Eccentric vs Concentric Foundations: A UK Homeowner's Guide

Concentric and eccentric are not two different foundations — they describe where a wall's load lands on its foundation. Concentric means the line of action of the load passes through the centre of the foundation, so the pressure under it is spread evenly edge to edge. Eccentric means the load sits off-centre, so one edge carries most of the pressure and the other barely any. The distinction decides whether a wall's existing foundation is good enough, or whether new "special foundations" have to be designed. You'll hear both phrases the moment a structural engineer starts drawing up work to a party wall, a side return, or a new outbuilding — "we'll need an eccentric foundation here," or "that wall sits on a concentric footing" — so here is the whole idea, in plain English.

That one question — centred or off-centre — decides whether a wall's existing foundation is good enough, or whether new foundations have to be designed. It's also the detail that the Party Wall Act wants you to put in writing in your notice, and the thing a Building Regulations inspection will turn on. So if you're about to hang a steel beam in a party wall, raise a wall to take a new roof, or build an outbuilding against a boundary, this is the piece that will let you follow your engineer's drawings and have a useful conversation with your party wall surveyor. I'll keep the structural bits at a guidance level — the actual design is the engineer's job — but you'll leave here able to follow the logic of a plan.

What Is a Foundation Actually Doing?

Before the two names make sense, it helps to see what a foundation is actually doing. A foundation's one job is to take the load of everything above it — the wall, the floors, the roof, and whatever the wall is asked to support — and spread that load out into the ground over a wide enough area that the ground doesn't get crushed, settle unevenly, or let the wall tip.

The measure that matters is the pressure under the foundation strip: how much load is pressing on each square metre of soil immediately below it. There are two limiting cases, and they're the whole of the distinction. If the line of the load lands right through the centre of the strip, the pressure spreads evenly from side to side. That's the concentric case. If the line of the load drifts off to one side, the pressure under the strip becomes uneven — one edge is hard pressed while the other is hardly loaded at all. That's the eccentric case. Everything else about foundations is about keeping that pressure inside what the ground can safely carry, and making sure the wall doesn't tip when the load is off-centre.

So "concentric" and "eccentric" are not two kinds of foundation you can order from a brickwork catalogue. They're a description of a loading condition. The same brick wall on the same concrete strip is either concentric or eccentric depending entirely on where its loads are coming from — and that's the trap people fall into, because it's easy to assume a wall that's been standing for a hundred years is permanently fine, right up until you hang something from it.

What Does "Concentric" Mean?

A load is concentric when its line of action passes through the centre of the foundation. For an ordinary straight wall that stands vertically over its strip, this is the default: the wall's own weight, plus whatever sits directly above it — floors, roof, the odd heavy shelf — all load straight down the wall's line. The resultant ends up right over the middle of the strip, and the pressure under the foundation is spread evenly from edge to edge. That's the concentric case, and it's the easy one, because the ground under the whole width of the strip is doing its share.

Most low-rise terraces and semi-detached houses are built on this assumption, and for a normal party wall carrying a normal storey height it holds comfortably. It's also why a standard 9-inch party wall on its normal footing looks unremarkable on a drawing: the loads are centred, and the footing is sized for an even pressure. When a plan just shows a wall on a simple strip with no special detail, that's the concentric, low-drama case — and it's the one you don't lose sleep over.

What Does "Eccentric" Mean — and Why Does It Bite?

An eccentric load is one whose line of action lands off-centre on the foundation. It doesn't have to be wildly off — even a moderate offset makes one edge of the strip carry most of the pressure, and the far edge carries little or none. The practical consequence is that the effective area of ground doing the work shrinks, so the ground has to carry more per square metre than it would under the same total load in the concentric case. In weaker soil that extra pressure can push the ground past its safe limit, and the wall will settle, tilt, or crack.

The reason eccentricity bites so often in domestic work is that it's almost always created by the work itself, not by the existing house. Here are the situations I see most:

What Does a Side Return Beam Do to the Party Wall?

This is the classic. You cut a hole through a wall to run a steel beam (an RSJ) and then build a side return out of it. The beam's end reactions, and the weight of the new structure beyond it, push the resultant load clear of the old wall's line — sometimes well clear of the existing footing, which may even run in a different direction to the new beam. The engineer's job is to carry those loads somewhere the ground can take them: typically new padstones under the beam ends, or a new ground-bearing strip or pad, or a combination, so the added load doesn't ride on the old party-wall footing alone. That new element is a "special foundation" in the Party Wall Act's language.

What About a New Wall or Outbuilding on the Boundary?

If you're building a new wall right on the line of junction — say for an outbuilding, a boundary wall, or a new lean-to — the whole load is new and concentrated at a point where there may be little or no existing support. Here "concentric" and "eccentric" are less the issue than simply sizing the new foundation for the load it must carry, and checking where it lands relative to any neighbouring structure. But the same logic applies: get the resultant over the footprint, and keep it there under the full load.

Can Uneven Ground Make a Wall Go Eccentric?

