Do I need a structural engineer for my extension?

Yes, in almost every case. Any extension that removes a load-bearing wall, adds a steel beam, or alters foundations needs structural calculations signed off by a qualified engineer before building control will approve the work.

What does a structural engineer do on an extension?

A structural engineer works out whether the building can physically support what the architect has drawn. They size the steel beams over removed walls, specify lintels and foundations, and confirm the roof structure can carry wind, snow, and dead loads.

How much does a structural engineer cost for an extension?

A full package of initial calculations for a typical single-storey extension is around £500 – £1,500, with each later revision charged separately. Some architects bundle the calculations into their own fee instead of using an independent engineer.

What is the difference between an architect and a structural engineer?

An architect designs the space, deciding where walls, windows, and openings go. A structural engineer proves that design will stand up, calculating the beams, foundations, and load paths that stop it collapsing. They solve different problems on the same project.

How long do structural calculations take for an extension?

Initial calculations for a straightforward single-storey extension typically take two to four weeks, or one to three weeks where an independent engineer handles them directly. Mid-build design changes can trigger revisions that add further time.

When do I need an independent structural engineer rather than relying on the architect?

An independent engineer is worth it for more complex structures such as bifold openings, corner-post removal, two-storey work, or unusual ground conditions. For a simple rear extension with standard openings, an architect handling the calculations is often enough.

Back to Single Storey Rear Extension

Phase 1 · Planning & Permissions · Task 08 of 17

What Does a Structural Engineer Do for Your Extension?

Your structural engineer calculates the steels, foundations, and load paths that stop your extension falling down. Expect to pay £500 – £1,500 for initial calcs, plus £45 – £135 per revision.

12 min readUpdated 2026Free with email

Process infographic showing the structural engineer's involvement across the extension project lifecycle, from initial calculations through mid-build revisions to building control sign-off

Prerequisites

Complete this first

Finding an Architect

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 this guide covers

01What a Structural Engineer Actually Does
02Do You Need a Separate Structural Engineer?
03What You Actually Receive
04The Revision Problem
05What It Costs
06How to Appoint a Structural Engineer
07From Calculation to Fabrication
08When the BCO Disagrees
09The Communication Chain
10When Design Changes Hit Structure
11What Happens After the Calculations

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