Q: How much extra will my foundation cost if I have a full-grown tree near my new studio-size extension? A: Only £4,000 anywhere in the UK. And you will never dig deeper than 2.5m. This question comes up in almost every enquiry for a 40m² studio-size extension. Homeowners are often told foundation costs will “double” or “triple” because of a nearby tree — but this is simply not true. Here is the correct engineering answer.

1️⃣ Building Control Does NOT Ask for 2.5m of Concrete

When Building Control instructs you to “dig to 2.5m because of a tree,” they are not changing your foundation design.
They are doing one thing only:

✔ Ensuring you remove the active clay zone affected by tree moisture.

Tree roots do not go deeper than 2.5m in UK soil.
The soil above this level moves seasonally — shrinking in summer and expanding in winter.

Building Control wants you to bypass these unstable layers.

This has nothing to do with pouring 2.5m of concrete.

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2️⃣ Foundation Design Stays the Same

A single-storey studio-size extension uses a standard strip foundation, normally:

• 0.5m deep concrete foundation
• At a dig depth of around 1.0m

This structural design does not change because of trees.

Instead, the way we prepare the soil changes.

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3️⃣ The Real Solution: Soil Improvement (Not Deep Foundations)

When a tree is nearby, engineers use the correct approach:

Improve the soil — don’t overdesign the foundation.

Soil improvement involves:

• Excavating deeper to reach stable clay
• Removing weak, shrinkable clay
• Replacing it with engineered compacted fill
• Pouring the same foundation design on top

This is the correct method across the entire UK.

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4️⃣ Why the Maximum Depth Is Always 2.5m

Every UK tree species — oak, willow, poplar, pine, birch, ash — affects only the top 2.5m of soil.

Below 2m–2.5m:

• The clay is stable
• There is no moisture variation
• No shrinkage
• No risk

So engineers never need to dig deeper than this level.

This applies nationwide.

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5️⃣ Example Scenario: Tree 5m Away From Your Foundation

If you have a full-grown tree 5m away:

Here’s the real foundation process:

• Dig down to approx. 2.5m
• Improve 2m of soil
• Pour approx. 1.7m of concrete
• Install the normal foundation design
• Total extra cost: £4,000

This works for all UK domestic tree conditions.

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6️⃣ Why Homeowners Are Often Misled About Cost

Many builders mistakenly say:

“You need 2.5m deep foundations.”

This is incorrect.

You need 2.5m excavation, not a 2.5m foundation.

Concrete quantity does not multiply.
Reinforcement does not multiply.
Design does not change.
Costs stay controlled.

Soil improvement = £4,000 max for a studio-size extension.

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7️⃣ Summary — The Clear, Engineer’s Answer

Q: How much extra does it cost to protect against tree risk when building a studio-size extension?

A: Only £4,000 anywhere in the UK.

Because:

✔ You will never dig deeper than 2.5m

✔ You do soil improvement, not deeper foundations

✔ Foundation design stays standard

✔ Building Control only checks soil stability

✔ Works for every UK tree species

This is the most efficient, approved, and economical method for foundation safety near trees.