Structural Engineer vs Architect: When Do You Need Which?
You're planning a project — removing a wall, converting your loft, or building an extension — and two professionals keep coming up: structural engineer and architect. Most homeowners treat these roles as interchangeable, or assume one covers what the other does. They don't. Commissioning the wrong professional wastes money and delays your project; missing one entirely can invalidate your building regulations approval or create a safety risk. Here is exactly when you need each, what they charge, and what they will hand over.
What Each Professional Actually Does
An architect is a designer. Their job is to translate what you want into drawings — plans, elevations, sections, and specifications — that communicate the vision to planners, building control, and contractors. They understand space, light, proportion, and planning policy. In England and Wales, only someone registered with the Architects Registration Board (ARB) can call themselves an architect. Many architects also hold RIBA (Royal Institute of British Architects) chartered status, which signals a higher level of professional training and is worth looking for.
A structural engineer is a mathematician and physicist applied to buildings. Their job is to prove that a structure is safe: that beams are sized correctly, that foundations can carry the loads imposed on them, and that removing a wall will not cause a collapse. Structural engineers hold qualifications from the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Look for MIStructE, FIStructE, or MICE after their name.
The two professions overlap in project coordination but not in technical scope. An architect cannot sign off structural calculations, and a structural engineer does not produce planning drawings.
When You Need a Structural Engineer Only
Some projects are technically straightforward but structurally significant. In these cases, an architect adds no value — you need calculations, not design.
Removing a load-bearing wall. Every internal wall removal in a house requires a structural engineer to calculate the size of the steel or timber beam (RSJ, universal beam, or glulam) that will carry the load above. The engineer produces a calculation pack and a simple annotated drawing showing beam size, bearing lengths, and padstone specifications. Building control will not sign off the work without it.
Loft conversion steelwork. Even a basic Velux loft conversion requires new floor beams to carry the new floor load and, in most cases, a steel ridge beam. The structural engineer calculates beam sizes, connection details, and any changes to the existing roof structure.
Underpinning and subsidence investigation. If a RICS Level 3 building survey flags suspected subsidence or structural movement, the surveyor will recommend a structural engineer — not an architect — to investigate and specify remediation. This is important: a RICS Level 3 survey is a condition report, not a structural engineering report. The two are different documents prepared by different professionals for different purposes.
New openings in external walls. Adding a window or door to a brick or stone wall requires structural calculations to size the lintel above and assess the impact on the wall below.
When You Need an Architect Only
If your project is primarily about obtaining planning permission or creating a design, you need an architect — not a structural engineer.
Planning applications. Local planning authorities require scale drawings — existing and proposed floor plans, elevations, and a site plan — as part of every householder or full planning application. A structural engineer does not produce these. An architect does.
Lawful Development Certificates. If you want formal confirmation that your proposed works fall within permitted development rights, you will need plans drawn up to the required standard. An architect produces these.
Design-led projects. If you want to maximise natural light in a kitchen extension, reconfigure a layout, or create something architecturally coherent, an architect earns their fee through design thinking that a structural engineer is not trained to provide.
When You Need Both
For most substantial projects — a single or double-storey extension, a full loft conversion, a basement conversion, or a significant internal reconfiguration — you need both professionals working together.
The typical sequence is: architect first (design, planning), then structural engineer (technical input for building regulations), then both coordinate on working drawings.
On larger projects, the architect may act as lead consultant and appoint the structural engineer directly as part of a design team. On smaller residential projects, the homeowner typically appoints each separately.
Typical Fees
| Service | Typical UK Cost (2025) |
|---|---|
| Structural engineer — single beam calculation | £200–£500 |
| Structural engineer — full loft conversion calcs | £600–£1,200 |
| Structural engineer — extension (foundations + steels) | £800–£2,500 |
| Structural engineer — subsidence investigation + report | £500–£1,500 |
| Architect — planning drawings only (householder extension) | £1,500–£4,000 |
| Architect — planning + working drawings + spec | £3,000–£6,000 |
| Architect — full service (RIBA stages 1–6), % of build cost | 5–15% |
| Architect — design only, large project (£200k+ build) | £10,000–£30,000+ |
Full architectural service fees of 5–15% of build cost are common on new builds and major projects. For smaller residential works, most architects offer a fixed fee for planning drawings, which is more predictable for homeowners.
How to Brief Each Professional
Briefing a structural engineer: Be specific. Tell them what wall you want to remove (or what beam you need), the floor loading above, the span, and whether this is for a building notice or full plans application. Send photographs of the element. Ask for a calculation pack suitable for Building Control submission, a marked-up drawing, and any padstone or bearing specifications.
Briefing an architect: The broader the brief, the more useful your architect can be. Share your wish list, your budget, how you currently use the space, and what is not working. A good architect will challenge your assumptions and offer options you had not considered. Ask specifically for: existing drawings (or measured survey), proposed planning drawings, and an indication of whether full working drawings will be needed for Building Control.
What Each Professional Delivers
A structural engineer's output is a calculation pack (typically 10–30 pages of load calculations, beam sizing, and foundation assessments) accompanied by annotated technical drawings (beam positions, sizes, connection details). This is submitted to Building Control as part of a Full Plans application or held on site during a Building Notice route.
An architect's output depends on the stage:
- Planning drawings: existing and proposed floor plans (1:100 or 1:50), elevations, site plan, location plan
- Working drawings: detailed construction drawings (1:20 or 1:10) showing how everything is built — junctions, insulation, finishes, door and window schedules
- Specification: a written document describing materials, standards, and workmanship requirements
RICS Level 3 Survey vs Structural Engineer Survey
One point of confusion worth clearing up: a RICS Level 3 Building Survey is not the same as a structural engineer's report. A RICS surveyor assesses condition and flags risks using a visual inspection. If they find evidence of structural movement, significant cracking, or subsidence, they will recommend further investigation by a structural engineer — they do not provide the investigation themselves. If you are buying a property with known structural issues, you will likely need both: the Level 3 survey as a condition overview, and a structural engineer's report for the specific defect.
For further reading on structural professionals and their roles:
- What Is a Structural Engineer? When Do You Need One in the UK? — a deeper look at structural engineer fees, the structural calculation pack, and the red flags that mean you need one immediately
- How to Hire an Architect in the UK — ARB registration explained, RIBA Plan of Work stages, fee structures, and red flags when briefing an architect
- Loft Conversion Types: Dormer vs Hip-to-Gable vs Mansard — the projects where you need both professionals working together, from Velux conversions to full mansard builds
- How to Find and Work with a Structural Engineer in the UK — how to locate an IStructE or ICE-registered engineer, what their report delivers, typical fees, and the six red flags to watch for