On a sloping site, or where the ground level is different on the two sides of a boundary, even a wall's own self-weight can end up with a resultant that's slightly off-centre. In good ground this is usually nothing; in soft fill or compressible clay it can be exactly the thing that makes a wall lean over time. This is the kind of subtle eccentricity that a ground investigation exists to catch before it becomes a cracked wall.

Do Raising, Thickening, or Underpinning Change the Loading?

When a party wall is raised, thickened, or underpinned, you're changing where and how deep the foundation sits, so the new work has to be designed to carry the loads it will carry once complete. Any temporary supports put in during the work — dead shores or Acrow-type props — exist to keep the wall safe while the permanent design is put in place. The end state is what has to be structurally sound, and that's a design decision, not a DIY one.

How Much Do Ground Conditions Matter?

The load position is only half the story. The other half is how capable the ground is. The same eccentric loading that's perfectly workable over dense, hard clay near the top of the London clay can be critical over soft made ground or a deep fill. Ground bearing capacity — how much load a square metre of the soil can safely carry — varies enormously across Greater London, and it's the reason a competent ground investigation (desk study plus boreholes to check fill depth and strength) is standard on London projects involving party walls and extensions. Without knowing what's under the strip, nobody can honestly say whether an eccentric load will cause trouble; with it, the engineer can size the foundation confidently and you can stop guessing.

How to Read the Plan: Concentric or Eccentric?

You do not need to do any structural arithmetic to follow a foundation plan, but a few things are worth looking for, because they are usually the difference between a wall that is still carrying its load down the middle and one that the new work has quietly turned off-centre. First find the wall you are working on and the strip it sits on. Then ask where the loads are actually coming from. If every load — the wall's own weight, the floors directly above it, the roof — points straight down into the middle of that strip, the plan is describing a concentric wall, and you should expect a simple, continuous strip footing with nothing unusual bolted onto it. That is the version of the plan you want if no new load is being introduced.

The moment the drawing shows something coming in from the side — a steel beam landing on a block or padstone off the face of the wall, a cantilever, or a new element whose load lands clear of the existing strip — that is the eccentric case, and it is exactly where you should see new, separately drawn foundations. On a plan these show up as padstones, small pads, or a short strip that does not line up with the wall's existing line and often runs at right angles to it. If your engineer's drawing is full of these little details clustered around the new work, that is not fussiness: it is the engineer carrying a load that no longer lands where the old foundation was ever designed to take it, and it is the detail that has to make it into your notice.

There is a useful rule of thumb that is not a calculation and involves no figures at all: if the new work does not change where the load lands, you are on the concentric path; if it does, you are on the eccentric path, and something new has to be designed to catch the load where it lands. Everything else in the plan is the engineer working out how big that "something new" has to be — which is exactly the part that is theirs to produce, and yours to make sure has been described to your neighbour.

What Does the Party Wall Act Require My Notice to Say?

Now the part that makes these terms more than a structural curiosity. The Party Wall etc. Act 1996 is explicit about this. When you serve a notice for work to an existing party wall or party structure, the notice must give the nature and particulars of the work, including, where special foundations are proposed, plans, sections and details of the special foundations together with reasonable particulars of the loads they will carry. That is the Act's job and no more: it governs the notice and any dispute, not the structural design. Whether the work can safely carry the loads once it's built is a calculation for your structural engineer — and it's the detail that has to sit behind the notice, in the drawing.

Why does that matter to you? Because a party wall notice that just says "we're going to do structural work to the party wall" isn't enough. It has to tell your neighbour — and any surveyor later appointed to make an award — exactly what foundations are involved and what they're going to carry, so the impact on the shared wall and the ground can be judged. Eccentric versus concentric is the difference between a wall whose existing footing keeps doing its job and one that suddenly has to do a different job, and it's the single most useful fact in that notice. It's the information that turns a vague "structural work" notice into one your neighbour can actually assess, and it's the detail that prevents arguments about what was and wasn't disclosed.

The practical rule is simple. If your proposed work doesn't change the load on a party wall or its foundation, you're on the concentric, low-drama path and a straightforward notice for the work is the order of the day. If it does — if there are new or additional special foundations, or the way the load is carried changes — then the notice has to spell those out, and it's the point where your structural engineer and your party wall surveyor need to be working together, because one draws the design and the other makes sure the legal notice and the process keep up with it.

Do I Need Planning Permission, Building Regulation Approval, and a Party Wall Notice?

It's worth keeping the three processes separate, because homeowners constantly blur them. Planning permission is about the look and effect of the development on the neighbourhood — the massing, the appearance, the impact — and it does not judge your foundations. Building regulation approval is the separate process that covers whether the structure is sound: before work goes ahead, a competent structural engineer produces a design for the foundations, sized for the loads they carry on the ground that's actually there, and that drawing becomes the document of record. The Party Wall Act is a third, independent obligation: notifying your neighbour and resolving any dispute. It runs alongside the other two and replaces neither. So an eccentric foundation is a structural design issue first — settled by the engineer's drawing and reviewed under building regulation approval — and a Party Wall Act notice issue second. The same drawing feeds both, which is exactly why getting the engineering right first makes everything downstream easier. We do not interpret the technical requirements for you; your engineer and your building control officer own the figures, and our role is to make sure the legal notice around them is right.

