Structural Engineer vs Architect — When Do You Need Each? UK Guide 2025

Planning a house extension, loft conversion, or renovation? Two professionals you are likely to encounter are the architect and the structural engineer. Their roles are distinct but complementary — and on most significant building projects, you will need both.

This guide explains exactly what each professional does, when you need them, how they work together, and what it costs.

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What Does an Architect Do?

An architect is a creative and technical professional responsible for the design of buildings and spaces. Architects are registered with the Architects Registration Board (ARB) — the title “architect” is legally protected in the UK under the Architects Act 1997.

For a residential extension or loft conversion, an architect typically:

• Understands your brief — What spaces do you need? How do you want to live? What is your budget?
• Designs the layout — Plans, sections, and elevations showing how the new space is organised
• Manages planning — Prepares and submits planning applications, negotiates with planning officers, and manages the process to approval
• Produces building regulations drawings — Detailed technical drawings showing how the building meets Part A (structure), Part B (fire), Part L (energy), and other requirements
• Specifies materials — What brickwork, windows, roof finishes, and insulation will be used
• Procures contractors — Manages the tender process to find the right builder at the right price
• Oversees construction — Visits the site, inspects works in progress, certifies payments, and manages variations

An architect is primarily responsible for what the building looks like and how it works — the design, spatial quality, and user experience.

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What Does a Structural Engineer Do?

A structural engineer is a technical specialist responsible for how the building stands up. They are concerned with forces, loads, and structural integrity — ensuring that what the architect has designed can be safely built and will perform safely throughout its life.

For a residential extension or loft conversion, a structural engineer typically:

• Assesses existing structure — Investigates the existing foundations, walls, and roof structure to understand what is there and its capacity
• Designs foundations — Specifies the type, depth, and dimensions of foundations for a new extension
• Designs steel beams and columns — Where walls are removed or new openings are created, the structural engineer calculates the required steel sections and bearing conditions
• Checks floor and roof structures — Specifies new joists, rafters, or engineered beams to carry the required loads safely
• Produces structural calculations — A formal set of calculations submitted to building control alongside the architectural drawings
• Issues construction details — Drawings showing how structural elements are to be built, including bearing lengths, connection details, and pad stone requirements

A structural engineer is primarily responsible for whether the building is safe — ensuring it will not collapse, deflect excessively, or fail under the loads it is subjected to.

!Structural engineer reviewing plans

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The Key Differences

	Architect	Structural Engineer
Primary focus	Design, function, planning	Structural safety and integrity
Regulated title	Yes (ARB)	Yes (CEng, IStructE)
Responsible for	What it looks like and how it works	Whether it will stand up safely
Typical involvement	Design through to completion	Design stage and technical approval
Building control role	Produces architectural drawings	Produces structural calculations
Planning role	Leads planning applications	No direct planning role

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When Do You Need an Architect?

You need an architect (or at minimum an architectural technologist) when:

• You want a thoughtful, well-designed layout rather than a basic addition
• You need planning permission and want the best chance of approval
• You need building regulations drawings
• You want independent professional oversight during construction
• Your project is complex, constrained, or in a conservation area

For most projects over 15m2, professional architectural input is strongly recommended — not just for planning and building regulations, but for the quality of the design outcome.

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When Do You Need a Structural Engineer?

You need a structural engineer when your project involves:

• New foundations — Any extension requires new foundations. The structural engineer specifies the type and depth
• Removing or modifying load-bearing walls — An engineer calculates the required steel beam and ensures the wall above is safely supported
• Creating large openings — New bifold doors, wide window openings, or RSJ installations all need structural calculations
• Loft conversions — New floor joists and ridge beam or structural ridge require engineering design
• Flat roofs — Structural calculation for the flat roof deck and its supports
• Basement conversions — Complex structural engineering for underpinning and tanking
• Side return extensions — Typically involves removing the existing flank wall and installing steelwork

On virtually every extension, loft conversion, or structural alteration, a structural engineer’s input is required for building regulations compliance.

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How Do Architects and Structural Engineers Work Together?

On most residential projects, the architect leads the design and coordinates the structural engineer’s input:

1. Architect designs the layout — Proposes wall removals, new openings, extension structure
2. Architect appoints or introduces structural engineer — Often from their network of trusted collaborators
3. Structural engineer reviews the design — Advises on feasibility, foundation type, steel requirements
4. Architect incorporates structural requirements into drawings — The architectural drawings reference the structural engineer’s details
5. Both submit documents to building control — Architectural drawings plus structural engineer’s calculations together form the building regulations package
6. Structural engineer may visit site — To inspect critical structural elements such as foundations or steel erection

The relationship is collaborative, not competitive. Crown Architecture works with a trusted panel of structural engineers across London to deliver coordinated design and structural packages.

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Do You Need Both on a Simple Extension?

For most extensions — even simple ones — yes. Even a straightforward single-storey rear extension involves:

• New foundations (structural engineer specification required)
• New load path from the roof of the extension (structural engineer specifies joists and wall plates)
• Potentially a new steel beam if the internal wall is removed
• All of this must be submitted to building control with structural calculations

For a very small addition (e.g., a porch or minor internal alteration), structural calculations may not be required. Your architect will advise.

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How Much Does a Structural Engineer Cost?

For residential projects in the UK, structural engineer fees typically range from:

Project Type	Typical Fee
Simple single-storey extension	£800–£1,500
Two-storey extension	£1,200–£2,500
Loft conversion	£1,000–£2,000
Wall removal (single beam calculation)	£400–£800
Basement conversion	£2,500–£6,000+

These are approximate. Always get a fixed fee quotation before appointing.

See also: House extension services by Crown Architecture | Loft conversion services

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Crown Architecture: Coordinating Your Design and Structure

Crown Architecture manages the full design and technical process for residential projects across London, coordinating architectural design, planning, structural engineering, and building regulations into a seamless package.

We work with a trusted panel of structural engineers and can introduce you to the right specialist for your project — with all structural inputs coordinated into our building regulations drawings.

Call or Text: 07443804841 to discuss your project today.

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FAQ: Structural Engineer vs Architect

What is the difference between a structural engineer and an architect?

An architect designs the building — layout, planning, appearance, function. A structural engineer ensures the building is safe — foundation design, steelwork calculations, load capacity. Most significant building projects require both.

Do I need a structural engineer for a house extension?

Yes — in almost all cases. New foundations and any structural alterations (wall removals, new openings) require a structural engineer’s calculations for building regulations approval.

Can an architect do structural calculations?

No — structural calculations must be produced by a qualified structural engineer. Architects and structural engineers have different training and professional responsibilities.

Who do I appoint first — architect or structural engineer?

Appoint the architect first. They will design the project and then appoint or introduce a structural engineer to provide input on the structural elements. The architect coordinates the two packages for building regulations.

How much does a structural engineer cost for an extension?

Typically £800–£2,500 for a residential extension or loft conversion. Wall removal calculations are usually £400–£800. Always get a fixed fee before appointing.

Is a structural engineer required for a loft conversion?

Yes. A loft conversion requires new floor structure calculations, ridge beam or structural ridge design, and potentially new foundation assessment. A structural engineer’s calculations are mandatory for building regulations.

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