For practical purposes the answer is yes. Almost every extension involves creating an opening in an existing external wall, and that means removing part of a load path and replacing it with something designed. Building Control will want calculations for that, and calculations are an engineer's work.
The more useful question is when, and the answer is earlier than most people think.
What actually needs designing
- The opening between the existing house and the new space - almost always a steel beam, with its bearings, padstones and any propping sequence
- Foundations for the new structure, sized for the ground and matched to the existing house so the two settle together
- The new roof structure, and how it connects to the existing roof
- Any new floor structure, including joist sizes, spans and support
- Lintels over new windows and doors
- Anything supported off the existing building, which means checking the existing structure can take it
- Temporary works - what gets propped, and in what order - which is the part that keeps people safe on site
Where the engineer sits between the architect and the builder
An architect or a designer produces the design: what the extension looks like, how the spaces work, and what goes where. That drives the planning application. An engineer makes it stand up: sizes the structure, produces the calculations, and provides the details that satisfy Building Regulations.
They are separate roles and the work overlaps at exactly one point - when the architectural design assumes something structural that turns out not to work. Bringing the engineer in while the layout is still movable is much cheaper than bringing them in afterwards, because a five-metre opening and a four-metre opening are the same drawing and quite different beams.
The sequence that saves money
- Agree what you want, and get the design far enough along to know the spans
- Planning permission if you need it - many extensions are permitted development, and planning and Building Regulations are separate systems
- Structural design and calculations, before the builder is on site
- Building Regulations submission - full plans, determined within five weeks or eight by written agreement, or a building notice for simpler work
- Build, with the engineer available for the queries that arise when the ground is open and the reality does not match the assumption
- Completion certificate. Get it. Its absence is one of the most common reasons a sale stalls years later.
The two mistakes we get called in to fix
Almost every remedial job on extension work traces back to one of two things.
The first is bearings. A beam that is strong enough still fails if it does not sit far enough onto the masonry either side, or if the padstone specified in the calculations was left out as a saving. Failures at bearings are more common than failures in the middle of a beam.
The second is foundations that differ from the existing house. An extension founded shallower, deeper or differently from the building it joins settles by a different amount, and a crack opens at the junction a year or two later. It is the single most characteristic extension defect, and on soft or made ground it is close to inevitable if the design ignores it.
What we need from you to quote
- The address, and roughly what the building is - age, construction, storeys
- A sketch or the architect's drawings, however early
- Photographs of the wall coming out, from both sides
- The spans: how wide the opening is, and how far the floor and roof above span
- Whether the house has been altered before, and whether there is paperwork
- Your deadline, if you have one
What it costs
Design work is quoted individually, because a single beam over a rear opening and a foundation design on made ground are not comparable jobs. You get a fixed figure in writing before anything starts.
Where an inspection is needed first - because the house has already been altered, or something has cracked - a structural survey and report is a fixed £580 + VAT with the report inside 10 working days of the inspection, guaranteed.
Frequently asked questions
Can my builder just work it out?
An experienced builder often has a good instinct for what size steel is needed, and that instinct is not a calculation. Building Control requires calculations from someone competent to produce them, and the liability for getting it wrong sits with whoever specified it. Most of the builders we work with prefer having the numbers behind them.
Do I need an architect as well?
Not necessarily. For a straightforward extension plenty of people go from a designer or the builder's drawings straight to an engineer. For anything where the layout, the light or the planning position is difficult, an architect earns their fee. The engineer is needed either way.
When should I bring the engineer in?
While the layout is still movable. A five-metre opening and a four-metre opening are the same drawing and quite different beams, and the structural implications of a layout are far cheaper to accommodate than to fix. Before the builder starts is the absolute latest.
Do I need planning permission?
Many extensions are permitted development and need none, but that is a separate question from Building Regulations, which structural work needs regardless. Getting planning permission does not get you Building Regulations approval, and a great deal of confusion comes from treating them as one system.
What if the ground turns out to be different from what we assumed?
It happens, and it is exactly why the engineer should still be available once the ground is open. Foundations are designed on the best information available at the time; if the trench shows something different, the design gets revisited before concrete goes in rather than after.