Rear extension structural guide for London homes
A rear extension is more than a new room. It changes the back of the house, the ground beneath and sometimes the neighbour's wall. Here is what the structural design covers.
What the engineer designs
A rear extension needs a structural engineer for three main things: foundations that suit the ground, the steel beams that open the back of the house into the new space, and the roof structure. In London, where much of the ground is shrinkable clay and gardens are often lined with trees, foundations frequently need to go deeper than the standard minimum. Building Control will ask for the calculations and drawings before approving the work.
- Foundations for the new walls, sized to the ground, trees and drains
- Steel beams, posts or frames where the existing rear wall is opened up
- Floor slab or suspended floor, depending on ground and levels
- Flat or pitched roof structure, including rooflights and glazing
- Connections between the new structure and the existing house
- Lateral stability where large areas of wall are removed
Foundations in London Clay
London Clay shrinks as it dries and swells when it wets, so foundations have to go deep enough to sit below the zone affected by seasonal change. On clay, that usually means a minimum depth of around 1 m, often more. Trench fill foundations, where a narrow trench is filled with concrete almost to ground level, are the most common solution for domestic extensions.
Trees make a big difference. Their roots draw water from the clay, and the required foundation depth increases with the tree's species, size and distance from the extension. Engineers use established depth guidance that relates these factors to the clay's shrinkability. Where trees have recently been removed, the clay can swell as it rehydrates, so the design may need compressible material along the sides and under the slab to protect against heave. Our guide to London Clay, trees and foundations explains this in detail.
Where the required depth becomes very large, mini piles with a ground beam can be a more practical option. The engineer will set out the choice once the trees and ground are understood. Drains running close to the new foundations matter too, because a leaking pipe can soften clay next to the footing, so the drainage layout should be checked at the same time.
Opening up the back of the house
Most extensions remove some or all of the existing rear wall to create one open space. That wall usually carries the floors and roof above, so a steel beam takes over. Where a wide opening is needed, the engineer may design a steel goalpost frame: a beam on two steel posts, each sitting on its own pad foundation.
In a Victorian terrace, side return and wraparound extensions are common. Here the side wall of the back addition is removed, and a long steel beam carries the outrigger's upper floors. The ends often bear on the party wall, which needs checking and notification.
Removing large areas of wall also removes some of the house's resistance to sideways loads. If the back of the house becomes mostly glass, the engineer may use a rigid moment frame to keep the structure stable.
Floors and roofs
The extension floor is usually a ground-bearing concrete slab, insulated and set to match the existing floor level. On clay with trees nearby, or where levels drop away, a suspended floor on beams can be more reliable.
Flat roofs are typically timber joists or, for longer spans and big rooflights, a mix of timber and steel. Large glazed roofs and rooflights need trimming around them, and frameless glass adds weight. If you are considering a green roof or solar panels, tell the engineer early, because the extra load affects the joist and beam sizes.
Drains, sewers and neighbours
Many London gardens have a shared or public sewer running across them. Building over or within 3 m of a public sewer usually needs approval from Thames Water before work starts, and the foundations may need to bridge or protect the pipe. Our guide to building over a public sewer covers the process.
The Party Wall etc. Act 1996 often applies too. Building a new wall on the boundary line needs a line of junction notice at least one month before work starts. Excavating within 3 m of a neighbour's building, to a depth below their foundations, needs an adjacent excavation notice, also at least one month ahead. Cutting into a party wall for a steel needs a party structure notice two months ahead.
Building Control and site inspections
Building Control inspects at key stages. The most important structural one is the foundation excavation, which must be inspected before concrete is poured so the inspector can confirm the depth and the ground. They will also want to see the steelwork in place before it is covered.
We send our calculations and drawings to Building Control and deal with any questions they raise, so your builder is not held up waiting for answers. At the end, the completion certificate confirms the work meets the regulations. Keep it with the approved drawings, because a future buyer's solicitor will ask for both.
Tell the engineer straight away if the excavation reveals something unexpected, such as roots, made ground, an old drain or soft clay at the planned depth. A quick decision on site, agreed with Building Control, is far cheaper than discovering the problem after the concrete is poured.
Planning the structural design
Bring the engineer in once the architect's layout is fairly settled, usually after planning permission or confirmation of permitted development. Earlier input helps where the design depends on wide openings or there are large trees nearby, because both can affect what is buildable. A trial pit to expose the existing foundation is sometimes worth doing early on clay sites.
Our extension structural package covers foundations, steel beams, floor and roof, with calculations and drawings ready for Building Control. Every design passes our three-step quality control, with final sign-off by our Head of Engineering. We have designed extensions across London, from the Victorian terraces of Wandsworth to 1930s semis in the outer boroughs.
Frequently asked questions
Do I need a structural engineer for a single-storey rear extension?
In almost all cases, yes. Even a modest single-storey extension needs foundations designed for the ground and a beam over the opening into the house. Building Control will expect calculations for both. The only extensions that might avoid an engineer are very simple lean-to structures that do not open up the existing house, and even then the foundations still need to suit the ground.
How deep do extension foundations need to be in London?
On London Clay, the usual starting point is around 1 m below ground, deeper where trees are close. The final depth depends on the tree species, its height, its distance from the extension and how shrinkable the clay is. Building Control confirms the depth on site at the excavation inspection. On sandy or gravelly ground, shallower foundations may be acceptable.
What is a goalpost frame?
A goalpost frame is a steel beam supported on two steel columns, shaped like a football goal. It is used where an extension opens up a wide section of the rear wall and there is no masonry left to support the beam ends. Each column sits on its own pad foundation. The frame can also provide stability where much of the original wall has gone.
Can I build my extension right up to the boundary?
Often, yes. You can build a wall entirely on your own land up to the boundary, or astride the boundary with your neighbour's consent. Both need a line of junction notice under the Party Wall etc. Act 1996 at least one month before work starts. Planning rules may also limit height and depth near the boundary, so check those with your designer or the council.