Most people appoint a structural engineer once, for one project, and reasonably want to know whether the appointment is genuinely necessary before they make it. This guide explains what structural engineers do, the situations that normally require engineering input, the situations that usually do not, and how the roles of architect, builder and structural engineer differ in a typical UK project.
What a structural engineer actually does
A structural engineer is concerned with how a building carries load and remains stable. That covers the weight of the structure itself, the imposed loads from people, furniture, storage, plant and vehicles, and environmental actions such as wind and snow. The engineer establishes how those loads travel through the building down to the ground, and confirms that every element along that path has adequate strength, stiffness and stability.
In practice the output is usually a set of structural calculations, supported by sketches or drawings that describe new beams, columns, connections, bearings and foundations. On existing buildings the work often also involves appraisal: understanding what is already there, what it is likely to be capable of carrying, and what may need to change.
A structural engineer is not a general building consultant. They are not responsible for the appearance of the space, the specification of finishes, the planning application, or the commercial management of the contract, although they routinely coordinate with the people who are.
Common residential situations
The large majority of residential enquiries fall into a small number of recognisable categories. If your project appears in the list below, structural engineering input is likely to be required.
- Removing or reducing a wall, or forming a new opening in one, where the wall may be carrying load
- Single or two-storey extensions, including any new openings formed in the existing rear or side wall
- Loft conversions, where new floor loading, new dormers and altered roof structure are introduced together
- Basement works, lowering floor levels, or underpinning
- Chimney breast removal, where the masonry above must be supported
- New openings for bi-fold doors, roof lights, or large glazed elements
- Garage conversions where the existing foundations and lintels were designed for a lighter use
Alterations to existing buildings
Alterations are usually more demanding than new construction, because the starting point is uncertain. Record drawings may not exist, previous alterations may not have been recorded, and the construction may differ from what is visible. The engineer's role therefore includes establishing a reasonable understanding of the existing structure before designing anything new.
This is why an engineer will ask about the age and construction of the building, the direction the floors span, what sits above the area being altered, and whether any previous structural work has been carried out. Those questions are not administrative — they determine the load path, and therefore the design.
New construction
For new build work the structural scope is usually clearer: foundations, ground floor construction, superstructure framing, floors, roof, and lateral stability. Even on small projects, the ground conditions, the proximity of trees and drainage, and the intended use of the building will influence the design.
On commercial and industrial new build, the engineer is typically engaged earlier, because the structural grid, frame type and foundation strategy influence cost, programme and buildability more than almost any other decision on the project.
Cracking and structural concerns
Cracking, sloping floors, sticking doors, deflecting lintels and damp-related deterioration are all reasons people contact an engineer. In these cases the appointment is for appraisal rather than design: an inspection and assessment intended to establish whether there is a structural issue, how significant it is, and what should happen next.
It is worth noting that a great deal of cracking is not structurally significant. An appraisal that concludes no structural work is required is still a useful outcome — particularly when a property is being bought, sold or let.
Commercial and industrial projects
Commercial and industrial projects generally involve engineering input as a matter of course, because loading, plant, access, operational continuity and statutory duties all interact. Typical triggers include change of use, new mezzanines, roof-mounted plant, heavier production equipment, new openings in structural frames, and any works taking place while the facility remains in operation.
On these projects the engineer's involvement often extends into the construction stage, covering temporary stability, sequencing and coordination with other disciplines.
Structural calculations and Building Regulations
In England and Wales, structural safety is addressed by Part A of the Building Regulations (Part D in Northern Ireland; Section 1 of the Scottish Technical Handbooks). Where notifiable building work affects the structure, the building control body — whether a local authority or an approved inspector — will normally expect supporting structural calculations prepared by a competent person.
Planning permission and Building Regulations approval are separate processes. Planning is concerned with the principle, appearance and impact of the development; Building Regulations are concerned with how it is built. A project can require one, both, or neither, and structural calculations relate to the second of these, not the first.
Calculations also have a long life. They are frequently requested during a later sale, remortgage or insurance enquiry, so they are worth keeping with the property records alongside the completion certificate.
Architect, builder and structural engineer: different roles
An experienced builder may know from practice roughly what section size is normally used in a given situation. That is not the same as a design: it does not account for the actual loads in your building, and it will not usually satisfy building control.
- Architect or architectural technologist — develops the design, prepares planning and construction drawings, and coordinates the overall proposal.
- Structural engineer — establishes the load paths, designs the structural elements, and produces the calculations that demonstrate the structure is adequate.
- Builder or contractor — prices and constructs the work, controls temporary stability on site, and raises buildability queries.
- Building control — checks that the completed work complies with the Building Regulations; it does not design the structure on your behalf.
Information worth preparing before you enquire
The quality of an initial response is directly related to the quality of the information supplied. Even a handful of clear photographs and a dimensioned sketch will usually allow an engineer to give a meaningful view on scope and cost.
- A short written description of what you are planning and why
- Existing and proposed drawings, if any have been prepared
- Photographs of the affected areas, inside and out, including any cracking
- Approximate dimensions — spans, wall thicknesses, storey heights and opening sizes
- The age and construction type of the building, as far as you know it
- Any previous reports, calculations or survey information
- Your programme, and whether the work is being priced, tendered or already on site
The typical BES process
- Initial enquiry, with a review of whatever drawings and photographs are available
- A fee proposal setting out scope, deliverables and programme
- Site visit or desktop appraisal, depending on the complexity and the information available
- Load take-down and structural design of the affected elements
- Issue of calculations and sketches suitable for building control and for pricing
- Support during construction, including responses to contractor queries
Key considerations
- If the strength, stability or load path of a building is affected, engineering input is almost certainly required.
- A wall's structural role cannot be reliably established from its appearance alone.
- Planning permission and Building Regulations approval are separate; calculations relate to the latter.
- Good preparatory information materially improves the speed and accuracy of any initial advice.
- Calculations are frequently requested years later during sale or remortgage — keep them with the property records.
Related BES tools and services
Related guidance
Unsure what engineering input your project needs?
General guidance only goes so far. Describe your building and what you are planning, and you will receive project-specific advice from a Chartered Structural Engineer.
Important limitations
This publication provides general information only and does not constitute structural engineering advice, design or assessment. Project-specific engineering requirements should be established by an appropriately qualified structural engineer.
BES-TG-001 · Version 1.0 · Reviewed August 2026 · Next scheduled review August 2027