What Does a Structural Engineer Do for Your Extension?

When you remove the rear wall of your kitchen to open up into the new extension, the only thing stopping the bedroom above from dropping through the floor is a steel beam. Your structural engineer is the person who works out exactly what size, type, and position that beam needs to be. Get the calculation wrong, and your house is structurally compromised. Get it right, and nobody ever thinks about it again.

That makes this one of those roles where the value is invisible. You'll pay a few hundred pounds for some paperwork and wonder what you got for it. What you got was a house that stays standing.

What a Structural Engineer Actually Does

An architect designs the space you want. They decide where walls go, where windows sit, how big the opening to the garden will be. A structural engineer works out whether the building can physically support what the architect has drawn. They're solving a different problem entirely.

For a typical single-storey extension, the SE calculates:

• Steel beam sizes for supporting the existing first floor where you're removing or cutting through load-bearing walls. These are specified by their Universal Beam designation (e.g. "305 x 102 x 33 UB"), which tells the fabricator the exact dimensions and weight per metre.
• Lintel sizes over new openings, including windows, external doors, and bifold doors.
• Foundation depth and width, based on soil conditions, tree proximity, and the loads the walls will carry.
• Roof structure adequacy, confirming that timber sizes, spacing, and connections can handle wind, snow, and dead loads.
• Any unusual structural features, such as cantilevers for corner-post removal or long-span beams for full-width glazing.

The SE doesn't design your extension. They prove it won't collapse.

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Do You Need a Separate Structural Engineer?

Not necessarily. Some architects (particularly architectural technologists with structural experience) produce their own structural calculations. Others subcontract to an independent SE and pass the cost through. You need to know which arrangement you're getting before you appoint.

If your architect produces structural calculations in-house, ask whether they carry professional indemnity insurance covering structural work. An independent SE will have their own PI insurance through their IStructE or ICE membership.

For a straightforward single-storey rear extension with standard openings and foundations, your architect can usually handle the calcs. But if you're removing corner posts or installing full-width bifold doors, get an independent SE. Same applies if you're dealing with unusual ground conditions or going to two storeys.