The short answer
For a typical UK house extension, a structural engineer usually costs around £500–£1,500, with a simple single-storey rear extension often £400–£900 and a larger double-storey or wrap-around design more commonly £900–£2,000+. That fee normally buys the structural calculations and drawings Building Control needs for the steel beams (RSJs), foundations and lintels, plus a specification sheet. A site visit, if needed to inspect existing structure or ground conditions, often adds £150–£400. Most engineers price per project rather than per hour for this work, and a chartered engineer (IStructE or ICE) sits at the upper end. Region matters — London and the South East typically carry a 15–25% premium. The honest figure depends on how many beams and foundations the design needs.
An extension almost always alters how the building carries load, so Building Control will want structural calculations signed off by a competent engineer. The figures below are typical ranges for guidance, not quotations.
Typical UK extension fees
- Single-storey rear£400–£900
- Double-storey / side return£700–£1,500
- Wrap-around / large£900–£2,000+
- Site visit (if needed)£150–£400
- London / South East15–25% premium
What the fee covers
For an extension, a structural engineer's fee normally covers the calculations and drawings that prove the new structure is safe and that satisfy Building Control under Part A (structure) of the Building Regulations. In practice that usually means sizing any steel beams or RSJs over new openings, designing the foundations for the extension, checking padstones and bearings, and producing a written specification the builder can follow. Some engineers issue a single calculation package; others split design and inspection into separate stages. It is worth confirming at the outset which model the engineer uses, because a quote that looks lower may exclude site visits or revisions that a fuller quote bundles in — comparing fees only works when you are comparing the same scope.
- Beam and lintel calculations for any new openings or removed walls.
- Foundation design sized to the ground conditions and the load above.
- Structural drawings and a specification sheet for the builder and Building Control.
- Optional site visit to inspect existing structure, trial holes or ground.
What moves the price
Two extensions of the same floor area can attract very different fees depending on how much structural design is involved. A clean single-storey rear extension with one steel beam is far cheaper to engineer than a wrap-around that removes several load-bearing walls and needs a complex frame of beams and columns. The number of separate calculations is the biggest driver, followed by ground conditions, the property's age and whether the engineer has to attend site.
| Extension type | Typical engineer fee | Notes |
|---|---|---|
| Single-storey rear | £400–£900 | often one beam plus foundations |
| Side return / infill | £500–£1,200 | narrow access, party wall common |
| Double-storey | £700–£1,500 | more beams, heavier loads |
| Wrap-around / large | £900–£2,000+ | multiple beams and columns |
Indicative UK figures for guidance. Sources: Checkatrade structural engineer cost guide; typical 2025/2026 ranges.
Party wall and Building Control
If your extension is near or on a boundary, you may also need a Party Wall Act 1996 notice and award — that is a separate surveyor cost, not the structural engineer's fee, though the engineer's calculations often feed into it. The structural calculations themselves are submitted with your Building Regulations application (full plans or a building notice), and Building Control may ask the engineer to confirm details before signing off. Budget for the engineer as one line in the wider extension cost, alongside architectural drawings, the planning route and the build itself.
Hourly versus fixed fees
Most engineers quote a fixed project fee for a domestic extension because the scope is reasonably predictable from the architect's drawings. Where the work is open-ended — for example ongoing inspections during a tricky build, or investigating an unexpected problem mid-project — an engineer may revert to an hourly or daily rate, commonly around £70–£150 per hour for a chartered engineer. If your drawings are still changing, expect the fee to be quoted against the final scope, since each redesign means re-running the calculations. A common saving is to have your architectural design reasonably settled before the engineer starts, so the calculations are produced once rather than revised through several rounds.
How the engineer fits the wider extension budget
It helps to see where the structural engineer sits in the full cost of an extension, because their fee is usually one of the smaller lines. The headline build cost — groundworks, walls, roof, windows, fit-out — dwarfs the design fees, and within the design fees the architect (or architectural technologist) producing the planning and construction drawings is typically a larger figure than the structural engineer's calculations. On top of those sit the planning application fee where permission is needed (or the simpler route of permitted development), the Building Regulations application fee paid to Building Control, and, where the work is near a boundary, a party wall surveyor. Treating the structural engineer as a standalone 'extra' overstates its weight in the budget; it is a modest but essential line that keeps the rest of the spend safe and compliant.
- Architect / technologist: planning and construction drawings — usually the larger design fee.
- Structural engineer: calculations and structural drawings for Part A.
- Planning fee: only where the extension needs full planning permission.
- Building Control fee: for inspecting and signing off the work.
- Party wall surveyor: if you build on or near a shared boundary.
Frequently asked questions
Do I always need a structural engineer for an extension?
If your extension removes a load-bearing wall, adds steel beams, changes the roof load or needs new foundations, Building Control will want structural calculations from a competent engineer. A purely cosmetic or very small extension that does not alter how loads are carried may not, but most extensions do.
Is the structural engineer fee separate from the architect's fee?
Yes. The architect produces the design and planning drawings; the structural engineer produces the calculations that prove it stands up. They are normally billed separately, though some practices offer both under one roof.
Does the fee include the Building Control submission?
Usually the engineer provides the calculations and drawings, and your builder, architect or you submit them to Building Control as part of the Building Regulations application. Confirm who is handling the submission when you agree the fee.
Sources & further reading
- Checkatrade — structural engineer cost guide
- Planning Portal — Building Regulations
- IStructE — find a chartered structural engineer
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.