Structural Engineer for Home Extensions: What You Need and When
If you’re planning a home extension, you will almost always need a structural engineer. Any project that changes how the house carries load needs proper calculations and drawings. This guide explains what a structural engineer does, when you need one, what calculations are involved, and what you should expect in the design process.
What a Structural Engineer Does in a Home Extension
A structural engineer makes sure your extension is safe, buildable, and approved by Building Control.
Checking how the existing house carries load
Every home has its own load path. A structural engineer checks which walls are load bearing, what the floors and roof sit on, and how the weight travels down to the foundations.
Confirming what can be removed or altered
If you are opening up the kitchen, extending the back of the house, or removing a wall, the engineer checks how much load the altered area carries and what support is needed.
Designing the new structural elements
This includes:
- steel beams
- steel posts
- padstones
- new foundations
- roof supports
- floor joist design
You get calculations and drawings that builders and Building Control accept.
When You Need a Structural Engineer for an Extension
You need a structural engineer when your extension changes load in any way.
Removing internal or external walls
Most extensions involve the removal of load bearing wall, which means you will need steel beams to support the load above.
Fitting steel beams or posts
RSJs or universal beams are needed when the opening is more than 1 metre, or when the wall is load bearing.
Building a rear, side, or wraparound extension
All extension types change how the house behaves structurally. New beams, new roofs, and new foundations need checking.
Adding a new storey or loft
If you are adding weight above, the floors and foundations must be assessed.
Key Structural Calculations Needed for Extensions
Steel beam design
Most homes use UB or UC steel beams for openings. The engineer carries out a full steel beam calculation to check the load from the upper floors, roof, and bearing lengths.
- load from the upper floors
- roof load
- lateral forces
- bearing lengths
- deflection limits
Foundation checks
For an extension to work long-term, the foundation must carry the load safely. Engineers check soil type, depth, and bearing capacity.
Roof and floor design
The engineer designs rafters, joists, and connections.
Lateral stability and load paths
This ensures the house doesn’t lean or shift after you open up walls.
Structural Drawings You Will Receive
A structural engineer prepares the home extension structural design pack, which includes calculations, drawings, and the details Building Control needs.
Your pack normally includes:
- beam layout plan
- section drawings
- steel sizes and positions
- padstones
- connections
- construction notes
- Building Control compliance notes
These drawings help builders follow the exact design.
How Structural Engineers Work with Building Control
What Building Control checks
They review:
- beam sizes
- bearing details
- foundation sizes
- lateral stability
- fire safety details
Documents submitted
You normally send:
- structural calculations (PDF)
- structural drawings (PDF)
- any soil or foundation notes
Typical approval timeline
Most councils approve within 5 to 14 days.
Cost Guide: How Much Do Structural Calculations Cost for Extensions?
Simple single-storey extension
£350 to £650
Large rear extension
£600 to £900
Two-storey extension
£800 to £1500+
What affects the cost
- number of beams
- span length
- foundation conditions
- roof type
- complexity
Step-by-Step Process
- You send the floor plan and photos
- We confirm which walls are load bearing
- We design all beams and supports
- We create drawings
- You submit to Building Control
- Approval granted
Common Mistakes Homeowners Make
- Starting work without calculations
- Letting builders guess beam sizes
- Ignoring foundation issues
- Copying someone else’s beam sizes
FAQ – Structural Engineer for Home Extensions
Do I always need steel beams in an extension?
Most openings need a beam, especially if the wall is load bearing.
Can the architect do the structural calculations?
No. Only structural engineers should design beams and foundations.
How long do calculations take?
Most projects take 2 to 4 days.
Can I use standard beam tables online?
No. They don’t account for real load paths or Building Control rules.
Conclusion
A home extension always needs proper structural design. Steel beams, foundations, and load paths all need checking before any work starts. With clear calculations and drawings, your builder can work safely and Building Control can approve your project without delays.
For more information on Building Regulations for extensions, you can check the official guidance on the Planning Portal.
Get Your Extension Calculations
If you want the calculations for your extension, I can prepare the beam design, drawings, and full pack ready for submission.