Foundation design is one of the areas where party wall questions become technical very quickly.
Owners often ask: “Can my foundations project under my neighbour’s land if I’m building on the boundary?”
The Party Wall etc. Act 1996 does provide a limited right to project foundations in certain circumstances—but the key point is that this is not an automatic entitlement. The wording of the Act is important, and modern structural design can usually avoid the need to project.
Below is a practical explanation of how this works, why the word “necessary” matters, and what typically happens when you build a wall up to the boundary line.
Building a wall up to the boundary line: Section 1(5)
Section 1(5) deals with a common scenario: the building owner wants to construct a new wall on the line of junction but placed entirely on their own land.
In plain terms, this is the “build up to the boundary but don’t straddle it” option. It is often the preferred route where a neighbour is unlikely to agree to a wall astride the boundary, or where you want to keep construction clearly on your own land.
The design consequence: eccentric foundations
When a wall sits right at the edge of your land, the foundation is often required to be an eccentric foundation (sometimes thought of as an offset/edge foundation). This is because the wall load is near the boundary edge, so the foundation design has to manage stability without simply spreading equally on both sides.
In practice, engineers commonly design foundations that:
- remain entirely within the building owner’s land, and
- still safely support the wall load and any associated structure.
This is where careful engineering and early coordination matters.
Projecting foundations under the neighbour’s land: Section 1(6)
Section 1(6) allows the building owner, in certain cases, to place below the adjoining owner’s land projecting footings and foundations that are needed for construction of the wall.
But the crucial phrase in the Act is “as are necessary”.
That wording matters because it frames the right as a limited allowance, not a default design choice.
What “as are necessary” means in practice
In most modern projects, structural engineers can design around boundary constraints so that foundations:
- do not project, or
- project only in a highly limited way (if genuinely unavoidable), or
- are reconfigured entirely (for example through eccentric design, alternative reinforcement, or other structural solutions suited to the build)
So while Section 1(6) exists, it does not mean:
- “you can always project under your neighbour’s land whenever it’s convenient,” or
- “a standard strip footing can automatically extend beyond the boundary.”
The test is necessity, and in the vast majority of cases, necessity is difficult to justify where viable alternatives exist.
Why this matters: neighbour relations, design risk, and dispute potential
Foundation projection is one of the quickest ways to create neighbour tension, because it involves:
- working beneath land that is not yours
- concerns about excavation stability and risk
- practical access and sequencing issues
- future disputes if the neighbour later wants to develop
Even where the Act provides a route, surveyors and engineers will generally seek the most straightforward solution—one that respects the boundary and reduces conflict and complexity.
That is why it’s so common for competent designs to keep foundations within the building owner’s land.
The typical approach on real projects
Step 1: decide the wall position early
- wall astride boundary (requires neighbour consent)
- wall up to boundary and wholly on your land (Section 1(5))
Step 2: instruct engineering with the boundary constraint in mind
If you tell the engineer early that the foundation must stay inside your title, they can design accordingly.
Step 3: avoid default “strip footing assumptions”
A basic strip detail may not be appropriate right at the boundary. A boundary wall commonly needs:
- an eccentric/offset footing
- specific reinforcement
- careful set-out and quality control
Step 4: document clearly in notices and drawings
If the wall and foundations are being designed to stay within your land, make that clear in the party wall notice drawings. Neighbours are more comfortable when the design is transparent.
Key takeaway
The Party Wall Act recognises that projecting foundations can sometimes be required, but it only permits projections to the extent they are necessary. In most cases, modern engineering can design foundations to avoid crossing under the adjoining owner’s land—particularly where a wall is built up to the boundary line under Section 1(5), commonly using an eccentric foundation design.
Getting this right early reduces risk, avoids delay, and keeps neighbour relations far smoother.
Need guidance on boundary wall foundations under the Party Wall Act?
Email mail@howorth.uk or call 07794 400 212. Share your proposed wall location (astride or wholly on your land), any foundation sketches, and whether an engineer has been appointed. We can advise on the party wall implications and the best practical route to keep your project compliant and straightforward.