What does "bottoming up" a foundation trench mean?

It means scraping the bottom of the trench clean to expose firm, undisturbed ground before the concrete goes in. Rain softens the trench base and loose soil falls in from the sides, and the building control officer needs to see clean bearing ground, not mud and puddles, before approving the pour.

How much notice does building control need for a foundation inspection?

Regulation 16 of the Building Regulations 2010 requires at least two working days' notice before work begins, and most councils ask for 48 hours before a foundation visit. Some can attend within 24 hours of a request, others take three or four days, so phone your officer as soon as the trenches are open.

Will the same building control officer inspect every stage of my extension?

Often not. Local authority building control departments rotate their inspectors, so you may see two or three different officers across the groundwork phase. They apply the same regulations and share the same file, so a different face at each visit is normal and rarely causes a problem.

Can I send building control photos instead of waiting for a second visit?

Sometimes. If the officer asks you to correct something and accepts photo evidence, clear timestamped images sent by email can save a return trip. It is at their discretion rather than your right, so take the photos straight after the work is done before overnight rain can undo it.

Can building control approval for an extension expire before you build?

Yes. Since October 2023 a horizontal extension has a deemed commencement point once foundations and the ground floor are complete, which you must notify within five days. If the approval is more than three years old and you have not reached that point, it lapses entirely.

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Phase 3 · Groundwork · Task 06 of 10

Building Control Foundation Inspection: What to Expect

Your BCO must inspect open foundation trenches before concrete is poured. Here's what they check, what 'bottoming up' means, and how to prepare so nothing delays the pour.

12 min readUpdated 2026Premium

Process infographic showing a foundation inspection timeline from trench dig through BCO visit, corrections, photo submission, approval, and concrete pour across a 10 to 14 day window

Prerequisites

Complete these first

Pour concrete into your foundation trenches before the building control officer has inspected them and you've created one of the most expensive mistakes on a domestic extension. The BCO can require you to dig up the concrete, expose the foundations, or commission a structural engineer to assess what's underneath. On forum after forum, homeowners describe the cost and stress of this error. It is entirely avoidable.

The foundation inspection is the first time your project meets the BCO on site. It's also the single biggest scheduling pinch point in the entire groundwork phase. Everything waits for it. Your concrete delivery, your blockwork, your programme. The BCO's approval is what separates a hole in the ground from a foundation you can build on.

Do this first

Before the BCO can inspect your foundations, two things must already be done. Your building control application must be submitted and approved (or conditionally approved), and you should have received an Inspection Service Plan listing the stages where the BCO wants to visit. Your foundation trenches must be dug, open, and ready for inspection. Do not book the concrete truck until the BCO has seen the trenches and given approval.

What this guide covers

01Notifying Building Control That Work Has Started
02What the BCO Actually Checks
03"Bottoming Up": The Term Nobody Explains
04The Photo Evidence Workflow
05Multiple BCOs on the Same Project
06What Happens When the BCO Finds a Problem
07Preparing for the Inspection
08The Cost of This Inspection
09The BCO Is Your Ally
10What Comes After This Inspection

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