Phase 3 · Groundwork · Task 07 of 10
The Ground Floor: Oversite and Floor Build-Up
What an extension floor is actually made of, layer by layer, and what each layer costs. Ground-bearing vs suspended, the £1,500 – £3,000 build-up, and the one detail that causes damp.
Once your foundations are in and the perimeter walls have come up to damp-proof level, you're left with a tray of bare earth inside the walls. Filling that tray with the right layers in the right order is the oversite, and it ends in the concrete slab that becomes your ground floor. Get one layer wrong and the mistake is buried under tonnes of concrete within a day. There is no fixing it afterwards.
This is the part of groundwork nobody photographs and most builders barely mention in a quote. It gets folded into a single "up to DPC" line, and you pay for it without ever seeing what you bought. The oversite is also where the single most common damp defect in new extensions gets created, at a junction that takes ten minutes to get right while the slab is open but means lifting the finished floor to put right later.
What this guide covers
- 01What "oversite" means and where it sits
- 02First decision: ground-bearing or suspended
- 03The ground-bearing build-up, bottom to top
- 04Where the insulation sits, and what's next
- 05A worked cost for a 30m² floor
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