Key Takeaways
- A report typically costs £500 to £2,000. The average is about £700 plus roughly £300 for the site visit.
- An inspection alone, for one defect, is £175 to £350 and is often all you need to know whether to worry.
- The report is what building control inspects against. Without it, notifiable structural work cannot be signed off.
- It contains calculations, a specification and drawings. It is a design, not an opinion.
- Your contractor should build to it and never produce it. A builder who sizes their own steel is a warning sign.
Once you know you need a structural engineer, the next question is what you actually receive for the fee. The answer is a design that someone is professionally accountable for, and that is why building control will not proceed without one.
What is in a structural engineer's report?
Three things, and the proportions vary with the job.
- Calculations sizing the structural members: the beam, its section, the padstones it bears onto and the loads it carries
- A specification saying what is to be built and how, including bearing lengths, propping requirements and the sequence of work
- Drawings or sketches showing the arrangement, which is what the contractor builds from and building control inspects against
What it is not is a general opinion about the house. If you want that, you want a survey, and our guide to what a structural survey covers explains the difference. The engineer's report answers a specific structural question and takes professional responsibility for the answer.
Where the question is diagnostic rather than a design, for example whether a crack matters, you may only need an inspection and a short written opinion. That is a cheaper product and is frequently the right one.
What does a structural engineer cost?
Around £700 for the report plus about £300 for the site visit is the common shape of it, with the full range running £500 to £2,000 depending on complexity.
| What you are buying | What it gives you | Typical cost |
|---|---|---|
| Inspection only | One defect looked at, brief opinion | £175 to £350 |
| Site visit | Attendance and measurement | about £300 |
| Report, average | Calculations and specification for a straightforward job | about £700 |
| Report with recommendations | Diagnosis plus a designed remedy | £500 to £1,500+ |
| Full range | Simple opening to complex multi element design | £500 to £2,000 |
| London and the South East | Regional uplift on all of the above | 20 to 40 per cent higher |
Set that against the work it governs. A single knock through with a steel beam is typically several thousand pounds of building work, so the engineer is a small share of the total and the one part that stops the rest being guesswork.
Why will building control not proceed without one?
Because the calculations are the evidence that what is being built will carry the load. Structural alterations are notifiable work, and the inspector's job is to check the built work against a design, not to design it themselves on site.
In practice the sequence is that the engineer produces calculations, those go in with the building regulations application, and the inspector then attends at the stages that matter: the propping, the bearings before they are covered, and completion. The completion certificate is what a buyer's solicitor asks for years later, and it is far cheaper to obtain now than to explain away at sale.
Our building regulations guide sets out the inspection stages, and the RSJ and load-bearing wall guide covers what happens on site.
Who appoints the engineer?
Either you or your contractor, and it matters less than people think as long as the engineer is genuinely independent of the person building the work.
GS Renovation appoints structural engineers on behalf of clients as a normal part of a project, builds strictly to the design produced, and coordinates the building control inspections. We do not produce calculations ourselves and never have. That separation is the point: the person designing the structure should not be the person deciding how much steel they feel like buying.
So a builder who offers to size a beam themselves, or who says an engineer is not necessary for a knock through, is telling you something useful about how they work. Our guide to choosing a builder in London covers the other checks worth making, and architect or structural engineer covers which professional you need in the first place.
When do you need one?
- Removing or forming an opening in any wall that carries load, including most spine walls
- Removing a chimney breast, where the stack above has to be supported
- Altering, replacing or notching floor joists, or forming a new floor opening for a staircase
- Loft conversions, where a new floor structure and often new steel is required
- Extensions, for the foundations, the opening into the existing house and any goalpost frame
- Underpinning, restraint straps and repairs to movement
- Any crack, bulge or deflection where you need to know whether it is active
If you are trying to work out whether your situation is one of these, the diagnostic guides are the place to start: cracks in walls, bowing walls, failed lintels and floor joists.