Yes, almost every house extension needs structural calculations to comply with UK Building Regulations, regardless of size. The myths below are the ones that most often lead homeowners astray on cost, timing, and who actually does what.
Here’s what’s actually true when it comes to hiring a structural engineer for extensions in London.
Myth: My Architect Can Handle the Structural Side Too
Fact: Architects design layouts, spatial flow, and aesthetics. Structural engineers calculate whether load-bearing walls, beams, and foundations will actually hold up the building safely, a different discipline entirely. Both typically need to work together on any extension involving wall removal or a new structure. What varies is whether they’re part of the same team or separate consultants you have to coordinate yourself, and that coordination is often where projects lose the most time.
Myth: Structural Engineers Are Only Needed for Big Extensions
Fact: Even a modest single storey rear extension usually needs structural calculations, particularly if a wall is being removed to create an open plan space or a new opening is being formed for bi-fold doors. The size of the extension matters less than whether the structure is being altered. Removing a chimney breast, installing solar panels, or knocking through to extend a kitchen can all trigger the same requirement as a full extension, which surprises a lot of homeowners planning what they assume is a small job.
Myth: A Cheap Structural Engineer Is Just as Good as an Expensive One
Fact: Cost varies for good reason. A structural survey or site visit typically runs £250 to £400, structural calculations £300 to £800, and a full structural report £450 to £900, with London prices generally 10 to 20% higher than the national average. Choosing purely on price without checking accreditation, such as membership with the Institution of Structural Engineers or the Institution of Civil Engineers, is one of the more common mistakes that leads to costly revisions later, sometimes costing more in the end than the fee difference ever saved.
Myth: You Can Skip Insurance Checks If the Engineer Comes Recommended
Fact: A personal recommendation doesn’t replace confirming professional indemnity insurance. If a calculation error causes a problem after construction, insurance is what protects the homeowner from bearing that cost directly. This is a five minute check worth doing regardless of how the engineer was found, and a legitimate practice will never hesitate to provide proof of cover.
Myth: A Basement Extension Doesn’t Need Anything Different From a Rear Extension
Fact: Basement work involves underpinning, waterproofing, and excavation near existing foundations, structural considerations that go well beyond what a standard rear extension requires. Basement extension specialists work through soil conditions, groundwater, and the structural impact on neighbouring foundations as standard practice, since getting this wrong risks the stability of the existing building, not just the new space being created.
Myth: Hiring a Separate Structural Engineer Doesn’t Really Slow Things Down
Fact: In practice, it often does. When structural engineering sits with a separate consultant rather than the same team handling the architecture, every revision to the calculations becomes a back and forth between two parties instead of one conversation. That coordination step is one of the more common causes of delay on projects generally, often adding weeks that nobody budgeted for at the outset.
Myth: Once the Structural Report Is Done, the Engineer’s Job Is Finished
Fact: A structural report gets a project through building control on paper, but the engineer’s involvement often needs to continue once construction starts. Site conditions sometimes differ from what a desk based survey assumed, particularly on older properties, and unexpected findings during groundwork or wall removal can require the calculations to be revisited.
Myth: Planning Permission and Structural Sign Off Are the Same Process
Fact: They’re entirely separate. Planning permission concerns whether a proposal is acceptable in terms of scale, appearance, and impact on neighbours. Structural sign off concerns whether the building will actually be safe once altered, assessed under Building Regulations rather than planning policy. A project can have full planning permission and still fail building control if the structural calculations aren’t submitted and approved separately.
Myth: All Structural Engineers Offer the Same Level of Service
Fact: Some engineers provide calculations only, a document handed over with no further involvement. Others stay engaged through the design process, attend site visits during construction, and adjust calculations as the project develops. The difference matters most on properties with structural uncertainty, older buildings, unusual ground conditions, or work near a shared boundary.
What to Actually Check Before Hiring
- Confirm IStructE or ICE membership, not just years of general experience
- Ask for examples of similar projects, particularly if the work involves a basement or unusual structural conditions
- Request at least two to three quotes for comparison
- Confirm professional indemnity insurance is in place
- Ask directly whether structural calculations are handled in house or subcontracted separately
- Ask whether the engineer stays involved during construction or only delivers the initial report
Getting Structural Advice That’s Actually Joined Up
With 17 years of experience and over 1,800 planning approvals across London and Surrey, Extension Architecture keeps structural engineering in house alongside architectural design and building regulation drawings, a setup only around 1 in 20 firms genuinely offer. This means structural calculations, planning applications, and design decisions happen within the same conversation rather than being passed between separate consultants at each stage.
Speak to the team on 0203 409 4215, or a free consultation will cover what your specific project actually needs structurally before any drawings are commissioned.