If you're planning a house extension — whether that's a single-storey rear addition, a double-storey side extension, or a loft conversion — one of the first questions you'll face is whether you need a structural engineer. The short answer is almost certainly yes. Here's why, and what's involved.
Why do extensions need a structural engineer?
Most extensions involve structural work of some kind — new foundations, a new roof structure, or alterations to existing load-bearing walls and beams. Building Control (part of your local council) requires that these structural elements are designed to a safe standard, and that's proven through structural calculations produced by a qualified engineer.
Without approved structural calculations, Building Control won't sign off your extension. Without sign-off, you'll have problems when you come to sell the property — solicitors and mortgage lenders will ask for evidence of compliance.
What does a structural engineer do for an extension?
When you instruct a structural engineer for an extension project, they typically:
- Review your architect's or designer's drawings
- Visit the site to assess the existing structure and ground conditions
- Determine the appropriate foundation type and depth
- Calculate beam sizes for any new openings or load-bearing alterations
- Specify padstones, lintels, and structural connections
- Produce structural drawings and a full set of calculations
- Liaise with Building Control as required
The calculations and drawings are then submitted as part of your Building Regulations application — either by you, your architect, or directly by the engineer.
What about loft conversions?
Loft conversions almost always require a structural engineer. The existing roof structure typically needs to be altered or strengthened, new floor joists are needed to create a habitable floor, and the additional load has to be transferred safely down through the building to the foundations.
An engineer will assess whether your existing walls and foundations can cope with the extra load, and specify the steel beams (RSJs), timber, and connections required to make it work safely.
💡 Even if your builder says they "know what size beam to use," Building Control will still require certified structural calculations. Don't skip this step — it protects you legally and structurally.
Do I need planning permission as well?
Structural calculations and planning permission are separate things. Many extensions fall under Permitted Development — meaning you don't need planning permission — but you always need Building Regulations approval, which is where the structural engineer's calculations come in.
Your architect or planning consultant can advise on whether your extension needs planning permission. Your structural engineer handles the Building Regulations side.
How long do structural calculations take?
For a typical single or double-storey residential extension, most structural engineers can turn around calculations within 1–2 weeks of receiving the architect's drawings. More complex projects take longer. At SurveyCORE we aim to confirm timescales clearly at the outset so there are no delays to your project programme.
Can I use the same firm for the survey and the structural work?
Yes — and it's often more efficient to do so. If you're buying a property with extension potential and want a survey first, SurveyCORE can carry out both the building survey and the subsequent structural engineering work. One point of contact, consistent advice throughout.
📋 Planning an extension in South East London? Get a fixed-fee quote for structural calculations from SurveyCORE — covering Bromley, Beckenham, Sidcup, Croydon, Dulwich, Bermondsey and beyond.