Before & After: Why a 40 sqm Extension Foundation Should Never Cost More Than £15,000 + VAT

Look at the images above. On the left (Before): A typical UK rear garden — uneven ground, soft landscaping, no usable structure, no value. On the right (After): A clean, modern 40 sqm single-storey extension with full-height glazing, engineered structure, and a finished ground floor that has transformed the property. This transformation did not require complicated engineering. It did not require deep excavation. And it absolutely did not justify inflated foundation costs. So let’s talk facts.

The Truth About Foundations for a 40 sqm (Studio-Size) Extension

A 40 sqm single-storey extension — what we often call studio size — is one of the simplest structural builds in the UK.

Yet homeowners are routinely quoted £25,000–£35,000 for foundations and ground floors alone.

That makes no technical sense.

Here’s why 👇

---

Excavation: It Never Needs to Be Deep

For a single-storey extension:

• Typical required excavation depth: 600–700 mm
• This already exceeds frost protection and load requirements
• Tree influence is addressed horizontally, not by digging deeper
• You are not building a basement
• You are not carrying multi-storey loads

👉 There is no justification for excavations beyond this depth in normal UK soil conditions.

---

The Trench Is Done in Hours — Not Days

For a 40 sqm extension:

• Trench length is typically 18–22 linear metres
• Machine excavation time: 2–3 hours
• Spoil removal is predictable and priced upfront

Once dug, the geometry is fixed.
There is nothing “unknown” left to price.

---

Foundations Are Standardised — Not Custom Guesswork

Once excavation is complete:

• 600 mm deep concrete strip foundation
• Steel reinforcement (where required) comes prefabricated
• Concrete is ready-mix, delivered to site
• Pour time: 2–3 hours
• Cure time is irrelevant to labour cost

This is repeatable construction, not bespoke art.

So ask yourself:

If the depth is fixed,
the trench length is known,
the steel is prefabricated,
and the concrete price is public —
why isn’t the foundation price fixed?

---

Ground Floor Slab: Also Predictable

A typical ground floor build-up includes:

• Compacted hardcore
• DPM
• Insulation
• Reinforcement mesh
• Concrete slab

Again — all measurable, all standard, all known before work starts.

No surprises.
No guesswork.
No reason for cost inflation.

---

The Real Cost Breakdown (Reality, Not Sales Talk)

For a 40 sqm extension:

• Excavation & disposal
• Concrete strip foundations
• Reinforced ground floor slab
• Labour (2–3 days total)
• Materials at market rates

👉 £15,000 + VAT is the realistic ceiling, not the starting point.

Anything beyond this is usually:

• Over-digging
• Unnecessary “tree panic”
• Inflated provisional sums
• Or contractors pricing uncertainty instead of engineering

---

Why Ours Is Fixed Price — And Others Aren’t

At Amtaargc, we set foundation prices because:

• We design before we dig
• We control the sequence
• We use standardised engineering details
• We don’t price fear — we price facts

If your extension is single storey, your foundation should be simple, fast, and predictable.

That’s how we deliver projects like the one you see above.

---

Final Thought

A foundation is not where your extension should become expensive.
It’s where it should become controlled.

If someone can’t fix the price of your foundation, it usually means they don’t understand it.

---

Thinking of a 40 sqm Extension?

If you’re planning a studio-size extension anywhere in the UK, and you’re being quoted inflated foundation costs — challenge it.

Or better yet, speak to engineers who build these every week.