Do I need a structural engineer for an extension?
3 min
7
September 2026
Key figures in this guide
One-minute decision
What are you trying to work out?
Use the checker first, then turn the result into a clearer brief before speaking to professionals.
Open the checkerYes. Almost every house extension needs a structural engineer, because building control will ask for calculations for the foundations, the beams over any new opening, and the new roof or floor structure. Expect £900 - £1,800 for a single-storey extension in 2026.
Your architect designs the space. The engineer proves it stands up. Building control will not sign off without that evidence, and no competent builder will price steelwork without it.
What the engineer designs on an extension
- Foundation type and depth, including near trees, drains or a neighbour
- The beam over the opening between the house and the new space, plus padstones and bearings
- Lintels over windows and doors
- Floor joists and the new roof structure
- Lateral restraint and how the new structure ties into the old
- Temporary support while a wall comes out
When you do not need one
- No load-bearing element is touched: a studwork partition, a refit, decoration
- Windows replaced in the same opening with the same lintel
- A garden room that is structurally independent and within permitted development
What it costs in 2026
| Extension type | 2026 engineer fee | Included |
|---|---|---|
| Single-storey rear extension | £900 - £1,800 | Foundations, beams, roof, full calculation pack |
| Side return or wraparound | £1,200 - £2,200 | Multiple openings and corner support |
| Two-storey extension | £1,400 - £2,600 | Floor and wall design as well |
| Extension plus loft conversion | £1,800 - £3,200 | Both structures, coordinated |
| Extra site inspections | £250 - £450 per visit | Foundation, steel and tie checks |
Add 15 - 25 per cent in London and the South East.
When in the process to appoint
- 01Settle the layout with your architect or designer.
- 02Appoint the engineer before you tender, not after.
- 03Issue calculations and drawings with the tender pack, so builders price real steel sizes.
- 04Submit to building control with the calculations attached.
- 05Book any site inspections the engineer recommends before foundations are covered.
What goes wrong without one
- Builders guess beam sizes and either overprice or undersize the steel
- Building control halts the job and asks for retrospective calculations
- Foundations near a tree or drain fail inspection and need digging deeper
- Quotes cannot be compared, because each builder assumed a different structure
What to ask before you appoint
- 01Are you chartered with IStructE or ICE, and what indemnity cover do you hold?
- 02Does the fee cover building control queries and builder questions?
- 03How many revisions if the layout changes?
- 04How long from instruction to issued calculations?
- 05Are site inspections included, or charged per visit?
Put the structure in your brief
Renovome writes your structural requirements into the brief that goes to every professional, so engineers quote for the same elements and builders price the same steel. Create your project brief, then find engineers near you.
Last reviewed: September 2026