Extension Groundworks & Foundations

Why the ground is the biggest variable on the job

Everything above the slab is reasonably predictable. Everything below it is not.

Above the slab, an extension prices fairly tightly. Brick and block go up at a known rate. A steel beam costs what a steel beam costs. Windows come off a schedule. Two competent builders pricing the same drawing will usually land within a few thousand pounds of one another.

Below ground there is no such agreement, because there is no shared knowledge of what is down there. One builder assumes a metre of dig. Another has walked the garden, looked at the oak two doors down, and allowed for two and a half. Both write one figure at the bottom of a page. One of those figures is a price and the other is an opening position.

Groundworks are also the one element that cannot be value engineered later. You can change the kitchen, downgrade the doors, paint it yourself. You cannot make the ground shallower. Whatever the clay and the trees require, the extension needs before a single brick is laid.

So the first question to put to anyone quoting your job is not what it costs. It is: what foundation depth have you allowed, how did you arrive at it, and what happens to the price if it needs more. A builder who has thought about it will give you a depth, a foundation type and a mechanism for dealing with the difference. A builder who has not will change the subject.

Trees, and the zone of influence

A mature tree is a pump, and it dries the ground far further out than its branches reach.

Across a summer a mature broadleaf draws a very large volume of water out of the soil and puts it into the air. The clay around its roots shrinks as that happens. Come autumn the tree stops, rain returns, and the clay swells back, every year, for as long as the tree is there.

The two things people get wrong are both about distance. The first is assuming roots stop where the canopy stops. They do not. Roots commonly extend well past the spread of the branches, and the drying effect reaches further still, because water migrates through clay towards the dry zone the roots create. As a rule the zone of influence runs out to something like the tree’s mature height, and further for the thirstiest species on the most plastic clay.

The second is judging by the tree’s current size rather than its mature size. A twelve year old oak in a neighbouring garden is a large problem that has not arrived yet, and the foundation has to assume it gets there.

Species matters enormously. Oak, poplar, willow and elm cause the most trouble around here, because each combines high water demand with real mature height. Poplar and willow are the worst for sheer volume of water. Oak is the one most likely to be protected and therefore immovable. Depth is set by species, mature height and distance together. The wrong combination puts you at two and a half metres or more, or off strip foundations altogether.

Protected trees, and what you are allowed to touch

The tree causing your foundation problem may be one you are not permitted to prune, let alone remove.

A Tree Preservation Order is made by the council and makes it an offence to cut down, top, lop, uproot or wilfully damage the tree without written consent. It applies wherever the tree stands, including in your own garden, and the penalties are serious. Consent is applied for separately from planning and it is not a formality. An application to remove a healthy protected oak because it is inconvenient is unlikely to be looked on kindly.

Conservation areas carry their own control even where no Order exists. Trees above a modest stem diameter are protected by virtue of standing in the conservation area, and you must give the council written notice of proposed works and then wait six weeks before doing anything. That window lets the council make an Order if it wants to keep the tree.

Roots are the part people forget. Severing major roots during excavation can damage or destabilise a protected tree as effectively as taking a saw to the trunk, and it carries the same liability. Where an extension goes in near a protected tree, the dig needs planning, and sometimes the foundation type is chosen to reduce how much root is disturbed.

Check the position on your address before design work starts. It changes what is possible, not just what it costs.

Made ground, old foundations and other surprises

Gardens have a history, and a lot of it is buried at exactly the depth you need to dig.

Made ground is anything placed rather than deposited by nature: demolition rubble, backfilled ponds and ditches, spoil from previous building work, old garden tipping, ash and clinker from when everyone burned coal. It is common on plots built on more than once, and it has no predictable bearing capacity.

You cannot found on it. The foundation has to go through it to competent ground beneath, which is why a site with a metre of old fill across it can double the depth of the dig with nothing else changing.

Old foundations turn up regularly: outhouse footings, the base of a demolished garage, a former boundary wall, a brick soakaway, a filled well. Some can be broken out. Some are easier to bridge with a reinforced foundation designed to span across them. Either way it is an engineering decision, not one for the person on the machine.

Services are the other find. Water, gas, electricity, telecoms and shared drains rarely run where the drawings say. Everything is scanned and traced before the first bucket goes in, and we hand dig where there is doubt. Sulfates in some made ground and clays attack ordinary concrete over time, and where testing shows that risk the concrete specification changes to suit.

The oversite: hardcore, membrane, insulation and slab

The floor build up is cheap to do properly and impossible to correct afterwards.

Once the foundations are in, the ground inside them is prepared. Topsoil and vegetable matter are stripped out entirely, because organic material rots and leaves voids. Hardcore goes in as clean crushed material, laid in layers and compacted as it goes rather than tipped in one lift, then blinded so nothing sharp touches the membrane.

The damp proof membrane sits over that. It is a heavy gauge polythene, lapped and taped at the joints and lapped into the damp proof course in the walls, so there is a continuous barrier from floor to wall with no break in it. A membrane punctured during the pour is a defect that shows up as a damp floor years later.

Then insulation, then the slab, or slab then insulation then screed depending on the design. Part L sets a limiting standard for the new floor, and meeting it takes a real thickness of rigid insulation rather than a token layer. Insulation is also carried down the edge of the slab, because the perimeter is where most floor heat loss happens.

This is the easiest place to save money quietly, since none of it is visible once the floor is down. It is also the one element you can never revisit. Adding underfloor insulation later means taking the floor out, and heating pipework is decided before the pour, not after.

What the dig actually involves

Machines, spoil and access, and on a tight site the access decides everything.

A typical dig starts with setting out and stripping topsoil, then excavating trenches with a small tracked excavator. Trench sides are battered back or supported once they get deep, because a collapsing clay trench is one of the genuine dangers of this trade.

Spoil is the surprise. A trench for a modest single storey extension at a metre deep produces several tonnes of clay, and clay bulks up when you dig it, so the heap is bigger than the hole. At two and a half metres the volume roughly triples. It goes into a skip, into a grab lorry off the road, or onto the garden if there is somewhere sensible for it. Disposal is charged by weight.

Access decides how all of that happens. A driveway or a side gate wide enough for a small machine and a grab lorry is straightforward. A mid terrace with no rear access means the machine comes through the house, arrives in pieces, or does not come at all and the trench is dug by hand. Barrowing spoil through a hallway on boards is slow, and it is priced accordingly.

Concrete arrives by mixer lorry and goes in by chute where it can reach, or by pump where it cannot. A pump is a fixed additional cost and on a rear garden with no vehicle access it is usually the only realistic option.

How we work

Look at the ground first, price the real job, and put the variable in writing.

Our team has 25+ years in the trade and comes out. The first visit looks at the ground, the trees, the drains, the access and how the existing house is likely to be founded, because those are the things that decide the number.

Where the ground is uncertain we dig a trial hole before quoting. The structural engineer designs the foundation off what is actually there. We apply for the build over agreement with Anglian Water where the drains require it, notify Colchester City Council Building Control, and book the excavation and oversite inspections rather than working around them. Where the depth changes, it changes against a rate you already have, agreed before the work is done.

We carry £2m public liability cover, we are based in West Bergholt about three miles north west of Colchester, and we work across the city and roughly thirty miles around it. The survey and the quotation cost nothing.