Structural Engineer vs Architect for a London Extension — Who Do You Need and When?

What an Architect Does

An architect is a regulated professional whose role centres on design, planning compliance, and coordinating the design team. To call yourself an architect in the UK, you must be registered with the Architects Registration Board (ARB) and hold a recognised qualification.

In the context of a London extension or loft conversion, an architect will:

• Design the extension — creating a functional, attractive addition that suits your home, complies with planning policy, and respects the local character
• Produce planning drawings — these show the proposal from the outside, including elevations, sections, and location plans, prepared to the standard required by planning authorities
• Handle planning liaison — submitting the application, addressing officer comments, negotiating with the council on design issues
• Coordinate the design team — briefing the structural engineer, ensuring their calculations align with the architecture, managing the brief with mechanical and electrical consultants if needed
• Produce Building Regulations drawings — detailed plans showing how the building will comply with Building Regulations, including wall sections, construction details, and thermal performance schedules
• Act as contract administrator — if you employ them on a full contract, they will visit site, check quality of work, certify progress payments to your builder

Architects are used to working with planning authorities and can be crucial in negotiating approvals for visible additions or sensitive locations. They understand design policy in conservation areas, local precedent, and what councils expect to see.

What a Structural Engineer Does

A structural engineer is a qualified professional (typically chartered, CEng or IEng with ISTRUCTE or similar body) who calculates the strength and stability of the building structure. Structural engineers may be civil engineers with structural specialism or dedicated structural designers.

For a London extension or loft conversion, a structural engineer will:

• Assess the existing building — carrying out a site survey to understand the current structure, foundation depth, soil conditions, and any defects
• Calculate structural loads — determining the weight of the new extension (roof, floors, walls, finishes, live load) and any imposed loads
• Design the structure — sizing beams, columns, foundations, and connections to safely carry those loads to the ground
• Produce structural calculations — detailed mathematical proof that the design meets the relevant structural standard (currently Eurocodes under Building Regulations)
• Produce construction details — drawings showing how beams are to be installed, connection details, rebar placement, and sequencing of works
• Manage Building Regulations — Design Stage — working with the architect to ensure structural solutions integrate with the planned details and finishes
• Party wall matters — in some cases, advising on party wall implications, though formal party wall awards are usually handled by a separate party wall surveyor

A structural engineer's calculations are the evidence that your extension or loft conversion will not collapse, will not cause damage to the existing building or neighbour's property, and meets Building Regulations Standards 1, 2A, and 2B (loading and stability).

When You Need Both — and Why

For any significant London extension — rear extensions, side extensions, loft conversions, new storeys — you need both an architect and a structural engineer. Here's why:

• Planning approval requires architectural design — the planning authority will assess the visual impact, scale, proportions, material, fenestration pattern, and relationship to the street scene. A structural engineer cannot prepare these drawings to planning standard.
• Building Regulations approval requires structural calculations — the Building Control officer must see evidence that the structure is stable and safe. An architect cannot produce structural calculations to the required standard.
• They work together in detailed design — the architect's detailed sections must align with the structural engineer's beam and foundation designs. Coordination is essential to avoid clashes on site.
• Risk allocation — the architect is responsible for design quality, planning compliance, and overall coordination. The structural engineer is responsible for structural safety and calculations. Clear separation of responsibility is important.

Planning Application Drawings

Planning authorities require drawings that show the proposed design clearly, in relation to existing features and the surrounding streetscape. These drawings are prepared to a defined standard: 1:100 or 1:200 scale for elevations and plans, 1:500 for location plans.

Planning drawings typically include:

• Location plan (1:500, showing property in context of surrounding roads)
• Existing block plan (1:200, showing site boundaries, existing building footprint, trees, access)
• Proposed block plan (1:200, showing new addition and relationship to boundaries, trees, driveways)
• Existing and proposed ground floor plans (showing room layouts, dimensions, internal/external areas)
• Existing and proposed elevations (all four sides, showing new addition in context, materials, fenestration, roof lines)
• Sections (showing new addition in relation to existing building, garden levels, neighbouring properties)
• Materials schedule (describing proposed finishes — brickwork, tile, glass, etc.)

