Rear extension structural engineering
A rear extension nearly always means removing the back wall to open the house into the new space, which is the moment the load path has to be redesigned. We provide the calculations and drawings that make that opening safe and satisfy Building Control, for clients across the UK.
Do I need a structural engineer to remove my rear wall?
Yes. The rear wall is usually load-bearing, carrying the floor and roof above, so removing it needs a beam designed to take that load. Building Control will ask for calculations for the beam, its supports and the foundations below.
The rear opening is the heart of the job
On most rear extensions the structural work centres on the wall you take out to connect the house to the new room. That rear wall is usually load-bearing: it carries the first-floor joists, often a section of roof, and any masonry above the ground-floor windows. When it comes out, all of that has to be picked up by a beam and carried down through supports at each end into the ground. Get the beam and its bearings right and the opening is stable, the finishes stay crack-free, and Building Control passes it without a run of queries.
We calculate the load the removed wall was carrying, size the universal beam or pair of beams to span the opening, and check deflection so plaster and glazing above are not stressed as the floor loads up. For wide openings and bi-fold or sliding doors we look at how the beam is propped during the works and how it is padded down at each end, because a large clear span concentrates a lot of load onto two small bearing points.
Bearings, padstones and the walls below
A beam is only as good as what it sits on. At each end we design a padstone or spreader to distribute the reaction into the masonry, and we check the wall or pier below can carry that concentrated load down to the foundation. On a typical rear extension the existing external corner or a new pier takes the beam end, so we confirm it has enough section and that the foundation under it is adequate for the new point load rather than the spread load it was built for.
Where two beams meet at a corner, as they often do on an L-shaped rear return, one beam frames into the other and the connection has to transfer the load cleanly. We detail that junction, size any posts hidden in the wall, and make sure the goalpost or portal action, if used, is fully braced. These details are what stop a wide rear opening from sagging or cracking over the first year of use.
Foundations, drains and the ground
Rear extensions frequently run over or close to existing drainage, and they sit on whatever ground the original garden offered. We assess the soil, nearby trees and the drain runs, then design a strip, trench-fill or raft foundation to suit. Where a foul or surface water drain crosses the footprint we set out how the foundation bridges or protects it, so the extension does not load the pipe and the drain stays accessible.
The calculations are prepared to the Eurocodes, the relevant UK National Annex and Approved Document A, and cover the beams, bearings and foundations as one package. We work from your architect's or designer's plans and coordinate the structure with the layout before it is priced, so the beam suits the opening you actually want rather than a standard assumption.
What we check.
The points our calculations resolve for a project like this.
- Beam sizing for the rear opening with deflection controlled to protect finishes
- Padstone and bearing design where the beam lands on masonry or a pier
- Corner connection design where two beams meet on an L-shaped return
- Check on the walls and foundations below the new point loads
- Foundation solution where drains cross or run close to the footprint
- Temporary propping and sequence for removing the load-bearing rear wall
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Do I need a structural engineer to remove my rear wall?
Yes. The rear wall is usually load-bearing, carrying the floor and roof above, so removing it needs a beam designed to take that load. Building Control will ask for calculations for the beam, its supports and the foundations below.
How much does a rear extension structural design cost?
A single fixed fee covers the calculations and drawings, agreed as a fixed fee before we start. The price depends on how wide the opening is, whether one beam or several are needed, and any special foundations near drains or trees.
Can you design a wide opening for bi-fold doors?
Yes. A wide clear span is a beam and bearing exercise. We size the beam, control its deflection so the doors keep working, and design the padstones and the walls below to carry the concentrated end loads.
What happens if a drain runs under my extension?
We design the foundation to bridge or protect the drain so it is not loaded and stays accessible. Depending on depth and position this may mean a lintel over the pipe or a locally deepened footing, which we detail on the drawings.
Do you provide temporary propping details?
We can set out the sequence and where props are needed while the wall is removed and the beam installed. This keeps the house safe during the works and is often the part that reassures both the builder and Building Control.
Related in home & residential structural engineering.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project