Extension structural package
Everything structural for an extension, from foundations to roof.
About this package
The extension structural package covers all the structural engineering for a single or two-storey extension, from foundations to roof. That includes the steel beams that open up the back of the house, the foundations for new walls, the roof structure and any internal walls removed along the way. One fixed fee covers the site inspection, the design and Building Control's questions.
It suits homeowners building a rear, side return or wraparound extension, architects who need a structural engineer for their scheme, and builders who want the design done properly before they start digging. It is also useful where there are complications, such as a nearby tree on clay, a sewer under the garden or a party wall on the boundary, which need to be considered together.
An extension is a set of linked decisions. The size of the steel depends on what the back wall carries, the foundation depth depends on the ground and nearby trees, and the roof design depends on spans and the finish chosen. Handling all three stages together leaves no gaps between them. Our three-step quality control, ending with sign-off by our Head of Engineering, applies to every calculation in the package.
What the package brings together
- Site inspection
- Extension structural package
- Building Control queries
Stages
- Site inspectionWe inspect the existing rear wall, the floors and walls above, the garden, nearby trees and any visible drainage. This tells us what the new steel must carry and what the new foundations must deal with.
- Extension structural designCalculations and drawings for foundations, steel beams and columns, padstones, the new roof structure and any internal openings. Where trees or sewers affect the design, foundation depths and details are set to suit.
- Building Control queriesWe submit the structural information to Building Control and answer the inspector's questions, from foundation depth on site to steel connection details, so your project keeps moving through each inspection stage.
Who it suits
- Rear extension on a terraced or semi-detached house
- Side return infill on a Victorian terrace
- Wraparound extension combining rear and side
- Two-storey extension with bedrooms above
- Extensions near trees or over a sewer
How it works
Frequently asked questions
How deep will my extension foundations be?
It depends on the ground and nearby trees. On London Clay away from trees, a depth of around one metre is common, but near large trees foundations often need to go much deeper, and piles or a suspended floor can become more economical. Building Control confirms the depth on site. Our guide on rear extension structure covers the main decisions.
What if there is a sewer under my extension?
Building over or near a public sewer generally needs the water company's agreement, which in most of London means Thames Water. The foundation design may need to bridge the pipe or keep load away from it, and a CCTV survey confirms the route and condition. Our build over or near sewer technical package covers the application if you need one.
Can I have a large open corner without a column?
Often, yes. Corner openings in wraparound and side return extensions are popular, and they can usually be achieved with a steel frame or a goalpost and cantilever arrangement. The trade-off is larger or heavier steel and sometimes deeper foundations at key points. We discuss the options during design so you can weigh the look against the build cost.
Does the package cover the party wall?
The structural design will show any work to the party wall or along the boundary, which is needed for the notices. Serving notices and preparing a schedule of condition is a separate service, as is the role of the party wall surveyor. We can provide notices and a schedule of condition as an addition, or work with the surveyors you appoint.