Your team and the process
Do you need a structural engineer for an extension or alteration?
Short answer
You need a structural engineer whenever the work changes how your house carries its weight down to the ground. The usual triggers are removing or altering a load-bearing wall, forming a new or wider opening, converting a loft, digging a basement, and building an extension on new foundations with beams where it opens into the house.
Structural alterations are building work under the Building Regulations 2010, and Part A of Schedule 1 requires the building to carry its loads to the ground safely. Building control will want to see how the new structure does that. For anything beyond the simple rules in Approved Document A, that means calculations, which is what the engineer provides.
What the law requires
The building shall be constructed so that the combined dead, imposed and wind loads are sustained and transmitted by it to the ground (a) safely; and (b) without causing such deflection or deformation of any part of the building, or such movement of the ground, as will impair the stability of any part of another building.
Requirement A2 adds ground movement: the building must be constructed so that swelling, shrinkage or freezing of the subsoil, and foreseeable land-slip or subsidence, will not impair its stability. Requirement A3 deals with disproportionate collapse.
The reason an internal alteration is caught at all is regulation 3. It makes a "material alteration" building work, and an alteration is material if it would at any stage leave the building not complying with a relevant requirement where it did before. Part A is one of the relevant requirements. Taking out a load-bearing wall without adequate support would do exactly that, so the work needs building control approval. See do you need Building Regulations approval.
Where Approved Document A stops
Approved Document A (2004 edition, incorporating the 2004, 2010 and 2013 amendments) is the government's guidance on meeting Part A. Section 2 gives prescriptive sizes for certain residential and small buildings of traditional construction: thickness of masonry walls, certain timber floor and roof members by reference to published span tables, masonry chimneys and plain concrete strip foundations. Under section 7 of the Building Act 1984, proof that you followed an approved document can be relied on as tending to show compliance.
Section 2 does not size a steel beam, design a basement, or tell you whether a particular wall can come out. Outside its rules, compliance is shown by design to the codes and standards listed in Section 1, which is the engineer's work.
The common triggers
| Work | What the engineer designs |
|---|---|
| Removing or altering a load-bearing wall | The beam, its bearings and padstones, and the sequence of temporary propping |
| A new or wider opening, for example into an extension | Lintels or beams, and the load they bring down on each side |
| Loft conversion | New floor joists, any steel beams, the framing of dormers, and a check that walls and foundations can take the added load |
| Basement | Underpinning, retaining walls and the temporary works to keep the house and the neighbours' supported |
| Extension | Foundations suited to the ground and any nearby trees, the roof structure, and the beams where the extension opens into the house |
| Cracking or movement | An inspection and a view on the cause and repair |
Basements and deep foundations near a neighbour also bring in the Party Wall etc. Act 1996; see the 3 metre and 6 metre rules.
What an engineer produces
For a typical extension or alteration the output is a set of structural calculations and drawings or sketches: member sizes and grades, bearings and padstones, connections, and foundation sizes and depths. Building control checks the calculations, with a full plans application or, under a building notice, when it asks for them. The builder builds from the drawings. On a basement or complex alteration the engineer may also set out the temporary works and the sequence of construction.
How the engineer fits with the designer
The designer sets the layout, the heights and the openings. The engineer sizes the structure that makes them stand up. The two have to agree: a beam must fit within the ceiling zone the designer has drawn, and a foundation must suit the drainage runs. On most home projects the designer coordinates the engineer's information into the drawings and specification that go to building control and to the builders who price the work.
The Building Regulations put a duty on each of them. Regulation 11K(4) says that where a designer carries out only part of the design, they must consider other design work that directly relates to it and report any concerns about compliance to the principal designer. An early view from the engineer, before the design is fixed, also narrows the cost risk; see contingency.
What this means for you
If the work touches the structure, appoint the engineer after the measured survey and before the Building Regulations application, and ask for a short early view at a point when the design can still change. Check that the fee covers the calculations, the drawings, answering building control's queries and any site visits you want. Published fee figures are in design and professional fees.
Sources
- Building Regulations 2010, Schedule 1, Part A legislation.gov.uk
- Building Regulations 2010, regulation 3 legislation.gov.uk
- Building Regulations 2010, regulation 11K legislation.gov.uk
- Approved Document A: structure, 2004 edition incorporating 2004, 2010 and 2013 amendments gov.uk
- Building Act 1984, section 7 legislation.gov.uk