Structural alterations are building works that change how a home carries its weight — removing or moving walls that hold up the floors and roof above, fitting beams to bridge the gap, and stabilising foundations where the ground or structure has moved. Done properly, they need a structural engineer's calculations, the right beam or lintel, and building control approval. Done badly, they can crack walls, sag floors or, in the worst cases, cause partial collapse.
What counts as a structural alteration
A structural alteration is any change that affects how loads travel down through a building to the ground. "Load" simply means the weight a part of the structure carries — the floors, walls, roof and anything resting on them. If a job changes that path, it is structural.
Common examples include:
- Removing or partly removing a load-bearing wall to open up a space.
- Forming a new opening for a door, window or wider doorway in a supporting wall.
- Installing a steel beam (often an RSJ, short for rolled steel joist) to carry the load above a removed wall.
- Fitting or replacing a lintel above an opening.
- Removing a chimney breast and supporting what remains above it.
- Cutting through walls to join two rooms, or a house to an extension.
- Underpinning to strengthen or deepen foundations.
Not every wall is structural. Internal partition walls that only divide space can sometimes be taken out without beams. The difficulty is telling the two apart, because a wall that looks light can still carry a floor or roof load. That judgement should not be guessed at on site.
Removing load-bearing walls and fitting beams
Done properly, they need a structural engineer's calculations, the right beam or lintel, and building control approval.
When a load-bearing wall comes out, the weight it held has to go somewhere. A beam takes that load and transfers it sideways to supports at each end, then down through the structure to the foundations. This is the core of most "knock-through" projects.
The usual sequence is: an engineer calculates the loads and specifies the beam size and grade; temporary supports (often called Acrow props and strongboys) are set up to hold the structure while the wall is removed; the beam is lifted into position; and permanent supports are built at each end. Only then are the temporary props taken down.
The supports at the beam's ends matter as much as the beam itself. Each end sits on a "bearing" — a solid pad, usually concrete or engineering brick (a dense, strong brick) — sized so it spreads the load without crushing the masonry below. If a bearing is too small or sits on weak material, the beam can settle and crack the wall.
Steel is common for longer spans because it is strong for its depth, but it is not the only option. Engineered timber and concrete beams are used in some situations. The engineer chooses based on the span, the load and how much depth is available.
Lintels do a smaller version of the same job. A lintel bridges a single opening — a doorway, window or hatch — carrying the masonry directly above it. For modest openings a standard galvanised steel or concrete lintel is often enough, but a wide opening, or one carrying a heavy load above, may need a designed beam rather than an off-the-shelf lintel. Cracking above a window is sometimes a sign that an existing lintel has failed or was never adequate.
Two further points are worth knowing. First, removing internal walls can reduce a building's resistance to sideways forces, so the design has to keep the structure braced. Second, fire and sound separation can be affected when rooms are joined, particularly in flats, which has its own building regulations.
When underpinning becomes necessary
Underpinning strengthens or extends existing foundations downwards so they reach more stable ground or carry more weight. It is not part of every alteration — it is a specific remedy for foundation problems or for added loads the original footings cannot take.
Typical triggers include:
- Subsidence, where the ground beneath the foundation moves downward, often shown by widening, diagonal cracks.
- Foundations that are too shallow for the soil, sometimes worsened by nearby trees drawing moisture from clay.
- Building an extra storey or heavy structure that adds load beyond the original design.
- Digging out a basement or lowering a floor next to an existing footing.
The most common method is mass concrete underpinning, where short sections beneath the existing foundation are dug out one at a time and filled with concrete, working in a planned sequence so the building is never undermined all at once. Other techniques, such as mini-piling or resin injection, suit particular soils and access conditions.
Underpinning is disruptive and should be specified by an engineer after the cause of movement is identified. Treating the symptom without diagnosing the cause — for example, underpinning when the real issue is a leaking drain washing out the soil — wastes money and may not work. Where subsidence is involved, it is also worth checking the position with buildings insurance before work begins.
Why a structural engineer is involved
A structural engineer is the person who decides what is safe. They assess the existing structure, calculate the loads, and specify the beams, bearings, lintels or underpinning needed. Their drawings and calculations are what a builder works to and what building control checks against.
Most structural alterations require building regulations approval, which is separate from planning permission. Building control — run by the local authority or an approved inspector — reviews the design and inspects key stages, such as before beams are concealed. The engineer's calculations support that approval; a builder alone cannot usually sign off the structural design.
The engineer and builder have different roles. The engineer designs the solution; the builder carries it out. On site, the temporary support, the size and seating of bearings, and the sequence of work all need to follow the design. If anything is found during the work that differs from assumptions — unexpected loads, poor existing masonry, a hidden chimney flue — the engineer should be asked before continuing.
For a homeowner planning this kind of work, it is reasonable to ask whether a structural engineer has produced calculations, whether building control has been notified, and how the temporary support will be arranged. Those three questions cover most of what keeps a structural alteration safe. Where a shared wall with a neighbour is affected, the Party Wall etc. Act 1996 may also apply, requiring notice to be served before work starts.
Reviewed: June 2026