Single-Storey Rear Extension — 1960s Semi, Tilbury
Project type: Single-storey rear extension, 4.2m × 3.5m Property: 1960s semi-detached, Tilbury (RM18) Client: Homeowner extending kitchen-diner Turnaround: 5 working days from site visit to delivery Structural engineer fee: £950 (fixed)
The Brief
The owners of a typical 1960s three-bed semi in Tilbury wanted to push the back of their house out by 3.5m and full width — opening up a small kitchen and separate dining room into one big kitchen-diner with bifold doors onto the garden. The architect had drawings and planning was already approved under permitted development.
What they needed from us:
- Structural calculations for the new extension (foundations, floor slab, walls, roof)
- A steel "goalpost" frame across the existing rear wall to keep the kitchen and new extension as one open space — no dropped beam, no posts in the middle
- Building Control submission package
- Confirmation that the existing foundations on the rear wall could take the goalpost loads
What We Found on Site
A 45-minute site visit revealed three things that shaped the design:
- Made ground at the rear. Trial pit at the corner of the existing rear wall showed 600mm of made ground (likely fill from when the estate was built in the 1960s) over firm London Clay. Standard 1m strip foundations would not work — we needed deep trench fill or piles.
- The existing rear wall is a cavity wall but the inner leaf is single-skin block. The goalpost steel needs to bear on the outer leaf with proper padstones — bearing the steel only on the inner leaf would have crushed it.
- A drainage run crossed the proposed foundation line. Build-over consent was needed from the water authority and the foundation had to be deepened locally to clear the drain by the required margin.
The Structural Solution
Foundations
We specified 1.2m deep trench fill foundations founded into firm London Clay, with the upper 600mm of made ground discounted entirely as a bearing layer (in line with NHBC Standards Chapter 4.2, which treats made ground as unsuitable for founding). Locally over the drain run, we deepened to 1.5m with reinforcement to span the drain.
Floor
100mm reinforced concrete slab on 150mm hardcore, A142 mesh, on a 1200-gauge DPM. Insulation 100mm PIR over slab to meet current Approved Document L.
The goalpost steel
The opening between the existing kitchen/dining and the new extension was 4.2m wide. The owners wanted no dropped beam visible internally, so we designed a flush goalpost:
- Beam: 203x133x30 UB running along the line of the existing rear wall, picking up the first-floor joists and gable masonry above
- Posts: 100x100x6.3 SHS each end, encased in plasterboard
- Padstones: 215x215x215 concrete to each post base, bearing on the existing inner leaf with a minimum 150mm bearing length (NHBC 6.1)
- Connections: Welded base plates to the SHS, bolted end plates to the UB
The goalpost was propped during install with Acrow props on either side of the existing rear wall before the masonry was cut out, then dropped into position and grouted up. Bearing was confirmed by a trial brick removal during the site visit, which showed the inner leaf was solid block (not perforated) and capable of taking the padstone load.
Walls and roof
- Cavity walls to match existing: 100mm outer brick, 100mm full-fill insulation, 100mm inner block
- Flat roof with warm-roof build-up to meet U-value requirements
- Timber rafters at 400mm centres bearing on a wall plate over the new walls
How Building Control Reviewed It
The application went to Thurrock Council Building Control. They came back with two queries inside a week:
- "Confirm bearing on existing foundation." We submitted a calculation showing the increased load on the existing foundation under the new goalpost was within the original design capacity — passed.
- "Confirm drain build-over." We provided the deepened foundation detail with reinforcement spanning the drain — passed.
No revisions needed. The full structural design was approved without resubmission.
What it Cost the Homeowner
| Item | Cost |
|---|---|
| Structural engineering (SBS, fixed fee) | £950 |
| Architect | (not via SBS — already in place) |
| Building Control fees | ~£550 (Thurrock Council) |
| Build-over consent | ~£250 (Anglian Water) |
The structural engineering came in slightly above our typical £800 quote for a basic extension because of the made ground and drain — both of which we identified at the site visit and quoted for upfront. No surprises.
Lessons for Similar Projects
If you're extending a 1960s–1980s semi anywhere in Tilbury, Grays, Stanford-le-Hope, South Ockendon or the wider Thurrock area:
- Expect made ground at the rear of estates from this era. A trial pit at site visit pays for itself ten times over.
- Always check for drains. Build-over consent takes 4–6 weeks. Find out early.
- Check the inner leaf before specifying bearing. Some 1960s estates used perforated or aerated blocks; a quick trial brick removal at site visit avoids surprises.
- Foundation upgrades cost less if designed in from day one rather than discovered halfway through digging.
- Chimneys still in the gable wall? If the extension's new steel sits below a retained chimney breast, that breast needs supporting separately. See our chimney breast removal guide — sometimes it's cheaper to remove the chimney during the same project.
Get a Quote for Your Tilbury or Thurrock Extension
We're based in Ilford and cover Tilbury, Grays, Thurrock, Stanford-le-Hope, South Ockendon, Purfleet and the wider South Essex area. Most calculations turned around in 5 working days as a fixed fee.
📞 Call 07401 650600 or email us with your address, a few photos of the existing house, and the architect's drawings if you have them.
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