Do you need a structural engineer for your extension?
Almost certainly, yes. If your extension takes out a load-bearing wall, forms a new opening, adds a storey or needs new foundations, Building Control will want structural calculations — and in practice a structural engineer is the person who produces them. The rare exception is a small conservatory or lean-to that touches nothing structural.
The short answer
Nearly every house extension involves at least one of four things: new foundations, a new roof, a steel beam over a knocked-through wall, or an opening cut into an existing wall. Each of those changes how loads travel down through the building, and each has to be shown to work on paper before a Building Control surveyor will sign it off. That paper is a set of structural calculations, and producing it is the structural engineer’s job.
You are not legally required to hire someone with the words “structural engineer” on their card. What you are required to do is satisfy Part A (Structure) of the Building Regulations. For anything beyond the trivial, the only realistic way to do that is to pay an engineer for the calculations — a builder’s word that “a 152 will do” is not something a surveyor will accept.
When you need one, and the times you don’t
The trigger is structural change, not the size of the job. A modest rear extension that removes the back wall of the kitchen needs an engineer; a large conservatory bolted onto an untouched wall might not. As a rule, get an engineer involved if your project does any of the following:
- Removes or alters a load-bearing wall, or forms a new opening in one
- Adds a steel beam, lintel or column to carry the floor or roof above
- Needs new foundations, or ties new foundations into the existing house
- Adds a storey, a dormer, or a new roof structure
- Builds close to an existing structure, a drain or a boundary where loads interact
When is one genuinely not needed? A ground-floor conservatory or glazed lean-to under about 30m², sitting on standard strip footings, with no load-bearing wall removed and no new opening formed, can often go ahead without engineer’s calculations. If you are unsure which side of the line you fall, ask your Building Control body before you commit — it is a five-minute conversation that can save a redesign.
What the engineer actually produces
The deliverable is rarely glamorous: a calculation package and a set of structural drawings or sketches, sized to your specific extension. Inside it you will typically find beam sizes, foundation details, and the connections that hold everything together.
Take the most common job, removing the wall between a kitchen and a new rear extension. The engineer works out the load sitting on that wall — the floor above, any wall above that, the roof — and specifies a steel beam to carry it. You will see a section size such as a 203 × 133 UB (a Universal Beam; “RSJ” is the older trade name for the same I-shaped section), a minimum bearing length where each end sits, usually around 150mm, and a padstone to spread the load so the beam doesn’t punch into the masonry below. Steel is designed to Eurocode 3 (BS EN 1993-1-1), and the calculations check both strength and how much the beam will sag under load.
For foundations, the engineer takes the weight of the new extension and specifies a footing deep and wide enough to carry it into ground that can take the load — which is why nearby trees, drains and made-up ground all change the answer. All of this feeds your Building Regulations application. See our guide to building regulations for extensions for how the calculations sit within the wider approval.
Structural engineer or architect?
They are not the same person and they are not interchangeable. An architect or architectural designer shapes the space — the layout, the light, how the extension looks and how you move through it — and produces the drawings you submit for planning. A structural engineer makes that design stand up: the beams, the foundations, the calculations that prove it is safe. On a typical extension you use both, and the engineer usually comes in once the layout is broadly settled.
Look for a chartered engineer — the letters to want are CEng alongside MIStructE (Institution of Structural Engineers) or MICE (Institution of Civil Engineers). For a straightforward domestic job an experienced non-chartered engineer working under chartered supervision is often perfectly suitable, and can be cheaper. For anything unusual — poor ground, an awkward existing structure, a basement — pay for the chartered name.
What it costs, and when to bring one in
Structural engineers usually charge either a fixed fee for a defined package of calculations or an hourly rate for ad-hoc advice and site visits. For most extensions a fixed fee is the norm, and it is a small line in the overall budget — the calculations for a single-storey rear extension typically land in the hundreds, not the thousands.
| What you’re paying for | Typical 2026 range |
|---|---|
| Beam and foundation calcs, single-storey extension | £500–£900 |
| Full calculations pack, larger or two-storey extension | £900–£1,500 |
| Standalone site visit or inspection | £150–£300 |
| Ad-hoc hourly rate | £75–£150 per hour |
Indicative ranges from BestBuilders’ 2026 regional cost model (published national cost guides × a regional labour index, averaged across 519 UK towns); London and the South East sit at the top of each band. For a full fee breakdown by job type, see BestBuilders’ structural engineer cost guide.
On timing: bring the engineer in after your concept or planning drawings are settled but before you submit for Building Regulations and before anyone orders steel. Get that order wrong and you either hold up the build waiting on calculations or pay for the wrong beam. Budget for the fee alongside your building control fees — the two go hand in hand.
Frequently asked
Get quotes from extension builders near you
Tell us what you are planning and we will match you with up to three builders who cover your area. Many will bring in a structural engineer as part of the job. Free, no obligation, and you decide whether to go ahead.
Get free quotes