Two-storey extension structural engineering

Two-storey foundations carry more, so they are less forgiving of poor ground. We assess the soil, the proximity of trees and any existing drains, then design a strip, trench-fill, raft or piled solution sized for the doubled bearing pressure. On shrinkable clay near mature trees the foundation may need to go deeper than a single-storey job would, sometimes with clay heave protection to the faces, because the swelling and shrinking of the soil can lift or drop a shallow footing.

Where the new foundation runs alongside the existing house we check that old and new settle together. A modern deep footing next to a shallow Victorian one can move differently under load, opening a crack at the junction, so we detail the connection and, where useful, specify how the two are tied or separated. If a party wall is involved we set out the structural information a surveyor needs for the notices, though the Party Wall award itself sits outside our scope.

We work from your architect's or designer's plans and resolve the structure before anything is priced. On a two-storey extension the position of the first-floor beams and load-bearing walls often decides whether an open-plan ground floor is achievable, so it pays to settle the frame early. We flag where a proposed opening would need a transfer beam, where a wall above must land on support, and where the roof structure ties back into the existing roof.

The calculations are prepared to the Eurocodes, the relevant UK National Annex and Approved Document A, and issued as a package your builder can price and build from. If the layout changes we revise the calculations to match, because a beam sized for the old plan may be wrong for the new one, and it is cheaper to update a calculation than to change a steel that is already on site.