Most house extensions in London need a structural engineer. Building Control will expect structural calculations for the new foundations, for the steel beams where the existing house is opened up, and for the new roof. This guide answers the questions London homeowners ask us most often before an extension starts.
Do I need a structural engineer for my extension?
In almost every case, yes. Nearly all extensions need Building Regulations approval, even when they do not need planning permission, and Part A of the Building Regulations covers structure. Your building control body (the council, or a registered building control approver) will normally ask for structural calculations and details before or during the work. Very few extensions are exempt, such as some small conservatories and porches, and any work that removes or opens up an existing wall still needs an engineer.
What do extension structural calculations cover?
A typical single-storey rear or side-return extension needs calculations and details for:
- Foundations for the new walls, sized for your soil and any nearby trees
- The steel beam or beams where the back or side wall of the house is opened up, with the padstones and posts that carry them
- The roof: joists or rafters, roof lights and any steel needed for a flat or glazed roof
- The new floor, whether suspended timber or a concrete slab
- How the new structure ties into the existing house
- Stability, where large openings or glazed corners remove walls that used to brace the building
The result is a set of calculations plus structural drawings that show the builder exactly what to install. We explain what those drawings should include in what extension structural drawings must show.
Why do London extensions often need deeper foundations?
Much of London sits on shrinkable clay. Clay swells when it is wet and shrinks in dry summers, and tree roots dry it out further. Foundations on clay are therefore taken deeper than on sand or gravel, and deeper again near trees, including your neighbours' trees. Your engineer will ask about trees and may recommend a trial pit to see the soil and the depth of the existing foundations. Where trees are close or the clay is very changeable, piled or specially designed foundations can be the most economical answer. Our guide to subsidence in London explains why clay movement matters.
What happens when the back wall is opened up?
Opening the rear wall into a new kitchen is usually the most structural part of an extension. The wall above still carries floors, walls and sometimes the roof, so a steel beam takes that weight and passes it down through padstones or steel posts to foundations that can carry it. For wide openings, or where the side return is opened as well, a steel frame (often called a goalpost) may be needed. The engineer checks what sits under each end of the beam, not just the beam itself. See our guide to load-bearing wall removal in London homes for more on steel beams.
Does my extension need a party wall agreement?
Often, yes. Under the Party Wall etc. Act 1996 you must serve notice on your neighbours if you build on or at the boundary line, cut into a shared wall (for example to seat a steel beam), or dig foundations within 3 metres of their building and deeper than its foundations. A deeper dig within 6 metres can also need notice. Most London terrace and semi-detached extensions trigger at least one of these, and the structural drawings are usually shared with the party wall surveyor.
What if there is a sewer under my extension?
Many London back gardens have a shared or public sewer running through them. If you plan to build over, or within 3 metres of, a public sewer you will normally need a build over agreement from the water company (Thames Water across most of London) before work starts. The engineer designs the foundations to bridge the pipe or go below it, so that the extension does not load the sewer.
Do I need planning permission as well?
Planning permission and Building Regulations are separate. Many single-storey rear extensions to houses fall under permitted development, which limits their size: generally up to 3 metres deep for terraced and semi-detached houses and 4 metres for detached houses, or up to 6 and 8 metres through the council's prior approval process, with limits on height and materials too. Flats, maisonettes and listed buildings do not have these rights, and many London conservation areas restrict them. Check with your council or your architect. The structural design is needed either way.
What will the engineer need from me?
- Your architect's or designer's drawings: plans, sections and elevations
- Access to the house for a site survey
- Anything you know about the existing foundations, drains or earlier building work
- Details of trees in your garden and your neighbours' gardens
- Contact details for your building control body and party wall surveyor, if appointed
We survey the existing structure on site, so the design fits the house as it was actually built, not only as it appears on drawings.
When should I bring in a structural engineer?
Once the layout is settled, but before builders price the job. Builders can then quote for the actual steels and foundations instead of guessing, which avoids costly extras later. Bringing the engineer in early also shows whether a very wide opening or a glazed corner will add a lot of steel, while the design can still change.
Does the engineer stay involved during the build?
A good one does. Builders often uncover something unexpected once they start digging or opening walls: a shallow foundation, a drain, or a wall that is not what it seemed. During construction we stay on call for contractor questions, revisions and site checks, so problems are solved quickly and properly.
How do I choose a structural engineer for an extension?
Look for chartered engineers who explain things clearly, measure the house on site and give a written quotation that states what is included. Our article on choosing a structural engineer for an extension lists the questions worth asking, and the role of structural engineers for house extensions covers what happens at each stage.
Extension structural engineers in London
Laytoe Structures is a firm of chartered structural engineers based on Turnham Green Terrace in Chiswick, working across London. Since 2016 we have carried out more than 2,700 projects, including structural surveys, and we provide building-control-ready calculations and drawings for extensions and loft conversions, including our loft conversion FAQs. Quotes are free and with no obligation.
Book online or call 020 3983 5057 for a quotation.