House extension: structural engineer guide for UK homeowners
A house extension is the most popular home improvement project in the UK, but it involves more structural engineering than most homeowners expect. This guide explains exactly what structural work is required, what Building Control needs to see, and how the structural engineer fits into the process from first drawings to completion certificate.
For costs, see our extension cost guide. For planning rules, see our 3-metre extension planning guide. This page focuses specifically on the structural engineering requirements.
Why every house extension needs a structural engineer
When you build a house extension, you are making structural changes to an existing building. You are removing part of the rear wall to connect the new space to the existing house. You are building new foundations adjacent to existing ones. You are adding loads to the roof and floor structure. Every one of these changes needs to be engineered correctly and approved by Building Control before work can proceed.
Building Control requires a structural engineer's calculation pack as part of the Building Regulations submission. The calculations demonstrate that the structure is safe, that the beams are correctly sized, that the foundations will support the loads, and that the floor and roof structure meet the required standards.
What structural work does a house extension require?
The structural work in a house extension falls into four main areas.
1. Steel beam at the rear wall opening
When the back wall of the existing house is opened up to connect the extension, a steel beam (RSJ) spans the opening and carries the load of the wall, floor and roof above it. The beam must be calculated for your specific span, the number of floors above, and the wall type. It sits on padstones built into the brickwork at each end. This is the most critical structural element in the whole project.
For a typical 3 to 4 metre opening, the beam is commonly 152mm to 203mm deep, but the actual size must be calculated, not guessed. For full detail on beam design see our steel beams for house extensions guide.
2. Foundation design
The new extension walls need foundations to carry the loads safely into the ground. Most single-storey rear extensions use strip foundations 450mm wide and 900mm to 1,200mm deep, but the correct depth depends on soil type, proximity to trees, proximity to drains, and the loads from the new walls and roof. Clay soil requires deeper foundations than sandy or chalk soil. The structural engineer specifies the foundation type and depth in the calculation pack.
3. Floor and roof structure
The floor joists for the new extension must be sized to carry the intended loads without excessive bounce or deflection. The roof structure, whether flat or pitched, must be designed to carry the self-weight of the roof covering, snow loads and wind loads, and transfer them safely to the walls below. Both are included in the structural calculation pack.
4. Padstone design
Every steel beam end sits on a padstone, a concrete or engineering-brick bearing pad built into the wall that spreads the concentrated point load from the beam into the masonry. Padstones must be sized correctly for the beam reaction force and the wall construction. Too small a padstone causes local crushing failure in the brickwork below. The structural engineer specifies padstone dimensions, material and mortar specification for every beam bearing.
Structural engineer fees for house extensions
| Extension type | Typical SE fee | What is included |
|---|---|---|
| Single-storey rear, 1 beam | £345 to £550 | Beam design, padstones, floor joists, flat or pitched roof, foundations |
| Single-storey rear, 2 beams | £450 to £700 | Above plus second beam (e.g. internal wall opening) |
| Wraparound (rear and side) | £600 to £900 | Multiple beams, corner junction design, complex load path |
| Two-storey extension | £700 to £1,200 | Beams at both floor levels, increased foundation loads, floor and roof at two levels |
| Extension with basement | £1,200 to £2,500 | Retaining wall design, underpinning assessment, waterproofing specification |
All fees are fixed and confirmed in writing before work starts. The fee covers the full calculation pack, structural drawings and any Building Control inspector queries. Turnaround is typically 5 to 7 working days from receipt of drawings.
What the structural calculation pack includes
When a structural engineer designs a house extension, the Building Control submission pack typically contains the following documents.
- Beam design calculations for every steel beam, showing the span, load, beam section chosen, bending and shear checks, and deflection under load
- Foundation design showing the foundation width and depth, the soil bearing capacity assumed, and the loads from the walls above
- Floor joist calculation confirming the joist size, spacing and span
- Roof structure calculation confirming rafter sizes, ridge beam requirements (if applicable) and wind and snow load checks
- Padstone schedule listing the size, material and position of every padstone in the project
- Structural drawing (PDF) showing the beam layout, foundation details and connection details in a format your builder can follow on site
The Building Control inspector checks the structural drawing during the construction inspection visits. The steelwork must match the drawing before it can be covered with insulation and plasterboard.
Building Control for a house extension
Building Regulations approval is required for all house extensions, regardless of whether planning permission is needed. The two processes are separate: planning permission is about whether you are allowed to build, Building Regulations are about whether it is built safely.
