Single-storey extension structural engineering
A single-storey extension changes how loads travel through your home. We provide the structural calculations and drawings that prove the new structure works and satisfy Building Control, for homeowners and builders across the UK.
Do I need a structural engineer for a single-storey extension?
Yes, in almost every case. As soon as you form a new opening into the existing house or add new foundations and a roof, Building Control will want structural calculations for the beams, supports and foundations. We prepare that package to a fixed fee.
What a single-storey extension needs structurally
Most single-storey extensions look simple on the drawings, but they change the way weight moves through the existing house. When you open up the back wall to connect the extension to the kitchen, the load that wall used to carry, the floor above, the roof, sometimes a first-floor wall, has to be picked up by a new beam and carried down through posts and foundations. Getting that load path right is the whole job.
We work out the loads gathered from the roof, floors and any walls above, size the steel or timber beam that spans the new opening, check the padstones and bearings at each end, and design the foundations for the new walls to suit the ground. The output is a set of calculations and drawings your builder can price and build from, and that Building Control will accept without a string of queries.
Foundations and ground conditions
The foundation is where an extension most often runs into cost surprises, and it is set by the ground, not the drawing. Clay soils that shrink and swell with the seasons, nearby trees, made ground and existing drains all change what the foundation has to be. We assess the conditions and design a strip, trench-fill or, where needed, a raft or piled foundation to suit.
Where the extension sits against the existing house, we also check that the new foundation and the old one behave together, so differential movement does not crack the join between old and new.
Working with your drawings and Building Control
We work from your architect's or designer's plans and coordinate the structure with the design intent before anything is priced on site. If the layout changes, the calculations change with it, we would rather revise once than have a beam turn up that no longer fits the opening.
The calculations are prepared to the Eurocodes, the relevant UK National Annex and Approved Document A, so they demonstrate compliance for the Building Regulations submission and can be followed on site.
What we check.
The points our calculations resolve for a project like this.
- Beam sizing for the new opening, with deflection kept within limits that protect finishes
- Padstone and bearing design at each beam end
- Foundation design suited to the ground conditions and any nearby trees
- Load path from roof and floors down to the new foundations
- Coordination with the existing structure so old and new move together
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 for a single-storey extension?
Yes, in almost every case. As soon as you form a new opening into the existing house or add new foundations and a roof, Building Control will want structural calculations for the beams, supports and foundations. We prepare that package to a fixed fee.
How much does the structural engineering cost for an extension?
For a straightforward single-storey extension We agree a single fixed fee for the calculations and drawings, agreed as a fixed price before we begin. The exact fee depends on the number of openings, the span and whether special foundations are needed.
Can you work from my architect's drawings?
Yes. We work from your architect's or designer's plans, coordinate the structure with the design, and issue calculations and drawings your builder and Building Control can work from.
What foundations will my extension need?
That depends on the ground, nearby trees and drains. We assess the conditions and design a strip, trench-fill, raft or piled foundation to suit, rather than assuming a standard depth.
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