House Extension Structural Engineer: What You Need to Know
Building a house extension is one of the biggest investments you'll make in your home. Whether it's a rear extension, side return, or wraparound, you need a structural engineer to design the hidden structure that makes it safe and legal.
Here's everything London homeowners need to know about the structural engineering side of extensions.
Why Every Extension Needs a Structural Engineer
Even a single-storey rear extension under Permitted Development needs Building Regulations approval — and Building Control requires structural calculations.
Your structural engineer designs:
Foundations
The most critical part of any extension. Your engineer specifies:
- Foundation type — strip foundations, trench fill, or piled foundations depending on ground conditions
- Depth — typically 1m minimum, deeper if there are trees nearby (root protection zones)
- Width and reinforcement — calculated based on the load the extension places on the ground
- Ground bearing pressure — the soil's ability to support the structure
London properties face common foundation challenges: clay soils that shrink and swell, mature trees that extract moisture, and historic foundations on neighbouring properties.
Steel Beams
Where the extension meets your existing house, a large opening is usually formed. This needs:
- A steel beam (RSJ) spanning the full width of the opening
- Padstones where the beam bears onto the existing walls
- Connection details tying the new structure to the old
For wraparound extensions or those with bi-fold/sliding doors, there may be multiple beams at different levels.
Structural Walls & Roof
- Cavity wall design — insulation, damp proofing, and structural adequacy
- Roof structure — flat roof, pitched roof, or combination
- Wind bracing — especially for larger extensions
Drainage & Utilities
While not strictly structural, your engineer coordinates with the drainage design to ensure pipes and services don't undermine the structure.
Types of Extension and What's Involved
Rear Extension (Single Storey)
The most common extension in London. Typically extends 3–6 metres into the garden.
- Usually under Permitted Development (no planning needed up to 6m for terraced, 8m for detached under Prior Approval)
- Still needs Building Control approval
- Structural engineer designs: foundations, steel beam at junction with existing house, roof structure
Cost for structural calculations: £800 – £1,500
Side Return Extension
Popular with Victorian and Edwardian terraces that have a narrow passage along one side. Opens up the kitchen dramatically.
- Usually combined with a rear extension
- The side wall removal needs a steel beam
- Foundations often near the boundary — party wall considerations apply
Cost for structural calculations: £800 – £1,500
Wraparound Extension
Combines side and rear into an L-shaped or full-width extension. More complex structurally:
- Multiple steel beams at different positions
- Larger foundation footprint
- More complex roof structure
- Often involves removing the original rear wall entirely
Cost for structural calculations: £1,000 – £1,800
Double Storey Extension
Two floors stacked. Significantly more structural work:
- Heavier foundations (carrying twice the load)
- More complex steel connections
- First floor structure design
- Usually requires planning permission
Cost for structural calculations: £1,200 – £2,500
Basement Extension
The most complex and expensive option. Involves:
- Underpinning existing foundations
- Waterproof concrete (tanking) design
- Temporary works during construction
- Structural assessment of neighbouring properties
Cost for structural calculations: £2,000 – £4,000+
The Extension Process: Structural Engineer's Role
Step 1: Initial Consultation
Tell us about your project — what type of extension, how large, what you want to achieve. We'll give you a clear quote.
Step 2: Site Visit
We visit your property to:
- Measure the existing structure
- Assess the existing foundations and walls
- Check for trees, drains, and services
- Note any structural concerns
Step 3: Structural Design
We produce:
- Structural calculations — the engineering analysis proving every element is safe
- Structural drawings — detailed plans your builder works from
- Foundation design — including specification and depth
- Steel beam schedule — sizes, positions, and connection details
Step 4: Building Control Submission
We submit directly to your local Building Control or approved inspector. Our calculations are formatted to their requirements — we know what each London borough expects.
Step 5: Construction Support
During construction, we're available if your builder has questions or if site conditions differ from expectations. If Building Control raises queries, we handle them.
Timeline
| Stage | Duration | |---|---| | Structural calculations | 5–7 working days (SBS standard) | | Express service | 48 hours | | Building Control approval | 2–5 weeks (varies by borough) | | Construction | 8–16 weeks (depends on extension size) |
The structural engineer is rarely the bottleneck. At SBS, we deliver within one week as standard, keeping your project on track.
Do You Need an Architect Too?
For simple rear extensions under Permitted Development, you may not need an architect — your structural engineer and builder may be sufficient. For larger extensions needing planning permission, you'll typically need both.
At SBS, we offer both structural engineering and architectural design. Read our guide: Structural Engineer vs Architect →
Party Wall Considerations
If your extension is built on or near a boundary with a neighbour, the Party Wall etc. Act 1996 applies. You'll need to serve notice on your neighbours before work begins. This is separate from structural engineering and Building Control, but your structural engineer can advise on the requirements.
Common Mistakes to Avoid
- Starting without Building Control approval — your builder should not begin work until calculations are submitted. Retrospective approval is possible but more expensive and stressful.
- Choosing the cheapest quote — the cheapest structural engineer often delivers the slowest. Factor in turnaround time and quality.
- Ignoring drainage — building an extension over or near existing drains causes problems. Your engineer should check this early.
- Not considering trees — trees near the extension footprint affect foundation depth. London clay is particularly sensitive to moisture changes from tree roots.
- Forgetting about the existing house — the junction between old and new is where most structural issues occur. Proper beam design here is critical.
Why Choose SBS for Your Extension?
- Residential extension specialists — one of our most common project types
- 1-week turnaround as standard, 48-hour express available
- Fixed pricing — £800–£1,800 for most extensions, quoted upfront
- Same-day site visits available across London, Essex and Kent
- Building Control ready — formatted for immediate submission
- 70+ areas covered across London, Essex and Kent
Get a Free Quote
Planning an extension? Tell us about your project and get a clear, fixed-price quote:
