Structural Engineer — When Do You Need One for a Home Extension or Renovation?

A structural engineer is required whenever your project involves changes to elements of the building that carry load — walls, beams, columns, roofs, and foundations. The specific situations where structural engineer input is mandatory (or very strongly advisable) include:

**Wall removals and openings**: any wall removal or new opening through an existing wall requires assessment to determine whether the wall is load-bearing. If it is, a structural engineer must specify the steel beam (RSJ) required to support the load above. The Building Control inspector will not sign off on a structural opening without a structural engineer's specification.

**Loft conversions**: a loft conversion always requires structural engineer input. The existing ceiling joists are designed as ceiling joists (not floor joists) and must be upgraded to carry habitable floor loads. The structural engineer specifies the new floor joist arrangement, the ridge beam, and any purlin support requirements.

**House extensions**: all extensions require structural engineer input for the foundation specification and the structural connection to the existing house. Even a single-storey rear extension requires a foundation design and a specification for the structural opening from the extension into the existing house.

**Basement conversions and underpinning**: basements and underpinning are among the most structurally complex residential projects. The structural engineer specifies underpinning or retaining wall design, waterproofing, and the temporary propping sequence.

**Chimney breast removals**: removing a chimney breast (particularly at upper floor or lower floor level, where the stack continues above) requires structural engineer specification of the support system for the remaining stack above.

**Party wall works**: where works are close to or affect the party wall, the structural engineer's calculations are often required as part of the Party Wall Award documentation.

**Any project where Building Regulations apply**: virtually all projects requiring Building Regulations approval will require structural calculations as part of the Building Regulations submission.

A structural engineer's role on a residential project typically involves:

**Site visit**: inspection of the existing structure — walls, floors, roof, and foundations — to assess load paths, construction type, and current condition. For load-bearing wall removals and loft conversions, this usually takes 1–2 hours.

**Structural calculations**: the mathematical analysis of loads — dead loads (the weight of the building), imposed loads (furniture, occupants), and, for roofs, wind and snow loads. The calculations determine the required size and specification of any new structural elements (beams, columns, connections).

**Structural drawings**: a drawing (or set of drawings) showing the structural scheme — beam positions, dimensions, connections, bearing lengths, and temporary propping requirements. These drawings go to Building Control as part of the Building Regulations submission and go to the contractor for pricing and build.

**Written specification**: for beams (RSJ steel sections), the structural engineer specifies the exact section size, steel grade, bearing requirements, and any fire protection specification.

**Follow-up queries**: during construction, the structural engineer may be consulted when the contractor opens up the structure and finds conditions that differ from the survey (e.g., the existing floor structure is different from expected).

**Some engineers provide site inspections**: a structural engineer site inspection at key stages (e.g., when steels are installed) provides an additional quality check and is good practice for complex projects.

Structural engineering fees in London (2025):

**Single structural opening (load-bearing wall)**: £400–£900 Includes: site visit, calculations, beam specification drawing, and Building Regulations submission support.

**Loft conversion structural package**: £800–£2,000 Includes: floor joist design, ridge beam, purlin assessment, and structural drawings for Building Regulations.

**Single-storey rear extension**: £900–£2,000 Includes: foundation design, structural opening specification, and connection to existing structure.

**Double-storey extension**: £1,500–£4,000 Includes: all of the above plus first-floor structure, roof structure, and more extensive foundation assessment.

**Chimney breast removal**: £500–£1,200 Includes: support design for the remaining stack, calculation, and drawings.

**Basement conversion**: £3,000–£8,000+ Includes: underpinning or retaining wall design, waterproofing specification, temporary propping design, and often a site inspection programme.

• **What affects the fee**:
• •Complexity of the structural scheme
• •Whether intrusive investigation (opening up) is required
• •Volume of calculations required
• •Whether site inspections are included
• •Whether temporary works design is required

**Step 1: Appoint through your architect or contractor** In a design-and-build arrangement, the contractor often arranges the structural engineer directly and includes the cost in the pre-construction fee. If you have an architect, they will typically have a preferred structural engineer they work with regularly.

**Step 2: Provide access for a site survey** The engineer needs to visit the property to assess the existing structure. This usually takes 1–2 hours. They will inspect the floor, walls, loft space, and outside the building.

**Step 3: Allow 2–4 weeks for the structural package** Most residential structural engineers can produce calculations and drawings within 2–4 weeks of the site visit. Allow for this in your programme planning.

• **Step 4: Use the structural package for Building Regs and pricing**
• The structural engineer's output (calculations and drawings) goes to:
• •Building Control as part of the Full Plans application
• •The contractor as part of the final scope for pricing

**Step 5: Be available for queries during construction** When the contractor opens up a wall or floor during demolition, they may find conditions different from what was expected. The structural engineer should be available to respond to these queries quickly, as delays in structural sign-off hold up the programme.