Structural Engineers: What They Do and When Your Project Needs One

For a typical home project, a structural engineer provides:

• Structural calculations — mathematical proof that the foundations, beams, walls, and floors can carry the loads placed on them
• Foundation design — specifying the type, depth, and width of foundations based on the ground conditions and building loads
• Steel beam design — sizing and specifying steel beams (RSJs) for new openings in walls, open-plan spaces, and structural support
• Lintel specifications — sizing lintels above doors and windows
• Floor joist design — specifying new or strengthened joists for loft conversions
• Structural drawings — detailed drawings showing how the structural elements connect and are built

The structural engineer’s calculations and drawings are submitted as part of the building regulations application.

Almost always needed

• Rear extensions — foundation design and any steelwork for openings between the extension and the existing house
• Loft conversions — floor joist design, dormer structure, roof steelwork, staircase support
• Two-storey extensions — foundations, steelwork, loadbearing walls
• Removing a loadbearing wall — steel beam to replace the structural support

Sometimes needed

• Garage conversions — if the existing garage structure needs strengthening
• Underpinning — if existing foundations need reinforcing
• Chimney breast removal — supporting the chimney above if removing the breast on a lower floor
• Adding a bathroom upstairs — if the existing floor joists need strengthening

Usually not needed

• Garden offices and outbuildings — unless over 30m² or structurally complex
• Porches — unless unusually large
• Replacement windows and doors — unless the opening is being enlarged
• Decorating, flooring, and cosmetic work

These fees are for calculations and drawings only. They do not include the cost of the actual steel beams or construction work.

1. Appointment. Your architect or builder recommends a structural engineer, or you find one yourself. Provide them with the architectural drawings showing what you want to build.
2. Design. The engineer designs the structural elements — foundations, beams, lintels, joists. They may need a site visit.
3. Calculations and drawings. The engineer produces a calculations document and structural drawings. These are submitted with your building regulations application.
4. Building control review. Building control checks the structural design as part of the Full Plans approval.
5. Construction. Your builder follows the engineer’s specifications. Building control inspects the structural work at key stages.

• Institution of Structural Engineers (IStructE) — the professional body. Search at istructe.org.
• Institution of Civil Engineers (ICE) — many civil engineers also handle structural work. Search at ice.org.uk.
• Your architect — most architects have structural engineers they regularly work with.
• Your builder — experienced builders know local structural engineers.

What to check

• Chartered status — look for CEng (Chartered Engineer) or IEng (Incorporated Engineer) after their name
• Professional indemnity insurance — essential. Ask for confirmation.
• Residential experience — make sure they handle domestic projects
• Turnaround time — typically 1–3 weeks for straightforward work

Does my builder need to follow the structural engineer’s specification exactly?

Yes. The structural engineer’s design is part of the building regulations approval. If your builder uses a different beam size, shallower foundations, or different materials, the work will not comply.

Can my builder do the structural calculations?

No. Structural calculations must be prepared by a qualified structural engineer. Your builder builds to the engineer’s design — they do not design the structure themselves.

What if the ground conditions are different from expected?

This happens. The engineer designs foundations based on assumed ground conditions. When the trenches are dug, the actual conditions may be different. If so, the engineer may need to revise the foundation design. For difficult sites, a site investigation or trial hole before design can reduce the risk (costs £200–£500).