The short answer
For most extensions, loft conversions and new builds, yes — you need both, because they design different things. The architect (or technologist/designer) handles the layout, space, appearance and planning drawings; the structural engineer handles the beams, foundations, load paths and the structural calculations Building Control requires. They're not duplicating each other — they cover separate halves of the same project, and they usually bill separately. You won't always need both: a purely structural job (like inserting a steel beam) may need only an engineer, while a non-structural redesign may need only an architect or designer. But where a project involves both reshaping space and structural change — which most do — the two roles work in sequence and both are needed.
It's a common worry that hiring both means paying twice for the same thing. It doesn't — they do genuinely different work. Here's how the two roles split a project and when you can get away with just one.
Two roles, one project
- Architect designsSpace, layout, planning
- Engineer designsStructure, calculations
- Most projects needBoth
- Engineer onlyBeam / wall removal
- BillingUsually separate
Why most projects need both
The reason the two roles coexist is that a building project has two distinct problems to solve, and each profession owns one of them:
- The design problem — how the space is laid out, how light enters, how it looks, and how to get planning permission. This is the architect's (or technologist's/designer's) work.
- The structural problem — what beams and foundations are needed, how loads travel, and proving it all stands up. This is the structural engineer's work, ending in the calculations Building Control needs.
On a typical extension or loft, you have both problems at once: you're reshaping space and making structural changes. That's why you need both — not because the work overlaps, but precisely because it doesn't. Trying to skip the engineer on a structural job leaves you without the calculations Building Regulations require; skipping the designer leaves you without a considered layout or a strong planning case.
How the work splits between them
On a standard project the two roles run in a logical order, with the design settled first and the structure designed to suit it.
| Task | Architect / designer | Structural engineer |
|---|---|---|
| Layout and space planning | Yes | No |
| Planning application drawings | Yes | No |
| Beam and lintel sizing | No | Yes |
| Foundation design | No | Yes |
| Building Regs submission | Drawings | Calculations |
Indicative split for guidance only; arrangements vary. Sources: RIBA and IStructE guidance.
When you only need one
Not every project needs the pair. The honest exceptions:
- Engineer only: a purely structural job with no design questions — removing a single load-bearing wall, inserting a steel beam, or underpinning — often needs just a structural engineer (plus Building Control). There's no layout to design.
- Architect/designer only: a non-structural change — reconfiguring rooms without touching load-bearing elements, or a redesign that adds no structural work — may need only an architect or designer.
For everything in between — which is most extensions, lofts and new builds — you'll want both. A practical tip on coordination and cost: some architects engage the structural engineer for you as part of their service, which keeps the two sets of drawings aligned and saves you managing it. The honest bottom line: needing both isn't double-paying — it's two specialists doing two different, necessary jobs. Work out whether your project involves structural change, design change, or both, and engage accordingly. And remember both feed into the same Building Regulations approval, ending with a completion certificate.
Frequently asked questions
Am I paying twice if I hire both an architect and an engineer?
No. They do different work — the architect designs the space and handles planning, the engineer designs the structure and produces calculations. The fees cover separate tasks and add together rather than overlapping, and each is responsible for their own part.
Can I hire just a structural engineer?
Yes, for a purely structural job with no design questions — like removing a load-bearing wall, inserting a steel beam, or underpinning. There's no layout to design, so an engineer plus Building Control may be all you need. Most extensions and lofts, though, need a designer too.
Does the architect arrange the structural engineer for me?
Often they can. Many architects engage a structural engineer as part of their service, which keeps the drawings and calculations coordinated and saves you managing two separate appointments. Confirm whether it's included or a separate cost when you agree the fee.
Sources & further reading
- IStructE — when do I need a structural engineer
- RIBA — working with an architect
- Planning Portal — Building Regulations
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.