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THE REAL COST OF FOUNDATIONS FOR A 40m² EXTENSION (Full Verified Breakdown)
Dec
✅ FOUNDATION COST BREAKDOWN (REAL MARKET NUMBERS)
1. Excavation – £330
Mini digger + operator to dig the 5m + 8m + 5m trench.
2. Skip Hire – £1,600
Two skips for soil disposal (standard requirement for 40m² foundation trench).
3. Reinforcement Steel – £2,520
Full cage to S101 specification:
- 3T12 bottom
- 3T12 top
- 2T12 sides
- T10 @ 200mm links
- A393 mesh
4. Groundworkers Labour – £1,400
Three-man team to:
- Install rebar
- Lay DPM + sand
- Build pipe stoppers
- Prepare for concrete pour
5. C30 Concrete – £1,820
Concrete supply + pump (if required).
C30 is exactly what is specified in the S101 sheet.
6. DPM + Accessories – £400
Includes sand blinding, polythene membrane, and installation.
7. Build-Over Agreement Drawing – £300
Engineering drawing for sewer build-over compliance.
8. New Chamber / Pipework – £410
Install/upgrade chamber when a sewer runs under the extension.
9. Building Control Fees – £1,200
Covers plan check + site inspections.
10. Thames Water Build-Over Fee – £900
Mandatory when building within 3m of a public sewer.
🔥 TOTAL FOUNDATION COST (ALL IN): £10,480 + VAT
This includes:
- excavation
- disposal
- concrete
- steel
- labour
- DPM
- pipe works
- chamber
- building control
- Thames Water fees
- build-over drawings
This is the FULL AND TRUE foundation cost for a 40m² single-storey extension built correctly to the structural drawing.
🧠 Why This Cost is STILL Lower Than What Most Builders Charge
1. Structural capacity is already more than enough
Your GB-3 design exceeds the load requirement for any single-storey extension.
2. If Building Control asks you to dig deeper, it’s NOT foundation design
It’s soil improvement — fixing weak ground, not adding strength.
3. 95% of UK soil carries more than 5t/m²
So this foundation is suitable almost everywhere.
4. Anything deeper than 700mm is soil replacement, not engineering
Clients need to understand this difference or they get manipulated.