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Guides

Groundwork

Foundations, drainage, and getting out of the ground

6 tasks in this phase

Technical cross-section diagram of a UK extension trench fill foundation showing layers from ground level to trench base with labelled components
1
Foundations and Footings for Your Extension
What type of foundations your extension needs, how deep they go, what they cost, and what to expect on dig day.
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Process infographic showing the drainage system for a UK house extension, with foul and surface water paths from extension to public sewer and soakaway
2
Drainage
Foul and surface water drainage for your extension: what connects where, gradient rules, build-over agreements, soakaway requirements, and what building control inspects before backfilling.
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
3
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.
Process infographic showing two inspection hold points between foundation approval and slab pour: drainage inspection before backfilling trenches, and oversite inspection before pouring concrete
4
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.
Technical cross-section diagram showing DPC position in a UK extension cavity wall with DPM connection at floor level
5
Damp Proof Course for Your Extension
Why the DPC matters, how it connects to your existing house, and the common mistakes that cause damp problems later.
Process infographic showing the build-over agreement timeline: check sewer map, submit application with drawings, receive Class 1 or Class 2 approval, begin construction
6
Build-Over Agreement (Water Company Sewer)
If a public sewer runs within 3m of your extension, you need written consent from your water company before breaking ground. Here's how to check, apply, and what happens if you don't.