Rear Extension — Victorian Terrace, East Ham
Project type: 4m single-storey rear extension Property: Victorian mid-terrace, 2 bedrooms, East Ham E6 Client: Homeowner creating a larger kitchen with bifold doors onto the garden Turnaround: 6 working days Structural engineer fee: £1,400 (fixed)
The Brief
The homeowner wanted a 4m deep single-storey rear extension to replace a tired lean-to conservatory. The architect had already produced planning drawings showing an open-plan kitchen-diner with full-width bifold doors overlooking the garden. Our job: design the structural elements and get Building Control approval.
Challenges Specific to This Property
Victorian terraces in East Ham have some characteristics that affect structural design:
Shallow foundations. The existing rear wall had original Victorian foundations — just 400mm deep brick footings. The new extension foundations needed to be designed to work alongside these without causing differential settlement.
London Clay. The site sits on London Clay, which is highly susceptible to seasonal volume change (shrinkage in summer, heaving in winter). Tree proximity also matters — there was a mature sycamore in the neighbour's garden approximately 5m from the new extension footprint. NHBC guidelines require deeper foundations near trees on clay.
Existing rear wall removal. To create the open-plan space, the existing rear wall needed to come out. This meant designing a steel beam to span the full internal width (5.1m) at the junction between old and new.
What We Designed
Foundations
Given the London Clay and the nearby tree, we specified strip foundations to a minimum depth of 1.5m — significantly deeper than the 1m minimum for standard clay conditions. The foundation was designed as a 600mm wide × 225mm deep (minimum) mass concrete strip with trench-fill below.
We included a note for the Building Control inspector: if roots are found during excavation, the foundation depth may need to increase to 2m. This kind of contingency planning saves delays — the builder knows what to do if conditions on site differ from expectations.
Steel Beam at Junction
A 305×165×40 UB spanning 5.1m to replace the existing rear wall. This is a substantial beam — it's carrying the first floor joists and the roof loading above the original rear wall line. We designed it with 350mm concrete padstones at each bearing point.
At 5.1m span, deflection was the governing criteria, not bending. We checked deflection under dead load only (to ensure the ceiling line didn't sag before the builder plastered) and under full service load. Both were within limits.
Roof Structure
The extension roof was a flat roof with a slight pitch for drainage. We designed:
- C24 timber joists at 400mm centres spanning between the new rear wall and the new steel beam
- A timber wall plate bolted to the inner leaf of the new blockwork walls
- Restraint straps tying the roof to the blockwork at 2m centres
Lintel Over Bifold Doors
The full-width bifold doors (4.8m opening) needed a steel lintel. We specified a 203×133×25 UB recessed into the blockwork inner leaf, supporting the roof structure above. The lintel was designed as simply supported with 150mm bearings.
Connection to Existing Structure
Where new met old, we specified mechanical fixings — resin-bonded stainless steel anchors at 450mm centres tying the new blockwork to the existing brick. This prevents cracking at the junction as the two parts of the building settle at slightly different rates.
Building Control Submission
We submitted a comprehensive package to Newham Building Control:
- Full structural calculations (14 pages)
- Foundation plan showing depths, widths, and reinforcement
- Structural steelwork layout with sizes and padstone details
- Roof joist layout with sizes and connections
- General arrangement drawing showing how new connects to existing
Approved in 2.5 weeks. One minor query about the distance to the neighbour's tree — we confirmed it was 5m and referenced the NHBC Chapter 4.2 guidelines to justify our 1.5m depth. Accepted immediately.
What Made This Project Smooth
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We factored in the tree from day one. Rather than designing standard-depth foundations and having the Building Control officer ask us to redesign, we anticipated the requirement and specified deeper foundations upfront. Saved 2–3 weeks of back-and-forth.
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Deflection-governed beam design. The 5.1m junction beam could have been a size smaller if we'd only checked bending. But at that span, the ceiling would have visibly sagged over time. We went one size up for a beam that stays dead flat.
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Clear junction details. The builder said the resin anchor detail at the old/new junction was the most useful drawing on the set. Without it, he'd have had to improvise — and the junction would have cracked within a year.
Total Project Costs
| Item | Cost | |---|---| | Structural engineer (SBS) | £1,400 | | Builder (full extension build) | £42,000 | | Building Control fee | £900 | | Architect (planning + building regs drawings) | £2,500 | | Total | ~£47,000 |
The extension added approximately 15m² of ground floor space and an estimated £70,000–£90,000 to the property value.
Key Takeaways
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Clay soil + trees = deeper foundations. In East London, London Clay is almost universal. Always check tree proximity — it dictates foundation depth.
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Deflection matters more than bending at long spans. For beams over 4m, we always check deflection first. A beam that's strong enough can still sag noticeably.
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Design the junction carefully. Where new extension meets existing house is the most failure-prone area. Resin anchors and proper detailing prevent the inevitable crack line.
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Anticipate Building Control queries. If you can see a question coming, address it in your submission. It saves weeks.
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One structural package, one fixed fee. Our £1,400 covered site visit, calculations, drawings, Building Control submission, and handling any queries. No extras.
Planning an extension? We offer free consultations across East Ham, Newham, East London and 70+ locations.
- Call: 07401 650600
- Email: sbs.structures@gmail.com
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