Structural Engineer Guide London 2025
When you need a structural engineer, what they do, typical costs, and how to find the right one for your extension project.
TL;DR - The Quick Answer
Most London homeowners planning an extension will need a structural engineer at some stage. Their role covers foundation design, steel beam sizing, and producing the structural calculations that building control requires before approving construction. Engineers are typically engaged for load-bearing wall removals, loft conversions, and new extensions of any size. Fees range from £500 to £2,000 for standard work, with calculations usually delivered within 1-2 weeks.
Key Facts
- Typical cost: £500-£2,000 for extension calculations
- When needed: Steel beams, wall removal, foundations, loft conversions
- Turnaround: 1-2 weeks typically for calculations
- Qualification: Look for CEng, MIStructE, or MICE
A structural engineer ensures your extension is safe and stable. They design the structural elements-foundations, beams, lintels-and provide calculations that building control requires before approving your project. Most extensions need structural input at some point.
When You Need a Structural Engineer
Removing Load-Bearing Walls
Creating open plan spaces by removing walls. Engineer designs steel beams and specifies temporary support needed during work.
New Extensions
Foundation design, wall construction, roof structure, and any connections to the existing building.
Loft Conversions
Floor strengthening, dormer construction, staircase opening. Most loft conversions need structural calculations.
Basement Excavations
Complex structural work including underpinning, waterproofing design, and temporary works during construction.
Structural Engineer Costs
| Service | Typical Cost |
|---|---|
| Single beam calculation | £300 - £500 |
| Small extension (rear) | £500 - £1,000 |
| Loft conversion | £600 - £1,200 |
| Two-storey extension | £800 - £1,500 |
| Basement conversion | £2,000 - £5,000+ |
| Site visits | £150 - £300 each |
| Service type | When needed | Typical cost | Who appoints |
|---|---|---|---|
| Single beam calculation | Wall removal, opening | £300–£500 | Client or architect |
| Extension calculations (small rear) | Any new extension | £500–£1,000 | Client or architect |
| Loft conversion calculations | Loft conversion | £600–£1,200 | Client or architect |
| Two-storey extension | Two-storey extension | £800–£1,500 | Client or architect |
| Basement conversion | Basement works | £2,000–£5,000+ | Client or architect |
| Site visits | Construction stage | £150–£300 each | Client |
Source: Mayfair Studio project data • Data as of 2026
What You Get
Structural Calculations
Mathematical calculations proving the structure is safe. Building control requires these before approving work.
Structural Drawings
Detailed drawings showing beam sizes, foundation depths, steel specifications, and connection details.
Specification Notes
Written instructions for builders covering materials, workmanship standards, and construction sequence.
Site Inspections
Visits during construction to check work matches design. Often required for steelwork and foundations.
Finding a Structural Engineer
- Institution of Structural Engineers: Search istructe.org for chartered engineers
- Institution of Civil Engineers: ice.org.uk member search
- Architect recommendations: Your architect will have engineers they work with regularly
- Builder recommendations: Experienced builders know reliable local engineers
Almost all home extensions in London require structural engineer calculations - for removing load-bearing walls, placing steel beams (RSJs), new foundations, and roof structures. Building Control will not approve an extension without structural sign-off. Structural engineer fees run £1,000–£2,500 for typical projects. Mayfair Studio includes structural calculations in its £1,295 fixed fee, avoiding a separate structural engineer bill.
Frequently Asked Questions
Do I need a structural engineer for a home extension in London?
Yes - almost all home extensions in London require structural engineer calculations. Structural calculations are needed for: removing load-bearing walls to create open-plan layouts, placing steel beams (RSJs) to span openings, new foundations for extensions, roof structure changes for loft conversions. Building Control will not approve your extension without structural sign-off. Mayfair Studio includes structural calculations in its £1,295 fixed fee, avoiding a separate engineer bill of £1,500–£2,500.
How much does a structural engineer cost for a home extension in London?
Structural engineer fees for home extensions in London: £1,000–£1,500 for simple single-storey extensions; £1,500–£2,500 for standard projects with RSJ beams and load-bearing wall removal; £2,000–£4,000 for complex two-storey or loft projects. Mayfair Studio's £1,295 fixed fee includes structural calculations - the all-in price avoids a separate structural engineer bill that would otherwise add £1,500–£2,500 to your project costs.
What does a structural engineer do for a home extension?
A structural engineer for a home extension designs the structural elements: calculates the size and specification of steel beams (RSJs) needed to span wall openings, specifies foundation depths and widths for the extension, designs roof structures for loft conversions, and produces structural drawings for building regulations. Their calculations are legally required for Building Control approval - without them, construction cannot legally proceed.
Do I need a structural engineer if I use permitted development?
Yes - permitted development relates to planning permission only. Structural calculations are required for building regulations regardless of whether your extension is permitted development or has planning permission. If you're removing any internal walls to connect the extension to your house, or creating a large opening, structural engineer sign-off is essential. Building Control will not issue a completion certificate without it.