Extension Foundations Explained

The foundation is the only part of an extension designed specifically for your plot. The walls, the roof and the glazing are all variations on things built a thousand times before. What is under the ground at your address is not, and that is why the foundation is the item most likely to move the price, the programme and the risk. This guide is about how the decision gets made, by whom, and on what evidence, rather than about any single number.

What a foundation is actually doing

Two separate jobs, answered by two separate dimensions, and people routinely confuse them.

The first job is spreading load. A wall concentrates a great deal of weight along a narrow line, and most ground cannot take that concentration without squashing. The foundation widens the contact area until the pressure falls within what the soil can carry. That is a strength question, and the answer is the width of the foundation, set by the load per metre of wall against the bearing capacity of the ground beneath it.

The second job is staying still. Ground near the surface moves, and a building sitting on moving ground moves with it. The foundation has to reach down past the layer that changes, into ground whose moisture content stays broadly constant through the year. That is a stability question, and the answer is the depth.

Almost every foundation problem on a domestic extension is a depth problem rather than a width problem. Essex clay has perfectly respectable bearing capacity. What it does not have is dimensional stability, and it is the moving that cracks buildings.

Why depth is a moisture question

Clay changes volume with its water content, and the depth at which that stops is the depth you are looking for.

Approved Document A, which covers structure, treats 450mm below finished ground level as the bare minimum for a strip foundation, and that figure exists purely to get the concrete clear of frost. Where the ground is a clay subject to volume change it asks for more, with 750mm given as a minimum rather than as a design, and it says plainly that the figure has to increase to account for vegetation and trees.

Those are floors, not answers. A regulation minimum tells you the point below which a foundation is certainly wrong. The working figure on the clays around Colchester is meaningfully deeper than either number, and it is set by three things acting together: how plastic the clay is, what trees are within reach, and whether any of those trees have recently come down or are about to. The practical depth, and how it is arrived at, is dealt with in how deep extension foundations need to be.

The mechanism underneath all of it is worth holding onto, because it explains every decision downstream. Wet clay swells, dry clay shrinks, and a mature tree dries the ground far beyond the spread of its branches. Remove the tree and the process reverses over years as the clay recovers water it has not held for decades, which is heave, and heave lifts. The behaviour of the local subsoil is covered at length in building on Essex clay.

The family of solutions, and the problem each one solves

There is no best foundation. There is the one that answers the specific difficulty your site presents.

Trench fill is the default answer on this ground for a reason, and what it involves is set out in what trench fill foundations are. The one worth a caution is the raft. A raft works well under a standalone building, but an extension is not standalone: it is attached to a house founded at a different depth on a different date, and pairing a raft with a shallow Victorian footing invites the two to move independently. It gets used on extensions, but it is a considered choice rather than a default.

Who decides, and at what point

The drawing states an intention. The trench states the fact, and the trench wins.

The sequence runs roughly like this. Somebody looks at the site, the trees, the age of the house and the local geology and forms a view. Where there is doubt, a trial hole or a proper site investigation turns that view into information about your ground rather than about ground in general. The structural engineer then designs the foundation, and the design normally names a depth with the words “to be confirmed on site” attached to it, because no engineer can see through soil.

Building Control checks the design if you have gone the full plans route, and then inspects the excavation before any concrete is poured. That inspection is the moment the depth becomes real. The inspector looks at what the trench has actually exposed, and can require it to go deeper. This is not a failure of the design and it is not a builder inventing extra work. It is the system working as intended, and it is the last point at which a foundation can be corrected cheaply.

Two consequences follow. Full plans is the better route on anything with a significant foundation question in it, because a disagreement resolved on paper costs nothing. And any quotation that gives you a fixed foundation price with no stated depth assumption is not a price, it is an opening position.

The existing house is part of the design

You are not founding a building. You are founding an addition to a building that was founded differently, decades ago, by somebody with different rules.

Older houses around Colchester sit on shallow footings. A Victorian terrace commonly has a few courses of brick corbelled out below the wall at perhaps half a metre, and nothing else. Interwar semis are usually a little deeper, on a narrow concrete strip. Neither was designed against the tree that has grown up since.

