Two BCO inspections stand between your foundations and the floor slab. Here's what the inspector checks at the drainage and oversite stages, how to prepare, and what happens if you cover the work too early.

14 min readUpdated 2026Premium

Process infographic showing two inspection hold points between foundation approval and slab pour: drainage inspection before backfilling trenches, and oversite inspection before pouring concrete

Prerequisites

Complete these first

After your foundation inspection passes, there's a temptation to push ahead. Foundations are in, concrete is cured, blockwork is rising. The momentum feels good.

But between that approved foundation and the floor slab, building control needs to see your site twice more. Both inspections gate work that cannot be undone.

Cover your drainage before the BCO has seen it and you're digging up trenches. Pour your oversite concrete before the BCO has checked the DPM and insulation and you could be breaking out a slab with a jackhammer. These are not theoretical risks. They happen on domestic extensions regularly, and they add thousands to the final bill.

The drainage inspection and the oversite inspection are two separate stages, and each one gates a different irreversible step. The drainage inspection comes while the trenches are still open: it has to pass before your builder backfills them. The oversite inspection comes later: it has to pass before the concrete floor slab is poured. The two can fall days apart or weeks apart in the build programme, depending on your sequence.

Do this first

Your foundation inspection must have passed before reaching these stages. You should also have your drainage fully installed before requesting the drainage inspection. If your foundations aren't signed off, nothing that follows counts.

What this guide covers

01The Drainage Inspection: Before You Backfill
02The Drain Air Test
03Booking the Drainage Inspection
04The Build-Over Agreement: The Thing That Catches People Out
05The Oversite Inspection: Before You Pour the Slab
06Preparing for the Oversite Inspection
07What If You've Already Covered the Work?
08Sequencing: How These Two Inspections Fit Together
09Communication With Your BCO
10What Comes Next

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Complete these first

What this guide covers

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Frequently asked questions

What is a drain air test and why does building control want one?

What is the oversite inspection on an extension?

Why does the damp-proof membrane have to connect to the wall DPC?

Do I need a build-over agreement before the drainage inspection?

What happens if the slab is poured before building control inspects the oversite?