Extension Foundation Calculator — Depth, Width & Concrete Volume

Minimum foundation depth depends on soil type. Gravel, chalk and rock need a minimum of about 450mm, while shrinkable clay needs roughly 750mm (low plasticity), 900mm (medium) or 1,000mm (high). On clay near trees, NHBC Standards Chapter 4.2 can require foundations up to 2.5m deep depending on the tree's water demand, mature height and distance; anything deeper than 2.5m must be designed by an engineer (NHBC Technical Requirement R5). Building Control confirms the required depth during the excavation inspection.

NHBC Standards Chapter 4.2 'Building near trees' gives guidance on foundation depths for buildings on shrinkable clay near trees. Trees extract moisture from clay causing it to shrink, and when trees are removed the clay can swell (heave). The chapter classifies each species' water demand (high, moderate or low — Table 2/3), sets the zone of influence as a multiple of mature height (Table 3b: high ×1.25, moderate ×0.75, low ×0.5) and derives the depth from the distance-to-height ratio cross-referenced with the clay's volume-change potential. There is no single 'Table 4.2' — the depths come from the chapter's tables or the NHBC Foundation Depth Calculator app. It is the standard reference used by Building Control.

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.

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.

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.