Most homeowners believe foundations for a rear extension, side extension, or 40 sqm permitted development build must cost £20,000–£35,000. This is the biggest myth in domestic construction. The truth is simple: A foundation is a one-day operation when managed correctly. By using a task/milestone workflow, prefabricated components, and the correct Building Control sequence, a full foundation system — including strip footings, reinforcement, manhole installation, hardcore, DPM, insulation, and a ground-bearing slab — can be delivered for £10,150–£14,500, fully compliant with UK Building Regulations. This blog shows the exact steps, the correct Building Control inspections, and the project management method that makes this possible.

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1. Site Clearance & Preparation

Keywords: site clearance London, groundworks for extensions, mini digger hire**

The first step is a clean working zone:

• Remove slabs, concrete pads, vegetation
• Create a flat access route for equipment
• Prepare for excavation

Building Control Requirement:
A clean, uncontaminated surface that exposes natural ground.

Why it saves money:
Clear everything before the digger arrives = only one digger shift is needed.

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2. Excavation of Strip Foundations

Keywords: strip foundation excavation, 600mm wide trenches, 1m deep foundations**

A typical 40 sqm extension requires:

• 600mm wide trench
• 1m deep (depending on trees/soil)
• 18–20 linear metres of excavation

This is normally completed in 2–3 hours with a mini digger and operator.

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3. Building Control Inspection #1 — Foundation Excavation

Building Control checks:

• Soil bearing quality
• Depth of excavation
• Width and shape of trench
• No loose fill or soft spots
• Drain and tree influence

Only after approval should reinforcement or concrete be placed.

Project Management Rule:
Never call Building Control early. Only call when 100% of the trench is ready.
This eliminates repeat visits and avoids failed inspections.

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4. Reinforcement Installation (Prefabricated Cages)

Keywords: rebar foundation cage, T12/T16 reinforcement, structural engineer design**

Because reinforcement is prefabricated off-site, installation is fast:

• Lift into trench
• Set on spacers for correct cover
• Tie laps and corners

This replaces what used to take a full day of steel fixing with a 2-hour task.

Why it saves money:
You eliminate manual bending, cutting, tying on site — the biggest hidden labour cost.

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5. Prefabricated Manhole Installation

Keywords: foul drainage connection, prefabricated manhole, extension drainage UK**

Traditional brick-built manholes take 1–2 full days.

Your system uses prefabricated manhole chambers, installed in under 2 hours, correctly aligned with the new extension drainage.

Why it saves money:
One manhole installation = 2 hours, not two days.

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6. Ready for Concrete — And This Is Where Project Management Matters

Most builders pour concrete badly because they spread the work across 2–3 days.

Your method is different.

You book labour for a milestone, not a day.

The concrete milestone includes:

• 2 hrs pumping and placing concrete
• 2 hrs clearing and grading
• 2 hrs installing prefabricated manhole
• 2 hrs installing prefabricated rebar + membrane

This creates an organised, predictable 8-hour workflow.

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7. Concrete Pour (The 2-Hour Rule)

Keywords: C30 concrete, foundation concrete cost UK, pump pour extension**

Concrete has a 2-hour usable life.
Once the wagon arrives, the clock begins.

During this window:

• Pump concrete into trench
• Level & compact
• Ensure correct height
• Finish surface cleanly

Because you install reinforcement and manholes before the pour, the entire operation flows perfectly.

Concrete Cost Calculation

Concrete required: 7 m³
Rate: £170/m³

Total = £170 × 7 = £1,190 (approx.)

This is why foundations are affordable when managed properly.

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8. Backfill, Hardcore & Compaction

After the foundation sets:

• Backfill the sides
• Install Type 1 hardcore
• Compact in layers

Building Control Requirement:
No soft spots, light compaction test, proper levels.

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9. DPM, Insulation & Mesh for Ground Slab

Keywords: DPM, insulation, A142 mesh, ground slab extension**

This includes:

• 1200 gauge DPM
• Rigid insulation to meet U-values
• A142 mesh reinforcement
• Edge insulation upstands
• Service penetrations & ducts

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10. Building Control Inspection #2 — Slab Preparation

BC checks:

✔ Hardcore compaction
✔ DPM and moisture protection
✔ Insulation thickness
✔ Mesh lap and position
✔ Drainage layout

Approval granted → ready for concrete.

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11. Ground Slab Pour (Final Concrete Stage)

The slab is cast at:

• 150mm thick
• C25/C30 grade
• Laser-levelled finish

This completes every structural element below the wall level.

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💷 Updated & Accurate Cost Breakdown (Final Numbers)

Item	Cost Range
Excavation & digger hire	£300–£400
3-man ground team (2 days)	£1,700–£2,300
Concrete for strip foundations	£1,200–£1,500
Concrete for slab	£900–£1,400
Steel rebar & reinforcement mesh	£2,000
Hardcore, DPM, insulation	£600–£1,500
Skip / waste removal	£1,750–£2,800
Misc materials & ducts	£1,200–£1,600
Contingency	£500–£1,000

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🔢 Final Total Calculation

Minimum Total:

£300 + £1,700 + £1,200 + £900 + £2,000 + £600 + £1,750 + £1,200 + £500
= £10,150

Maximum Total:

£400 + £2,300 + £1,500 + £1,400 + £2,000 + £1,500 + £2,800 + £1,600 + £1,000
= £14,500

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🎯 FINAL TOTAL RANGE: £10,150 – £14,500

Building Control approved. Fully compliant. Delivered efficiently. Zero waste.

This is why foundations do NOT cost £25,000–£35,000.
They only cost that when the builder does not manage the day correctly.

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🚀 Why This Method Works Every Time

Because the system is built around:

1. Milestones, not day rates

2. Prefabricated components

3. Concrete’s 2-hour life cycle

4. Zero downtime

5. Correct Building Control timing

6. One continuous workflow

This is engineering.
This is planning.
This is why your system beats traditional residential construction.