How the process works
- You (or your architect) submit a Building Regulations application to the local Building Control authority or an approved inspector
- The application includes the architectural drawings and the structural engineer's calculation pack
- Building Control reviews the submission and may raise technical queries
- Once approved, construction can begin
- A Building Control inspector visits at key stages: foundation dig and depth, steel beams before covering, roof structure before felt and tiling, and final completion
- On completion, a Building Control completion certificate is issued
Party wall considerations
If your extension is close to or on the boundary with a neighbouring property, the Party Wall etc. Act 1996 may apply. You may need to serve a Party Wall Notice on your neighbour before work starts. A Party Wall Surveyor advises on whether the Act applies and, if so, prepares the Party Wall Award. Allow at least two months for this process before work is due to start.
Flat roof vs pitched roof extension: structural differences
The roof type affects the structural design and the structural engineer fee.
Flat roof extensions
Flat roof extensions use timber joists spanning across the building at the roof level, typically at the same depth as the ceiling joists. The joists must be sized to carry the roof covering loads, insulation and any imposed loads. Flat roofs also need a slight fall (minimum 1:40) towards the drainage point, which affects the joist depths. The structural design is relatively straightforward.
Pitched roof extensions
A pitched roof extension with a ridge and rafters requires rafter sizing, a ridge beam assessment and sometimes collar and purlin design. If the extension is wider than about 3 metres, a ridge beam may be needed to prevent the rafters pushing outward on the walls below. The ridge beam design adds to the structural engineer's scope but typically only a modest amount.
Lantern roof or glazed roof
A glazed lantern or bifold door-to-roof extension involves a more complex structural design because the glass roof imposes both dead loads and significant wind loads. The structural frame must be designed to carry these loads, and connections to the existing structure must be carefully detailed. These projects are at the higher end of the structural engineer fee range.
Common structural problems with house extensions
Undersized beam over the rear wall opening
This is the most common structural problem in house extensions. A builder who estimates the beam size without engineering calculations frequently underspecifies it. An undersized beam will deflect visibly over time, cracking the plasterboard above the opening and potentially causing the masonry above to settle. Correcting this after the extension is built requires lifting the structure, removing finishes and replacing the beam, at far greater cost than getting it right first time.
Inadequate foundations
Foundations dug to the minimum depth without a soil investigation can perform poorly in clay soils, which expand and contract seasonally. Differential movement between new and old foundations can cause cracking at the junction. The structural engineer specifies foundations based on the actual site conditions, not a minimum standard guess.
No padstones
A steel beam sitting directly on a standard brick without a padstone concentrates the beam reaction force onto a very small area. This can cause local crushing of the brickwork, especially in older properties with lime mortar. Padstones must always be specified and installed.
Opening too close to an existing opening
If a new extension opening is created close to an existing window or door in the rear wall, the pier of brickwork between them may be too narrow to carry the combined loads. The structural engineer checks this and either redesigns the opening positions or strengthens the pier.
Frequently asked questions
Do I need a structural engineer for a small extension?
Yes. Even a small single-storey rear extension requires structural calculations for the beam over the opening, the foundations and the roof. The size of the extension affects the complexity and cost of the calculations, but the calculations are always required for Building Control.
Can my builder design the steel beams?
No. A builder can suggest beam sizes based on experience, but only a Chartered Structural Engineer can produce a calculation pack accepted by Building Control. Building Control will not sign off a project with no calculations or with calculations produced by someone without structural engineering qualifications.
How long does it take to get structural calculations?
Typically 5 to 7 working days from receiving the architectural drawings. For simpler jobs the turnaround is sometimes faster. We confirm the timescale when we confirm the fixed fee.
What drawings do I need to send the structural engineer?
Floor plans and elevations showing the extension layout, the existing house layout, and the proposed opening positions. These do not need to be final architect's drawings. Sketch drawings with dimensions, estate agent floorplans, or early-stage architect drawings are all sufficient to price the job and begin the structural design.
What happens if I build without structural calculations?
Building Control can issue a notice requiring the work to be opened up and inspected, or to be demolished and rebuilt correctly. Without a completion certificate, the extension significantly complicates any future sale or remortgage. Retrospective structural calculations are possible in some cases, but they may require intrusive inspections of the as-built structure.
Get structural calculations for your house extension
If you are planning a house extension and need structural calculations for Building Control, here is how we work:
- Send your drawings, sketch or estate agent plans with the opening positions marked
- We confirm a fixed fee in writing before any work begins
- We produce the full calculation pack: beam design, foundations, floor joists, roof structure, padstone schedule and structural drawing
- Ready in 5 to 7 working days
- Inspector queries answered as part of the fixed fee
Structural engineer fees for a house extension start from £345. The fee is confirmed as a fixed price before we start. Related pages: