Structural Engineer for Extensions London — Foundations, Frames and Building Control

Homeowners extending their property to the rear or side, architects requiring structural engineering coordination for a planning or Building Control submission, and contractors who need an engineer-designed structural frame and foundation specification before work begins. If your extension involves a new foundation, a structural opening at the junction with the existing building, or a structural roof frame, you need a structural engineer.

Rear, side, and wraparound extensions require structural design at every stage: from the foundations supporting the new structure, through the structural frame and roof, to the openings between old and new at the junction.

The interface between existing and new construction is where most problems arise — including differential settlement, inadequate bearing, and poorly designed openings. Proper structural engineering addresses these risks before they become costly site problems.

Foundation design: The extension foundation must transfer all new loads into the ground. In London, this requires site-specific design: made ground in inner areas, London clay in the south and east (susceptible to shrinkage and heave near trees), and chalk in parts of South London. We design foundations appropriate to the specific site conditions, incorporating ground investigation findings where available.

Structural frame: Single-storey extensions typically require strip or pad foundations, structural walls or columns, and a structural roof frame. Double-storey extensions are more complex — the loads are greater, the foundation design is more critical, and the frame must carry floor and roof loads simultaneously. We design the complete structural frame, coordinated with your architect's layout.

Opening at the junction: The opening between the new extension and the existing building requires a structural solution — typically a steel or timber beam over the width of the opening — to carry the existing structure above. This opening design is a critical element of the extension structural package.

Single-storey extensions are the most common type in London — typically rear extensions to Victorian and Edwardian terraces that create open-plan kitchen-diner spaces. Double-storey extensions add an upper-floor bedroom or bathroom above the ground-floor extension. Each storey adds loading and complexity. We scope the appropriate structural package at the quotation stage.

Building Control requires structural calculations and coordinated drawings for all extensions involving new foundations, structural openings, or roof steelwork. We prepare the full structural submission pack — calculations, drawings, foundation details, connection details — ready for submission. We also support Building Control queries after submission.

Most of our extension structural work involves Victorian terraces in South, East, and North London undergoing rear single-storey extensions to create open-plan ground floors. We also work on side-return extensions, wraparound extensions, and double-storey additions. In areas with significant tree cover — common in Wandsworth, Croydon, and outer boroughs — clay heave analysis is an important element of the foundation design.

Civil engineering design (drainage, SuDS, retaining walls and external works) and planning application support are available alongside this structural package, so the below-ground and planning elements of your project can be coordinated with the structural design under a single appointment.