What If the Existing Foundation Has Already Gone Eccentric?

So far we've assumed your new work is what tips a wall from concentric to eccentric. But the distinction also runs the other way, and it's the one that catches homeowners by surprise, because the problem predates the work they're only just planning. A wall can already be carrying an off-centre load — or the ground under one side of its foundation can have weakened — and the tell is rarely in the new work at all. It's in the building: a crack that's noticeably wider at the top than it is at the bottom, a wall that's slightly out of plumb, a floor that slopes, or a doorway that's begun to jam. Each of those is a signature of a wall losing support on one side, which is eccentricity that's been working quietly for a while, not a fresh event.

If that's what you're seeing, the honest answer is that it's a structural diagnosis question first — for your structural engineer to work out what the existing foundation is doing and whether it can keep doing it — and, if remedial work is the answer, a building regulation approval matter second. It is not something a party wall notice on its own resolves, because a notice tells your neighbour what you intend to do; it does not make a weak foundation sound. The common remedial route is underpinning: extending or reinforcing the existing foundation so the wall is supported again where it's losing support. And here's where the three obligations genuinely collide, because underpinning a party wall is itself notice-able work. The engineer diagnoses and designs the repair, building regulation approval covers whether that repair is structurally adequate, and the party wall notice covers your neighbour. Get the diagnosis right before anything else, or you'll be designing around a guess.

A Worked Example: A Side Return on the Corner of a London Terrace

Let's put the whole idea on a page, because it lands much better as a specific case than as a set of definitions. Picture a two-storey terraced house sitting on London clay, where a rear party wall carries the two storeys above it the way the wall did when it was built. For most of its life that wall was a textbook concentric case: its own weight and the storeys directly above it all loaded straight down through the middle of its strip, and the footing underneath was sized for exactly that, even pressure from edge to edge. Nobody in the room is thinking about eccentricity, because there isn't any.

Now you add a side return off that party wall. The beam that supports the new roof lands its end reaction on a padstone that sits off the face of the wall, well clear of the old strip — and in soft ground at the corner it may need a small pad or a short pile to reach something that will actually take it. The instant you do that, the party wall is no longer carrying a purely concentric load: at the level of the beam, a significant share of the load is being handed over to a new foundation at one side of it. That is eccentricity created by your work, and it is the exact moment the Party Wall Act wants you to describe it in writing. The engineer's drawing shows the padstone and the new element; the party structure notice then carries the same information to your neighbour — the nature of the work, the special foundations, and the loads they carry. Nothing is exotic about it, and that's the reassuring part. The wall doesn't stop working; it simply takes on a different job, and once you can see which job it is, the engineering, the notice, and the conversation with your neighbour all follow in an order that's easy to keep straight.

Key Takeaways

• Concentric means the load lands in the centre of the foundation; eccentric means it lands off-centre. It's a description of the loading, not a different type of foundation.
• The same wall can be concentric or eccentric depending on what's asked of it — hang a beam or cantilever from it and it's no longer carrying a centred load.
• Eccentric loading concentrates pressure on one edge of the foundation, so the ground has to do more work per square metre and the wall is more prone to tilt or crack — especially in weaker or made ground.
• The most common domestic triggers are steel beams for side returns, new boundary walls or outbuildings, and underpinning or raising. These are when "special foundations" get designed.
• Ground conditions decide how much it matters, which is why a competent ground investigation is standard before a foundation is sized.
• For work to an existing party wall, the Act requires your party structure notice to state the nature and particulars of the work — and, where new special foundations are proposed, details of the foundations and the loads they'll carry. Eccentric versus concentric is the detail that tells a neighbour what will actually happen to the wall they share.
• Planning, building regulation approval, and the Party Wall Act are three separate obligations — all three need to be met. An eccentric foundation is a structural design question first (the engineer's drawing, reviewed under building regulation approval) and a party wall notice question second.

What Should You Do Next on Your Project?

If your project involves work to a party wall or a new boundary structure, the sequence that keeps it clean is this. First, get the structural design — and get it early, because the foundation detail drives the notice. Second, if the ground is uncertain, commission the ground investigation before the foundations are finalised. Third, once the engineering is settled, have your party wall surveyor prepare a notice that spells out the special foundations and the loads they carry, rather than a generic "structural work" line. A party wall surveyor is usually a chartered surveyor; RICS is the profession's regulator and its find-a-surveyor service can help you locate one. And fourth, make sure your builder actually knows what the eccentric elements are, because a builder who doesn't understand why a padstone is under the beam end is a builder who will shortcut it. A short walk-through of the drawings with all three parties — engineer, surveyor, builder — before the work starts resolves more problems than any amount of paperwork afterwards.

Most of the time the answer is unglamorous: the existing wall keeps its concentric footing, one new foundation catches the added load, and the notice records it. But the difference between knowing that in advance and finding out on site is the difference between a smooth job and a dispute. When the loads don't line up, it's worth getting the foundations designed properly and the notice written to match — and if you'd like a second pair of eyes on whether your drawings and your notice are telling the same story, that's what we're here for.