Understanding Foundations: The Homeowner’s Guide to Ground, Depth & Design

Foundations rarely get discussed until they become a problem. That’s understandable — they’re underground, invisible, and most homeowners have no obvious reason to think about them until their extension is underway and the Building Control Officer asks a question nobody on site can answer.

This guide covers what foundations are, how they work, why ground conditions control almost every decision, and what you should know before you start any project that involves digging near your property. Getting foundation design right at the outset saves time and money. Getting it wrong mid-project is expensive — sometimes by an order of magnitude.

What a foundation is and what it actually does

A foundation transfers load from your building into the ground. Every wall, floor, column and beam above it generates weight, and that weight has to go somewhere. The foundation spreads that load over the soil beneath it at a level and pressure the ground can support without settling, shifting, or failing.

The key word is “without settling.” All ground compresses slightly under load. What matters is that it compresses uniformly, predictably, and within limits that don’t crack your walls or distort your structure. A structural engineer designing foundations isn’t just designing for strength — they’re designing for acceptable movement.

The three variables that drive foundation design are: what’s on top (the loads), what’s underneath (the ground type and condition), and how the two are affected by what’s nearby (trees, drains, neighbouring structures).

The common foundation types in residential work

Strip foundations are the standard solution for load-bearing wall construction. A trench is dug along the line of the wall and filled with concrete, giving a continuous footing with width and depth determined by the ground conditions and the wall loads. They’re the most commonly specified foundation type for extensions and new builds.

Pad foundations carry concentrated point loads — steel columns or timber posts. Instead of a continuous strip, a discrete block of concrete is poured at each load point. You’ll see pad foundations wherever columns are used and wherever loads are too high or too localised for a strip to be practical.

Raft foundations spread the building load across the full footprint of the building, rather than just along the walls. They’re used where ground conditions are poor, where differential settlement is a risk, or where underfloor heating makes a solid slab desirable. A raft needs careful design because the concrete slab and its reinforcement have to act as a structural plate.

Piled foundations go deeper — usually into load-bearing strata that can’t be reached economically by excavation. Mini-piles are common in constrained urban sites, on poor ground, or where proximity to neighbouring structures makes open excavation impractical. Piles are more expensive and require more specialist design than any of the above, but sometimes they’re the only viable option.

How to assess your ground conditions

The ground beneath your project controls which foundation type is appropriate and how deep it needs to go. You can get a reasonable initial picture from a trial hole — a dug inspection of the ground at the proposed foundation level. Here’s what you’re looking for:

Type I — Rock (sandstone, limestone, chalk, mudstone): Requires mechanical excavation to dig. This is excellent ground. Foundations can generally be shallower and carry high loads.

Type II — Medium-dense gravel or sand: Needs a pick to excavate. A 50mm square wooden peg is hard to drive beyond 150mm. Very good foundation ground.

Type III — Stiff clay or sandy clay: Can be indented slightly by thumb pressure. Very good ground, but clay shrinks and swells with moisture changes, so depth needs to account for this.

Type IV — Firm clay: Thumb makes an impression easily. Good ground, same tree/shrub caveats as stiff clay.

Type V — Loose sand or silty/clayey sand: Can be excavated with a spade; wooden peg easily driven. Ground of this type can work but needs checking carefully.

Type VI — Soft silt, clay or sandy clay: Finger pushed in up to 10mm. This is borderline — could be made to work but needs specialist attention.

Type VII — Very soft silt or clay: Finger pushed in 25mm or more. This is poor ground. Keep digging for firmer material, or consider piled solutions.

The BCO will inspect the excavation before concrete is poured. They’re checking visually that what’s at the bottom of the trench matches what the engineer assumed in the design. If it doesn’t match, the engineer needs to be informed before work continues.

Foundation depth: the rules and when they change

The absolute minimum depth for any spread foundation under Building Regulations is 450mm below ground level. This protects against frost heave — the expansion of frozen soil that can lift foundations over a winter. Most lightly-loaded foundations in good sandy or gravelly ground will be designed at 450–600mm.

Clay changes everything. Clay is expansive — it swells when wet and shrinks when dry, driven by seasonal moisture changes near the surface. In clay, foundations need to be at least 900mm deep to get below the zone of seasonal movement. In many London clay conditions, 1,000mm or more is standard. In areas with significant tree cover or history of drought, foundations may need to go to 1,500mm or deeper.