An architect will prepare these to planning submission standard. A structural engineer's drawings are too technical and detailed for planning purposes — they focus on internal structural arrangements, not on design quality and visual impact assessment.

Building Regulations and Structural Calculations

Once planning approval is granted, you must then obtain Building Regulations approval. This is a separate legal process from planning. Building Regulations require that the extension meets minimum standards for structural stability, fire safety, thermal performance, acoustics, and electrical safety.

For structural compliance, Building Regulations require:

• Structural calculations — calculated to Eurocode standards (BS EN standards), proving that beams, columns, foundations, and connections will safely carry the loads
• Construction details — drawings showing how the structure will be built, including rebar placement, concrete specification, beam bearing lengths, and connection details
• Specification — written description of materials, quality standards, and inspection requirements
• Inspection and testing schedule — identifying key structural points that must be inspected during construction before being covered up

A structural engineer's calculations demonstrate that the extension will not collapse, will not cause unacceptable settlement or cracking, and meets the Eurocodes. Building Control officers will check these calculations and may request revisions if they consider the design inadequate.

CDM and Principal Designer

Under the Construction (Design and Management) Regulations 2015 (CDM), any construction project lasting more than 30 days or involving more than 500 person-days of work must appoint a Principal Designer — a health and safety role focused on eliminating hazards during construction.

For most London extensions, the architect is appointed as Principal Designer. They must ensure that the design avoids or reduces hazards, and that information is coordinated between the architect and structural engineer so that the construction team has clear, hazard-aware instructions.

The Principal Designer will prepare a Health and Safety File, which contains information about residual hazards (e.g., location of loadbearing walls, electrical circuits, asbestos survey results) that the construction team and later occupants need to know.

Party Wall Surveying — A Separate Role

Under the Party Wall etc. Act 1996, if your extension is within 6 metres of a neighbour's building and goes deeper than 3.65 metres, or if you're cutting into a party wall, you must serve party wall notice and appoint an independent surveyor (or, in some cases, allow the neighbour to appoint one too).

The surveyor's job is to protect the neighbour's property, agree the method of work, and resolve disputes. This is separate from structural engineering — the party wall surveyor may have a surveying or structural background, but their role is legal/procedural, not technical design.

Party wall notices are often served simultaneously with the planning application, so the process runs in parallel. Failure to serve notice can result in injunctions and costly disputes.

Smaller Projects — Designers and Technicians

On smaller projects — single-storey rear extensions under 5 metres depth, simple internal works, garage conversions — some London homeowners use:

• Architectural technicians — qualified to produce technical drawings (RIBA Part III equivalent), but not registered as architects. They can produce planning drawings and Building Regulations details, but cannot act as Principal Designer.
• Structural technicians or technologists — qualified to produce routine structural calculations, often under the supervision of a Chartered Engineer. They can produce straightforward structural details for standard extensions.
• Package designers or package consultants — sometimes offered by specialist extension or loft conversion companies, who package the design, planning, and Building Regulations documents as a complete service.

This approach can reduce cost, but relies on the designer/technician having sufficient experience and professional indemnity insurance. If the project is straightforward (e.g., a standard single-storey extension with no complex structural issues), a competent technician may be adequate. If there are complexities — narrow party walls, shallow foundations, proximity to listed buildings or conservation areas — an architect and engineer are advisable.

Do You Always Need an Architect?

For a full-service relationship where the architect designs, takes the project through planning, coordinates Building Regulations, and supervises construction — yes, you typically need an architect for any significant extension.

However, on simpler projects or where you have less budget, you might:

• Engage an architectural technician instead of a full architect — they can produce drawings and Building Regulations submissions at lower cost, but lack the regulatory standing and design experience of an architect
• Use a builder-supplied designer or engineer — many extension and loft conversion specialists employ in-house designers who can produce plans and Building Regulations submissions, though they may lack independence and design flair
• Employ the architect only for planning, then use a technician for Building Regulations details — this separates the design/planning phase from the technical delivery phase

Planning authorities do not require an architect signature — they accept drawings from any competent professional. However, the quality of planning drawings, and your chances of approval, are usually better with an experienced architect who understands planning policy and local precedent.