What Is a Structural Engineer and When Do You Need One?

A structural engineer is a chartered professional who designs and analyses the load-bearing elements of buildings — the parts that hold the structure up and transfer loads safely to the ground. In the context of domestic building work, their role is to calculate whether the existing structure can safely accommodate the proposed changes, and to specify the new structural elements (steel beams, padstones, columns, foundations) that the works require.

Structural engineers in the UK are typically chartered members of the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). The chartered designations are MIStructE (Member), FIStructE (Fellow), MICE (Member of ICE), and FICE (Fellow of ICE). For domestic building work, you should appoint a chartered structural engineer — not a general builder or architectural technician, who may not have the professional indemnity insurance or the qualifications to certify structural calculations.

The distinction between a structural engineer and an architect is important. An architect designs the appearance, layout, and function of a building. A structural engineer designs the structure that makes the building safe. On most domestic projects, you need both — the architect produces the planning and working drawings, and the structural engineer produces the structural calculations and details that the Building Control officer and the contractor need to build the works safely.

For a house extension, structural calculations typically cover four main areas. Foundations: the engineer calculates the load from the new extension (dead loads from the structure, live loads from occupants and furniture) and specifies the foundation type and depth required to transfer those loads safely to the bearing ground. In Hertfordshire, where London Clay is common, foundations often need to be deeper than the standard 1 metre to avoid movement caused by clay shrinkage.

Structural openings: where the extension connects to the existing house, there will typically be one or more structural openings — a set of bifold doors, a wide opening between the kitchen and the new extension, or a doorway through a load-bearing wall. The engineer specifies the steel beam (Universal Beam or Rolled Steel Joist) required to span the opening safely, and the padstones (concrete or engineering brick bearing pads) required to distribute the beam's load into the wall below.

Roof structure: for a flat roof extension, the engineer specifies the roof joists and the point load connections to the existing structure. For a pitched roof extension, they specify the rafters, ridge beam, and any purlins or struts required. For a loft conversion, they specify the new floor joists (typically 47 x 220mm C24 timber at 400mm centres, or steel joists for longer spans), the dormer structure, and the ridge beam if the roof is being altered.

Connections to existing structure: the engineer specifies how the new structure connects to the existing building — joist hangers, wall plates, restraint straps, and the details for tying new masonry to existing masonry. These connections are critical for the structural integrity of the completed building and are inspected by Building Control at the relevant stage.

The most reliable way to find a structural engineer for domestic work is through the IStructE's Find an Engineer directory (istructe.org) or through a recommendation from your architect or builder. For domestic projects in Hertfordshire, you're looking for a local firm with experience in residential work — not a large commercial engineering practice whose minimum fee will be disproportionate to the scope of your project.

For a standard house extension or loft conversion, structural engineering fees run £800–£2,500. The fee depends on the complexity of the project, the number of structural elements to be designed, and the engineer's hourly rate. Some engineers charge a fixed fee for standard domestic projects; others charge on a time basis. Always agree the fee in writing before work starts, and confirm what the fee includes — calculations, drawings, and site visits are sometimes billed separately.

The structural engineer's calculations are submitted to Building Control as part of the Building Regulations application. Building Control will review the calculations and may raise queries — the engineer is responsible for responding to those queries and revising the calculations if required. This back-and-forth is normal and is included in the engineer's fee for a well-scoped appointment.

The division of responsibilities between architects and structural engineers is a source of confusion for many homeowners. In practice, the architect leads the design process — they produce the planning drawings, the working drawings, and the specification. The structural engineer works within the architect's design to ensure the structure is safe and buildable.

On a straightforward house extension, the architect produces the planning drawings (floor plans, elevations, sections) and the working drawings (construction details, specification). The structural engineer produces the structural calculations and structural drawings (beam schedules, foundation details, connection details). Both sets of documents are submitted to Building Control as part of the Full Plans application.

On more complex projects — basement conversions, large structural alterations, new builds — the structural engineer may be involved from the earliest design stages, working with the architect to develop a structural strategy before the detailed design is prepared. This early involvement is particularly important where the structural solution has a significant impact on the architectural design — for example, where a large open-plan space requires a complex steel frame.

TCM Building & Maintenance works with both architects and structural engineers as part of our integrated project team. For most domestic projects, we can recommend local professionals who we work with regularly and who understand our build methodology — which reduces the risk of coordination issues between the design team and the contractor.

Building work that requires structural calculations but proceeds without them is a serious risk. Building Control will not issue a Completion Certificate for work that doesn't comply with Building Regulations — and without a Completion Certificate, you'll have difficulty selling the property. A solicitor acting for a buyer will identify the missing certificate and require either a retrospective inspection (which may require opening up the works) or an indemnity insurance policy (which is increasingly difficult to obtain for structural work).

More seriously, structural work carried out without proper calculations can fail. A steel beam that's undersized for the span and load it's carrying will deflect excessively, causing cracking in the walls and ceiling above. In extreme cases, inadequate structural work can lead to partial or complete collapse. The consequences of getting structural work wrong are not just financial — they're potentially life-threatening.

The cost of a structural engineer's calculations (£800–£2,500 for a standard domestic project) is a small fraction of the total project cost and a negligible price for the certainty that the structure is safe and the Building Regulations are complied with.

TCM Building & Maintenance works with a network of chartered structural engineers across Hertfordshire. For every project that requires structural calculations — extensions, loft conversions, structural alterations — we coordinate the appointment of the structural engineer as part of our project management service.

For a client in Borehamwood who wanted to remove two load-bearing walls to create an open-plan ground floor, we appointed a structural engineer at the design stage — before the architect had finalised the drawings. The engineer's early input confirmed that the proposed steel frame was feasible within the existing floor-to-ceiling height, and identified a more efficient structural solution that reduced the steel tonnage and the cost. The project completed on programme and the Completion Certificate was issued without any queries from Building Control.

Our team includes in-house structural knowledge — our directors have managed projects requiring complex structural engineering, and we understand how to read and build from structural drawings. This means we can identify coordination issues between the architectural and structural drawings before they become problems on site, and we can have informed conversations with the structural engineer about buildability and programme.

If you're planning a project that involves structural work, read our Building Regulations Guide or contact us for a free feasibility assessment.