The short answer
It's often not a choice — they do different jobs and many projects need both. An architect designs the space: layout, light, proportion, how the building looks and works, and the drawings for planning. A structural engineer proves it will stand up: they size beams, design foundations and load paths, and produce the structural calculations Building Control needs. If your project is mainly structural — removing a load-bearing wall, inserting a steel beam, underpinning — you may only need an engineer. If it's about design and layout with little structural change, an architect (or technologist) may be enough. But for most extensions, loft conversions and new builds you'll want both: the architect to design it and the engineer to make the structure safe and compliant.
These two professionals are often confused, but they're not interchangeable. Understanding what each one does is the quickest way to work out who your project actually needs — and it's frequently both.
Architect vs engineer
- Architect designsSpace, layout, appearance
- Engineer designsStructure, beams, loads
- Architect forPlanning, design
- Engineer forCalculations, Building Regs
- Most projects needBoth
What each one actually does
The cleanest way to think about it: the architect decides what the space should be, and the engineer makes sure it stands up.
- An architect (a protected, ARB-registered title) handles concept and layout, how rooms relate, light and proportion, the look of the building, and the drawings for a planning application. Many also run the project through the RIBA Plan of Work.
- A structural engineer handles the bits that carry load — beam and lintel sizing, foundation design, load paths, and how alterations affect the building's stability. They produce the structural calculations that Building Control requires for any structural work.
An architectural technologist sits alongside both, focusing on the technical detailing and Building Regulations drawings. So the real question is usually "which of these does my specific job need?" — and for a lot of projects the answer is more than one.
Which does your project need?
Use the type of project as the guide. As a rule, the more structural change involved, the more certain you are to need an engineer; the more design and layout change, the more an architect helps.
| Project | Architect? | Structural engineer? |
|---|---|---|
| Remove a load-bearing wall | Optional | Yes |
| Loft conversion | Helpful | Yes |
| Single-storey extension | Helpful | Usually |
| Bespoke new build | Strongly | Yes |
| Internal redesign (non-structural) | Yes | No |
Indicative guidance only; every project differs. Sources: IStructE and HomeOwners Alliance guidance.
How they work together
On a typical extension or conversion, the two roles overlap in sequence rather than competing. A common flow is:
- The architect (or designer/technologist) develops the layout and produces drawings for planning permission.
- Once the design is settled, a structural engineer designs and calculates the beams, foundations and load paths for the chosen scheme.
- Both feed into the Building Regulations submission — the architect's/technologist's construction drawings plus the engineer's calculations.
Some architects bring a structural engineer in as part of their service, which can simplify coordination. The honest bottom line: don't frame it as architect versus engineer. Work out whether your job involves structural change (engineer), design and layout change (architect/technologist), or — as is usually the case — both, and engage accordingly. If you genuinely only need a beam sized and have no design questions, an engineer alone may be all you need; if you're reshaping space with structural work, you'll want the pair.
Frequently asked questions
Can a structural engineer design my extension?
A structural engineer designs the structure — beams, foundations and load paths — but doesn't typically handle layout, planning drawings or the look of the building. For the design side you'd usually use an architect or architectural technologist, with the engineer providing the calculations.
Do I always need both an architect and a structural engineer?
Not always. Purely structural jobs (like removing a load-bearing wall) may only need an engineer; purely design changes with no structural work may only need an architect or designer. But most extensions, lofts and new builds need both.
Who do I hire first, the architect or the engineer?
Usually the architect or designer first, to settle the layout and get planning sorted, then the structural engineer to design and calculate the structure for that agreed scheme. Some architects engage the engineer for you as part of their service.
Sources & further reading
- IStructE — when do I need a structural engineer
- RIBA — working with an architect
- HomeOwners Alliance — structural engineer guide
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.