How deep should extension foundations be?

Minimum foundation depth depends on soil type. Gravel and rock need a minimum of 450mm, while clay soils require 750mm to 1,000mm or deeper. On clay soils near trees, NHBC Chapter 4.2 may require foundations 1.5m to 3m deep depending on tree species, height and distance. Building Control will confirm the required depth during excavation inspection.

What is NHBC Table 4.2 and how does it affect foundations?

NHBC Standards Chapter 4.2 provides guidance on foundation depths for buildings on clay soils near trees. Trees extract moisture from clay causing it to shrink, and when trees are removed clay swells. The table cross-references tree species, height, distance from the building and clay plasticity to determine minimum foundation depth. It is the standard reference used by Building Control inspectors.

How wide should a strip foundation be?

A strip foundation for a single storey extension is typically 450–600mm wide. Two storey extensions usually need 600mm, and three storey structures may require 750mm or wider. The width depends on the wall load (kN/m) and the bearing capacity of the soil. Your structural engineer will specify the exact width.

How much concrete overpour should I allow?

An overpour allowance of 10% is standard for strip foundations. Trench sides are rarely perfectly straight, and the excavation may be slightly deeper in places. Some contractors allow 10–15% depending on ground conditions. It is better to have slightly too much concrete delivered than to run short during a pour.

Do I need a structural engineer for extension foundations?

While not legally required for simple extensions, a structural engineer is strongly recommended. They will design the foundation based on a site investigation, specifying depth, width, reinforcement and concrete grade. Building Control will require foundation calculations for anything beyond a straightforward single storey extension on good ground.

TradeCalculator

Extension Foundation Calculator — Depth, Width & Concrete Volume

Calculate foundation trench depth, width and concrete volume for UK extensions based on soil type, wall load and tree proximity per NHBC standards.

Foundation perimeter length

Soil type

Wall loading

Trench width

Nearest tree type

Tree distance from foundation

How We Calculate This

This calculator determines strip foundation dimensions based on UK Building Regulations and NHBC Standards Chapter 4.2.

Depth calculation

Base depth is set by soil type (450mm for granular soils, 750–1000mm for clay). Tree proximity on clay soils triggers NHBC adjustments based on the tree influence zone radius = tree height x species multiplier.

Concrete volume

Concrete fills a minimum of 225mm depth (or 60% of trench depth, whichever is greater). Volume = perimeter x width x concrete depth, plus overpour allowance.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.