When planning a home extension, loft conversion, or new build, a common question is: do I need a structural engineer, an architect, or both? These are different professionals with different roles, and understanding the distinction helps you get the right people involved at the right time — and avoid paying for services you don’t need.

What Does an Architect Do?

An architect is responsible for the design of a building — how it looks, how it flows, how it relates to its context, and how it meets your functional brief. Key architect responsibilities include:

• Feasibility studies and initial design concepts
• Planning drawings (floor plans, elevations, sections)
• Design and Access Statements for planning applications
• Building regulations drawings (co-ordinated with structural engineer)
• Interior layout and material specifications
• Contractor tendering and contract administration
• Site visits and project management

Only someone registered with the Architects Registration Board (ARB) can legally call themselves an “architect” in the UK.

What Does a Structural Engineer Do?

A structural engineer is responsible for ensuring a building is safe and structurally sound. They calculate loads, design structural elements, and specify the right materials. Key structural engineer responsibilities include:

• Structural calculations for beams, columns, and foundations
• Structural drawings showing steel or timber element sizes and connections
• Foundation design (especially for new builds and extensions)
• Structural surveys and reports (for purchase or pre-renovation)
• Party wall structural assessments
• Inspection and sign-off of structural elements during construction

Key Differences

When Do You Need an Architect?

• Planning applications — architects prepare the drawings and documents required
• Complex or bespoke designs — where the layout, aesthetics, and spatial quality matter
• Listed buildings and conservation areas — specialist knowledge is essential
• Full project management — if you want someone to oversee the build
• New builds — almost always require an architect

See our guide: Do I Need an Architect for a Loft Conversion UK?

When Do You Need a Structural Engineer?

• New structural openings — any time you remove a wall, you need a structural engineer to size the beam and pad stones
• Extensions — new foundations and structural frame need calculations
• Loft conversions — new floor joists, ridge beam, and steel connections need engineering
• Subsidence or cracking — structural inspection and report
• Party wall matters — structural engineer often needed alongside party wall surveyor
• Basement conversions — extensive structural work

When Do You Need Both?

For most significant home improvements — rear extensions, loft conversions, double storey extensions, and new builds — you will need both an architect and a structural engineer. They work together:

1. The architect designs the layout and prepares the concept and planning drawings
2. Once planning is approved, the architect and structural engineer collaborate on building regulations drawings
3. The structural engineer provides calculations for the building control submission
4. During construction, the structural engineer may inspect key structural stages

Can a Structural Engineer Replace an Architect?

For some straightforward projects — particularly if you just need structural calculations for a simple beam rather than a full design service — you might use a structural engineer without an architect. However, structural engineers are not trained in design, planning policy, or spatial layout, so this approach is limited to technically straightforward work.

Costs in the UK

• Architect fees (drawings and planning): £1,500–£4,000 for a typical extension
• Structural engineer fees: £500–£2,500 for a typical extension
• Combined total: £2,000–£6,500 for most residential projects

Read our full guide: How Much Does an Architect Cost UK 2025.

Frequently Asked Questions