What Do Structural Calculations Show?
Structural calculations are mathematical assessments that demonstrate a structural element — a beam, a wall, a floor, a foundation — is strong enough to carry the loads placed on it safely.
They are typically produced by a Chartered Structural Engineer (MIStructE or CEng) and submitted to Building Control as part of a Building Regulations application.
When Are Structural Calculations Required?
Steel Beams and Structural Openings
Whenever a load-bearing wall is removed or altered — for example, to create an open-plan kitchen or to install an opening between rooms — a steel or timber beam must be designed to carry the loads above.
Calculations are needed to specify:
- The beam size and grade (e.g. 203x133 UC or 254x146 UB)
- The bearing length and padstones required at each end
- Any temporary support requirements during installation
- The capacity of any existing masonry below
Loft Conversions
A loft conversion typically requires structural calculations for:
- New structural floor joists (replacing the existing ceiling joists)
- Any ridge beam or collar tie modifications
- Steel beams where existing walls are to carry increased load
- Structural checking of existing walls if a heavy dormer is proposed
Extensions
An extension requires structural calculations for:
- New foundations (pad or strip, depending on soil conditions)
- Lintels over openings (windows, doors, roof lanterns)
- Flat roof or pitched roof structure
- Any steel frames required to span large openings
Basement Conversions
Basement work requires the most complex structural input:
- Underpinning design (the method and sequence of work)
- Retaining wall design and waterproofing specification
- Temporary propping calculations during excavation
Other Structural Alterations
Any work that affects the load path through a building may require engineering input:
- Chimney breast removals (the stack above must be supported)
- Removing internal walls (as above)
- Adding a large roof lantern or skylight
- Installing underfloor heating in concrete slabs
When Are Calculations NOT Usually Required?
For simple works that do not affect structure — surface-level refurbishment, kitchen or bathroom refits, new electrics or plumbing, redecorating — structural calculations are not needed.
Standard lintels over small openings (windows up to 1200mm wide) can sometimes be specified from manufacturer tables rather than bespoke calculations, though Building Control will confirm.
The Building Control Process
Building Control (either via the local authority or an Approved Inspector) reviews structural calculations as part of the Full Plans Building Regulations application. They may:
- Accept the calculations as submitted
- Raise queries and ask for clarification
- Request peer review of the calculations in complex cases
Once accepted, the calculations form part of the approved documentation for the project. Any changes to the structural elements must be re-approved.
Choosing a Structural Engineer
For residential projects in London, a typical structural engineer's fee for an extension or loft conversion ranges from £500 to £2,000 depending on complexity. Look for:
- Membership of the Institution of Structural Engineers (IStructE)
- Chartered Engineer (CEng) status
- Experience with residential projects in London
- Professional Indemnity Insurance
Your architect or design team will often have a preferred structural engineer they work with regularly. This simplifies coordination between architect drawings and engineer calculations.
What to Expect From the Process
- Architect produces design drawings
- Structural engineer reviews the design and carries out site visit if needed
- Engineer produces preliminary structural scheme (beam positions, foundation type)
- Engineer produces formal calculations and issues to Building Control
- Building Control accepts or queries
- Engineer responds to queries; calculations approved
- Contractor can proceed based on approved calculations
RCB Design & Build
RCB coordinates structural engineering input as part of the pre-construction process on all relevant projects. We work with experienced structural engineers to ensure calculations are produced, submitted, and approved before work begins — not after it has started.