The critical factor with clay is volume change, not just bearing capacity. Clay can carry load perfectly well at shallow depth — the problem is the annual cycle of expansion and shrinkage near the surface. A foundation caught in that zone will move seasonally, and that movement transmits as cracking, sticking doors and distorted window frames.

Trees, shrubs and the shrinkage zone

A mature tree extracts substantial moisture from the ground through its roots. In clay soils this creates a zone of desiccated, shrunk soil around the root system. If your foundation is in that zone, two things can happen: if the tree is removed, the clay rehydrates and swells (heave); if the tree stays, the clay may be drier than expected and less capable than assumed at shallow depth.

As a rough guide, foundations within a horizontal distance equal to the mature tree height should be assessed carefully. The species matters — oaks and willows have much more aggressive root systems than, say, silver birch. Your structural engineer will flag this and may require deeper foundations or piling to get below the root influence zone.

Shrubs are less critical but not irrelevant. Small shrubs within 2–3m of a proposed strip foundation in clay should be removed before work starts, and the ground given time to recover before excavation begins.

Neighbouring foundations: proximity and party walls

If your extension abuts a shared boundary or neighbouring structure, the depth of your new foundation relative to the neighbour’s existing foundation matters. Where foundations are at significantly different levels — or where your new foundation would excavate below the level of a neighbouring foundation — there’s a risk of undermining that existing foundation.

This triggers a party wall requirement (under the Party Wall Act 1996) in most cases, and your engineer needs to know about neighbouring structures at the design stage. The specification for your foundation may include underpinning, or may require a different foundation type entirely to avoid the proximity problem.

The BCO will check the depth and condition of exposed neighbouring foundations during their site inspection. If what they see differs from what the engineer assumed, work stops until the engineer reviews.

What your structural engineer actually produces

For a foundation on an extension or new build project, a structural engineer produces:

– Foundation design calculations — bearing pressure checks, depth reasoning, sizing calculations
– A foundation drawing — shows plan location of all foundations, depth, width, concrete specification
– Specification notes — covers concrete grade (typically C25/30 or C30/37), reinforcement (if required), and workmanship requirements

The drawing is what goes to the BCO for approval and to your builder for setting out. The calculations are the technical backup — the BCO may ask to see them if there’s doubt about the specification.

For more complex ground conditions — soft ground, high tree influence, proximity to neighbouring foundations — the engineer may also specify a trial pit investigation programme and issue revised drawings once the pit results are known.

Building Control and foundation sign-off

One point that trips up homeowners regularly: a structural engineer does not “sign off” the foundations. The sign-off — the formal approval that the ground is suitable and the foundation design is acceptable — belongs to the Building Control Officer. The engineer designs and specifies; the BCO inspects and approves.

This means your BCO inspection for foundations is non-optional. The BCO needs to visit the site before concrete is poured so they can inspect the excavation. Pouring concrete before inspection is a common shortcut that can result in the BCO requiring you to expose the foundations for inspection regardless, or, in worst cases, requiring re-excavation.

If the BCO is unhappy with the ground conditions they see, they’ll request additional information from your engineer. The engineer can review photographs and site description and advise on whether the design needs modifying. At PorthouseDean this back-and-forth is included — we handle Building Control queries without additional charge and our engineer stays contactable throughout the construction phase.

Common foundation questions

Why do I need 1-metre-deep foundations when my neighbour’s extension only went to 450mm? Your ground conditions may be different (particularly if you’re in a clay area), your loads may be higher, or there may be trees nearby that weren’t a factor on your neighbour’s plot.

Can I reuse existing foundations for an extension? Sometimes — if the existing foundations are of adequate width and depth, and the new extension loads are within their capacity. This needs an engineer to assess. Never assume reuse without checking.

What if foundations come in at a different depth than specified? Tell your engineer before pouring concrete. A quick photograph and description of the ground at formation level is usually enough for the engineer to confirm whether the depth is acceptable or needs adjustment.

Do I need a geotechnical investigation? For most domestic residential extensions into known-condition ground, a desk study plus trial pit inspection is sufficient. Full geotechnical boreholes are generally required on larger projects, on sites with Ground contamination concerns, or where piling is proposed.

The right foundation for your project depends on what’s above it and what’s below it — two things that vary every time. Getting a structural engineer involved before ground is broken is the most cost-effective insurance you can buy.