How Does a Structural Engineer Design a Rear Extension in Islington?

A rear extension on an Islington Victorian terrace is one of the most common residential structural engineering projects in London. Despite being 'common', each one presents specific engineering challenges driven by ground conditions, the existing building's structural condition, and the desired internal layout.

Foundation Design on London Clay

Islington sits predominantly on London Clay, which has good bearing capacity (typically 100–150 kN/m²) but is susceptible to volume change. Foundations for a rear extension must be deep enough to avoid the zone of seasonal moisture variation — typically 1.0m minimum, increasing to 1.5m+ near trees. Where trees are close, the structural engineer may specify piled foundations instead of traditional strips.

Steel Beam Design

• The connection between old and new is critical — a steel beam (typically a UB section) spans the full width of the new opening where the rear wall is removed
• Beam sizing depends on the span, loads from above (floors, roof, masonry), and the support conditions at each end
• Padstones — concrete or steel bearing plates — distribute the beam's point loads into the existing masonry without crushing the brickwork
• For open-plan layouts extending into the original house, multiple beams may be needed to remove internal walls while maintaining the first-floor structure

Connection to the Existing Structure

The junction between the new extension and the existing Victorian rear wall is where problems often occur. The structural engineer designs this connection to accommodate differential movement — the new extension will settle slightly as its foundations compress the soil, while the existing house has long since finished settling.

Our engineers at Bourdon Hill design rear extensions across Islington every week. We know the local ground conditions, the typical terrace configurations, and what Building Control inspectors in Islington expect to see.