FOUNDATIONS FOR EXTENSIONS
Get the groundworks right on your home’s new addition with Tim Doherty’s guide
So you’ve got a great design in the bag and you’re keen to crack on with your extension. First things first: you need to decide how to build it, and that process starts with the foundations. Specifically, you need to understand what supports the existing house. This will have a major impact on the design of the new footings. Combine this with knowledge of soil conditions, trees, service/drain locations and more, and your team can plan the best solution for your project.
Typical existing foundations
The long hot summer of 1976 heralded a turning point in foundation design. Following a significant flurry of claims for subsidence, insurance companies insisted they were cast deeper to help stabilise new and replacement dwellings. Before that, houses built in the 1930s to 1970s may have had foundations little more than 600800mm in depth and the Victorian / Edwardian periods frequently only went to depths of just 450-500mm.
In fact, the Victorians, famous for their solid brick external walls, used to step the brickwork out by a quarter or a half of the brick’s width over several courses, so that the bottom of the wall ostensibly sat on a wider brickwork footing. This footing may in turn have sat on a shallow strip of concrete, but generally speaking not to any significant additional depth.
Houses built inter and post war may have used more concrete and less brick, but the trend would still have been for relatively shallow foundations. This was the case until changes to the Building Regulations introduced minimum notional target depths of circa 1m to try and help eliminate superstructure movement.
If you have an old house with an existing cellar, this will probably have pretty shallow foundations under the base of the external (retaining) walls beyond the cellar floor. There’s also unlikely to be any transition between the foundation design for the adjacent above-ground walls. So the cellar might be down to a depth of around 2.5m, and sit immediately next to a non-cellar wall foundation of only 500mm depth. Nowadays, it is common practice to transition these changes in depth over a short length of walling.
Topography, trees, services & drains
Next on the list should be a practical assessment of levels, planting and services on your property. Drains, in particular, have a nasty habit of being right in the way of where you’ll want to put your extension – as traditionally they sit close to the existing building and often wrap around several sides.
Sometimes drains on your land also cater for neighbouring properties, and here you’ll be under an obligation not to compromise overall performance. Technically, any installation that serves two or more properties is considered a public drain, and under regulations introduced in 2011, the water authority is responsible for its maintenance. So, should a diversion be needed to allow for your extension, plan well ahead and make sure that the water authority agrees with your proposals.
The same applies to services (gas, electric, water and broadband) leading to your property: always make sure these won’t be compromised by the addition, and speak to the network distributor if necessary.
If large trees are in close proximity to your home or extension, this can also be a problem. The NHBC (www.nhbc. co.uk) has a very useful guide that shows how deep your foundations will need to be according to the height of common species and how far away the house is from the tree. Topography is also important: if the ground is sloping, then stepped foundations may be required. Here, again, there are clear minimum standards to conform to in terms of the depth and length of steps, and how far they need to overlap.
Bear in mind, too, that if your extension is close to a boundary or within six metres of your neighbour’s property, you may need to come to an agreement under the Party Wall Act prior to excavating any trenches for your foundations. The Act provides a
framework for preventing and resolving disputes – find out more at www.selfbuild.co.uk/party-wall.
Soils
The type of subsoil will also influence the depth and width of foundations required; and sometimes affects more fundamental aspects of the foundation design. The ideal conditions to find are well-compacted sandy gravels (with the right mixture of porosity and compressive strength), chalks or sand/ siltstone. Generally, these will allow for straightforward foundations.
Clays, on the other hand, will shrink as they dry out in summer and expand when they soak up moisture in the wet seasons – so footings generally need to be deeper and wider. You may need to install specialist products, such as clayboard, in order to stabilise the foundations – or even an engineered solution. Similarly, if the soil adjacent to your property is soft, organic or generally unstable (such as filled, made-up ground), then you’ll probably need to appoint a structural engineer.
Can you use trench fill or strip footings?
The easiest type of foundation to build is an excavated trench which is then mass filled with concrete (hence the name trench fill). This is usually taken all the way up to 75mm below the top of the excavated trench walls, and then a course of blockwork is installed up to damp proof course (DPC) level – giving you a minimum of 150mm clearance above external ground level (subject to the detailed design for your project).
Strip footings work according to the same principles, but the depth of concrete is restricted to the bottom of the trench only, up to depths of around 200-400mm (usually with a couple of reinforcement bars). The rest of the foundation wall is constructed in dense concrete blockwork, directly under the above-ground external walls, with a DPC separation at floor level.
Concrete costs mean trench fill is a bit more expensive, but most people go for it as it’s so quick and easy to install. Widths are usually 450mm or 600mm to match excavator bucket sizes. The external wall system, which is typically around 300mm thick, then sit directly in the middle of the foundation. The trench sides should always be vertical, so you can be sure the concrete is substantial all the way to the base.
If access is tight and the trenches need to be hand dug, it’s fine to go for something between those two widths (unless your engineer says otherwise). The key thing is to ensure the setting out is accurate and the wall sits centrally on top of the foundation.
Using string lines and profile boards is the easiest way to accurately set out the site. If the excavation depth is more than 1.2m (3m being the maximum with these standard foundation types), and depending on soil conditions, then the sides of the trenches may need to be supported temporarily. Pumps might also be necessary to remove any water in the bottom of the excavations.
Engineered rafts
Where trench fill or strip footings aren’t suitable, your structural engineer might opt for a raft. This integrated foundation and floor slab is specially strengthened at the sides and mid-span (or in point loading locations) and all tied together with reinforcing bars and mesh.
It’s unusual for an extension to use a raft. As the name suggests, this option is designed to ‘float’, so there would need to be a purpose-designed movement joint at the junction between the new addition and the existing dwelling. But engineers are solutionoriented people, and if it’s the right move for your project and the site conditions, then they’ll specify it!
Piles & pads
More likely on troublesome sites will be concrete pad or piled foundations, with purpose designed engineered ground beams spanning from one pad/pile to another. The floor structure (which may be concrete or timber joists) is then suspended on these ground beams.
The choice between pads or piles is based upon soil conditions, with the latter being a more extreme, and thus expensive, option. Typical domestic piles can reach depths of 6m to 8m – or even further – to reach good ground bearing strata. Mini piles work on the same principle but are often confined to depths of 3m to 4m, with several different options of how these could be installed; some can be twisted into position much like a cork screw. The engineer will have to approve the eventual design and some piling firms have their own engineering services to offer a full package. A major cost is the pile caps, which sit on top of the piles to give a level base for the ground beam.
Concrete pads, by comparison, are often only one metre square and are built to normal depths of 1m to 1.5m, with reinforcement typically added to tie the ground beams to the top of the pads. One of the difficulties is shaping a pad of this size correctly – so this task is normally done by hand.
Right: Access to the site can be poor on an extension project, so much of the work often needs to be carried out by hand