Your new foundation will almost always be deeper than the old one, and that creates two problems at once. The first is physical: excavating alongside a shallow footing removes the ground supporting it, so the dig is sequenced in short lengths and backfilled as it goes rather than opened as one long trench. The second is behavioural. Two structures founded at different depths respond differently to the same dry summer, and the junction between them has to accommodate the difference.

That is why the connection between old and new is a deliberate decision rather than an afterthought. Tying the new masonry rigidly into the old works where the two will move together and causes cracking where they will not. The alternative is a designed movement joint at the junction, which looks like a defect to anybody who has not been told about it and is in fact the thing preventing one.

Boundaries, and your neighbour’s ground

A foundation near a boundary stops being purely an engineering question. Digging within three metres of a neighbour’s building or structure, to a level below their foundations, triggers a notice under the Party Wall etc. Act 1996, and a shallow footing next door means an ordinary trench can cross that line easily. There is a second trigger at six metres for deeper excavations.

There is also a restriction that catches designs rather than programmes. Foundations containing reinforcement cannot be placed on a neighbour’s land without their prior written consent, and reinforcement is exactly what an engineer specifies to bridge a drain or a soft spot near a boundary. Where consent is not forthcoming, the foundation is redesigned to stay inside your own line, which costs footprint. Both points, and the notice periods that go with them, are in the Party Wall Act explained.

What else is down there

Gardens have a history, and most of it is buried at the depth you need.

Drains are the most common complication. A foul run crossing the footprint has to be dealt with rather than concreted over, and where a public sewer is involved the water company’s build over process applies with its own timescale. Where a drain runs alongside the foundation, the trench is filled so the pipe is neither undermined nor loaded, and where the foundation must cross the line of a drain, it is reinforced to span across it. None of that is unusual. All of it needs designing before the digger arrives.

Made ground is the other frequent find: demolition rubble, a filled pond, spoil from earlier building work, ash and clinker from when everyone burned coal. It has no reliable bearing capacity and you cannot found on it, so the foundation goes through it to natural ground beneath. A metre of old fill across a plot doubles the dig with nothing else having changed.

Sloping sites bring their own rule. Foundations step down the slope rather than following it, in increments no greater than the thickness of the foundation itself, with a generous overlap of concrete at each step so the two levels act as one. It is straightforward when it is designed and awkward when it is improvised.

The concrete is a specification, not a commodity

Ground conditions decide the mix as well as the shape. Some clays and a good deal of made ground contain sulfates, which attack ordinary concrete over years and are the reason soil testing includes a sulfate assessment on suspect sites. Where the result requires it, the mix changes to a sulfate resisting specification.

Water is the other variable. A trench that fills with groundwater has to be pumped and poured promptly, and a trench left open for a week in Essex clay softens at the base and often has to be dug out again to reach sound material. The gap between excavation and pour is a quality issue, not a scheduling convenience.

Designing for heave, which is the expensive failure

Shrinkage settles a building. Heave lifts it, and buildings are far worse at being lifted.

Where a tree has recently been felled, or is coming down, or where a large tree stood on the plot within living memory, the clay beneath is drier than its natural state and will slowly recover. The design has to allow that recovery to happen without the building fighting it.

Two measures do most of the work. A compressible layer is placed against the vertical faces of the foundation, so swelling clay can move sideways rather than gripping the concrete and dragging it upwards. And the ground floor is suspended over a void or a compressible former rather than bearing on the ground, so the slab is not pushed up from below. Both are cheap at design stage and impossible afterwards.

How to read the foundation line in a quotation

Because the depth is confirmed at the trench rather than at the desk, the honest way to price it is with the assumption stated and the variation agreed in advance. What you want to see, in writing, is the depth allowed, the foundation type, the basis for both, and a rate for additional depth if the excavation inspection calls for it. That way nobody is negotiating in a hole in the rain, and the figure you are comparing between builders is a like for like one.

What the work itself involves, from setting out through to the finished insulated slab, and what it costs around Colchester, is covered on the extension groundworks and foundations page.

We look at the ground before we price it, dig a trial hole where the site is uncertain, and have the foundation designed off what is actually there rather than off an assumption. If you want a realistic view of what your plot is likely to demand before you commit to a design, the survey and the quotation cost nothing.

Foundation design is specific to a site and is a matter for the structural engineer and Building Control on your own project. Treat this as an explanation of how those decisions are made rather than as a specification for your extension.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.