Do I Need a Structural Engineer for a House Extension?
By Joseph Lawson MEng (Hons), Director / Structural Engineer, Lawson Cook Limited
If you are planning a house extension, you will often need a structural engineer as part of the design process.
An architect or architectural designer will usually develop the layout and appearance of the extension, while the structural engineer determines how the building will safely support the loads imposed upon it.
For many extensions, this includes designing the foundations, steel beams, loadbearing walls, roof structure and any structural alterations required where the new extension connects to the existing house.
When Is a Structural Engineer Required for an Extension?
Not every extension requires the same level of structural engineering input. However, a structural engineer will commonly be required where the proposed works involve:
Removing part of an existing external or loadbearing wall
Creating a large opening between the house and extension
Installing steel beams or columns
Constructing a new loadbearing wall
Designing new foundations
Supporting a new roof structure
Altering existing floor or roof structures
Creating large openings for bifold or sliding doors
Supporting masonry above new openings
Designing retaining walls
Constructing close to trees or on potentially shrinkable clay soils
Altering an existing structure where the load paths are not straightforward
The structural design will normally form part of the information submitted to Building Control.
What Does a Structural Engineer Design for a House Extension?
The precise scope depends on the proposed works and the existing property.
A typical structural design package for a residential extension may include the following.
Foundations
The engineer will consider the loads from the proposed extension and specify suitable foundations.
This may involve conventional strip or trench-fill foundations, although other solutions can sometimes be required depending on the building, ground conditions, nearby trees and site constraints.
Where trees are close to the proposed extension and the site is underlain by shrinkable clay, the required foundation depth can be influenced by the species, height and proximity of the vegetation.
Ground conditions should therefore be considered as part of the foundation design rather than simply assuming a standard foundation depth.
Steel Beams
If an existing rear or side wall is being opened up to connect the house with the new extension, structural steelwork is commonly required to support the structure above.
The structural engineer will calculate the loads acting on the beam and specify an appropriate steel section.
Depending on the size of the opening and the arrangement of the building, the design may require:
A single steel beam
Multiple beams
Steel columns
A goalpost frame
A moment-resisting frame
Padstones or bearing details
New foundations beneath columns
Large open-plan arrangements can therefore require substantially more engineering than a relatively small opening through a loadbearing wall.
When Is a Goalpost Frame Required?
A goalpost frame typically consists of a horizontal steel beam supported by steel columns at each end.
It may be required where there is insufficient masonry remaining to provide suitable beam bearings or where the proposed opening is particularly wide.
A frame may also be necessary where lateral stability needs to be considered.
For example, replacing almost the entire rear wall of a house with bifold or sliding doors can significantly change how the original building behaves structurally.
The engineer therefore needs to consider more than simply whether the beam is strong enough to carry the vertical load.
What About the Extension Roof?
The roof also forms part of the structural design.
Depending on the architectural arrangement, this might comprise:
Traditional timber rafters
Timber ceiling joists
Structural ridge beams
Purlins
Steel beams
Flat roof joists
Proprietary roof trusses
Where proprietary roof trusses are proposed, these are normally designed by the specialist truss manufacturer.
The structural engineer will typically design the supporting structure and ensure that the loads from the roof can be safely transferred through the building to the foundations.
Do Building Control Need Structural Calculations?
Where structural elements form part of the works, Building Control will normally require sufficient information to demonstrate compliance with the structural requirements of the Building Regulations.
This commonly includes structural calculations together with drawings showing the sizes, positions and construction requirements of the structural elements.
A calculation sheet on its own is not always particularly useful to the contractor.
For this reason, at Lawson Cook we generally prepare structural calculations together with structural drawings so that the proposed construction can be clearly understood on site.
Does the Structural Engineer Design the Whole Extension?
Usually, no.
The architect or architectural designer will normally be responsible for the architectural design, including matters such as:
Proposed layouts
Overall dimensions
Planning drawings
Building Regulations drawings
Insulation and thermal performance
Fire safety requirements
Doors and windows
External finishes
The structural engineer works from this information to develop the structural design.
For this reason, we normally recommend that the architectural drawings are sufficiently developed before the structural design begins.
Changes to layouts, opening sizes or roof arrangements after the structural design has been completed can result in the structural calculations and drawings needing to be revised.
Do I Need a Site Visit?
In many cases, yes.
A site survey allows the structural engineer to understand how the existing building has been constructed and how the proposed extension will interact with it.
Items that may be considered during the survey include:
Existing wall construction
Floor span and direction
Existing roof structure
Beam and lintel arrangements
Existing extensions or alterations
Available structural bearings
Changes in floor level
Drainage and manholes
Nearby trees
Site access and constraints
However, structural surveys are normally visual and non-intrusive unless opening-up works have specifically been agreed.
Existing foundations, concealed beams and other hidden construction cannot necessarily be confirmed from a visual inspection alone.
Opening-up or trial holes may therefore sometimes be required.
What About Drains and Manholes?
Drainage should be considered early when planning an extension.
Existing drains or manholes can affect the position and construction of new foundations. Where works are proposed close to or over a public sewer, a separate build-over agreement may also be required from the relevant water authority.
A structural engineer may design foundations around known drainage constraints, but the drainage system itself would normally be designed or coordinated separately unless specifically included within the engineer's appointment.
Should I Contact the Structural Engineer Before or After the Architect?
For most extensions, we recommend appointing the architect or architectural designer first.
Once a proposed layout has been developed, the structural engineer can review the drawings and design the structure required to make that proposal work.
Early structural input can nevertheless be useful where the proposal includes particularly large openings, unusual construction or significant alterations to the existing building.
In those situations, discussing the structural feasibility before the architectural design is finalised can avoid unnecessary redesign later.
What Information Does a Structural Engineer Need?
If you are requesting a quotation for structural design of an extension, it is helpful to provide:
The property address
Existing floor plans
Proposed floor plans
Proposed elevations
Proposed sections where available
Photographs of the existing property
Details of any trees close to the proposed extension
Information regarding drains or manholes where known
From this information, the engineer can usually establish the likely scope of structural work and advise whether a site survey is required.
Structural Engineering for House Extensions in Hertfordshire
Lawson Cook Limited is a structural engineering practice based in Hertford providing structural design services for house extensions throughout Hertfordshire and surrounding areas.
Our residential structural engineering work includes:
Rear and side extensions
Two-storey extensions
Open-plan alterations
Loadbearing wall removals
Steel beam and column design
Goalpost and moment frame design
Foundation design
Loft conversions
Chimney removals
Structural alterations to existing properties
We typically provide a coordinated package of structural calculations and structural drawings suitable for use by the contractor and submission to Building Control.
If you are planning an extension, send us the property address, architectural drawings, estate agent floor plans where available and photographs of the existing property, and we can review the proposal and provide a quotation for the structural engineering works.
Lawson Cook Limited
Structural Engineers
Hertford, Hertfordshire
info@lawsoncook.co.uk