Are Foundations Getting Deeper? A Look at Modern Building Practice and Party Wall Implications

At Peter Barry Surveyors, we have to consider how the foundations of a client’s kitchen extension interact with neighbouring properties and the Party Wall Act. It’s a fair question, especially when comparing older properties with modern builds. And while the regulations themselves haven’t explicitly mandated deeper foundations, the reality on site tells a different story.

Whether you’re planning an extension, underpinning works, or simply trying to understand your obligations under the Party Wall Act, foundation depth is more than just a technical detail — it can shape the entire approach to compliance.

What’s Changed Since 2010?

The short answer: quite a bit.

Modern foundation design is governed by British Standard 8004:2015, which replaced earlier guidance and placed greater emphasis on site-specific conditions. This shift has led to more conservative foundation depths, especially in areas with shrinkable clay soils, nearby trees, or variable groundwater levels.

In practice, this means that while a strip foundation in 2010 might have been 600mm to 1.0m deep, today’s equivalent is more likely to be in the 900mm to 1.2m range — and often deeper if trees are present. Trench fill foundations, once typically around 1.0m, now regularly exceed 1.5m. Raft foundations have also thickened, with depths increasing from 300–600mm to 500mm–1.0m.

These changes aren’t just about soil mechanics. They reflect broader trends in construction: heavier structural loads, more ambitious designs, and a growing awareness of climate-related risks like frost heave and subsidence.

Why It Matters for Party Wall Notices

This brings us to Section 6 of the Party Wall etc. Act 1996, which deals with adjacent excavation. The section applies when a building owner proposes to excavate within 3 metres of a neighbouring structure and to a depth lower than its foundations.

For Victorian or 1930s properties, this is usually straightforward. These buildings often sit on shallow strip foundations — sometimes as little as 450mm deep — meaning that almost any modern excavation will trigger Section 6. A new extension with trench fill foundations at 1.2m? That’s a clear case for serving notice.

But for post-2010 properties, things get more nuanced.

Modern homes are typically built on deeper foundations, especially if the site conditions demanded it. If the adjoining owner’s property was constructed with trench fill foundations at 1.5m, and the proposed works involve excavating to a similar depth, Section 6 may not apply — unless the new excavation goes deeper or closer than the thresholds set out in the Act.

This creates a grey area. Surveyors must assess not just the proposed depth, but the likely depth of the adjoining foundations — which may not be visible or documented. In some cases, trial pits or structural drawings are needed to confirm whether notice is required.

Practical Implications for Owners

If you’re planning works near a boundary, it’s essential to consider both the depth and proximity of your excavation. Even if your builder assures you that the foundations are “standard,” that doesn’t mean Section 6 won’t apply.

Here’s a quick comparison:

If in doubt, serve notice. The cost of compliance is modest compared to the risk of proceeding unlawfully — especially if damage occurs or relations with neighbours sour.

Final Thoughts

Foundation depth may seem like a technical footnote, but it’s increasingly central to how we interpret and apply the Party Wall Act. As building practices evolve, so too must our understanding of what constitutes “deeper excavation.”

At Peter Barry Surveyors, we’re here to help you navigate these complexities — whether you’re working on a Victorian terrace or a modern infill plot. If you’re unsure whether Section 6 applies to your project, or you need support preparing notices and drawings, feel free to get in touch on 020 7183 2578 or via email.