Most renovations reach a point where someone says “you’ll need a structural engineer for that.” This guide explains exactly when that’s true, what the engineer does, and what you should expect to pay — written from the inside, not by a firm trying to sell you the calculations.
What a structural engineer actually does
A structural engineer works out whether your building will safely carry the loads you’re about to change. On a typical home project that means:
- Sizing steel beams (RSJs) and lintels when you remove or open up a wall.
- Designing foundations for an extension, including how they meet the existing structure.
- Checking existing structure — joists, roof timbers, chimney breasts — before you alter or remove them.
- Producing calculations and drawings that Building Control will sign off against.
They are not the same as your architect. The architect decides what the space looks like; the engineer makes sure it stays up. On larger jobs you’ll use both.
When you genuinely need one
You almost certainly need a structural engineer if your project involves any of the following:
- Removing or notching a load-bearing wall (including most “knock-through” kitchen-diners).
- Building an extension — single or double storey.
- A loft conversion that adds floor loads and alters the roof structure.
- Removing a chimney breast on one or more floors.
- Underpinning, subsidence, or any visible structural cracking.
- Forming a large opening for bifold or sliding doors in an external wall.
If you’re redecorating, fitting a kitchen, or moving a clearly non-structural stud partition, you don’t.
Not sure whether a wall is load-bearing? That’s the most common trigger for a call-out — and it’s covered in its own guide on how to tell if a wall is load-bearing.
Structural engineer vs architect vs surveyor
These three get confused constantly:
| Role | What they’re for |
|---|---|
| Structural engineer | Calculations and design that keep the building standing — beams, foundations, structural checks. |
| Architect / architectural designer | The design, layout, drawings and (often) the planning application. |
| Surveyor | Assessing the condition of a property (e.g. a homebuyer’s survey) or party-wall matters. |
For a renovation you’ll usually engage a designer for the look and an engineer for the structure. A party-wall surveyor is a separate role that comes in if your work affects a shared wall with a neighbour.
What a structural engineer costs in the UK
Fees depend on complexity, but as a 2026 guide:
- Single beam / one calculation (e.g. one knocked-through wall): roughly £300–£600.
- Typical extension or loft package: roughly £600–£1,200.
- Site visit / inspection: often £150–£350 if charged separately.
Most domestic engineers quote a fixed fee once they’ve seen drawings. Always get the calculations and a stamped drawing — Building Control needs both.
How to find a good one
Look for a chartered engineer registered with the Institution of Structural Engineers (IStructE) or ICE. Ask for examples of similar domestic work, confirm professional indemnity insurance, and get the fee in writing. A good engineer will also flag where a cheaper design choice exists — for example a wider but shallower beam to avoid dropping your ceiling.
The bottom line
If your project changes how loads move through the building — walls, openings, roofs, foundations — you need a structural engineer, and it’s not a corner worth cutting. The fee is small next to the cost of getting it wrong, and the stamped calculations are what let Building Control sign your project off.
This is a starting draft. Expand each section with worked examples and internal links to the load-bearing-wall and building-control guides as they publish.