Do you need a structural engineer for an extension? Almost always. Opening the rear wall (the universal open-plan move), carrying new roofs and floors, and founding new walls all require calculations that Building Control will ask to see. Typical London fees for a single-storey package run roughly £500–£1,500 — a small line in the budget that determines whether approval, construction and the finished structure go smoothly.
What the engineer designs
- Openings — steels/beams carrying the existing rear or flank wall, posts, padstones, bearing checks
- New structure — roof, floor and wall design for the extension itself
- Foundations — sized for the ground, and on clay near trees, deepened per NHBC guidance
- Existing-structure checks — the old walls, lintels and footings now carrying new loads
- The package — calculations and structural drawings for Building Control and the builder
Foundations, clay and trees
Foundation depth is where London extensions get expensive quietly. On shrinkable clay, NHBC Standards Chapter 4.2 ties minimum foundation depths to tree species, mature height and distance — a substantial oak nearby can push required depths well beyond a standard trench, sometimes toward engineered solutions (trench-fill, piles, heave protection). Two rules follow: survey the trees before pricing the foundations, and never let anyone 'match existing' Victorian depths for new work — the existing shallow footing is grandfathered; yours won't be.
The classic pitfalls
- Late engagement — structural input after tender means redesign, re-quoting, delay
- Unknown existing structure — outriggers without foundations, decayed bressummers, discovered mid-demolition
- Differential movement — new deep foundations rigidly tied to old shallow ones without design for the difference
- Drainage collisions — Victorian runs exactly where the new footings want to go; build-over agreements missed
- Party wall timing — notices served weeks before excavation instead of the months the Act's process needs
Process and programme
Efficient sequence: architectural design → structural design and calculations (allow 1–2 weeks from complete drawings for standard schemes) → Building Regulations submission → tender on the full package → party wall procedure running in parallel → construction with the engineer on call for discovered conditions. The structural fee is typically fixed and quoted from the architect's drawings; changes of scheme after design are the main cause of fee growth, so settle the architecture first.
This guide provides general information only and is not formal structural advice for any specific property. The cause of movement or damage should always be established by proper professional assessment before repairs are commissioned.