Foundations For Extensions

When you're adding an extension to a house, it’s essential to get the foundations right. You need to make sure you choose the right foundations for the soil type and get the measurements correct – or the whole project could come crashing down (literally). Here’s our guide to some of the different types of foundations for home extensions:

Strip foundations

What: Trenches are dug at least 25cm deep, and then filled with a concrete base and blocks on top, up to ground level.

Why: This is the least expensive type of foundation and the default choice for adding an extension to a house.

Trench-fill foundations

What: Trench-fill foundations are similar to strip foundations: trenches are dug at least 15cm deep, but instead of a concrete base with blocks on top, they’re filled with concrete to just below ground level, and topped with a damp proof course.

Why: Trench foundations are easy and quick to dig – they’re less work than strip foundations, as you don’t have to be messing around laying blocks – but more expensive because of the extra concrete.

If the ground is difficult to work on – unstable, wet or contaminated – the simplest solution is just to dig the trenches deeper and fill them with more concrete. However, once you get down to around 2.5m deep, this becomes impractical, dangerous for the workmen, and prohibitively expensive, so you will need to consider other types of foundations.

Pile foundations

What: Piles are long columns, usually made of concrete, which are driven down into the ground, and linked near ground level by a concrete beam.

Why: Pile foundations are used for heavy buildings which can’t be supported by the soil; the load of the building has to be transferred down to stronger soil further underground. Piles are very flexible in terms of their size and shape and can be made from different materials, including wood and steel.

Raft foundations

What: A raft or mat is a thick concrete slab reinforced with steel which extends under the whole building.

Why: By spreading the weight of the building over the whole area, the load per square metre is reduced, making a raft suitable where the ground is weak. Rather than trying to anchor the building, in the event of subsidence or ground heave (where the ground moves upwards), the raft is designed to mitigate the effects of the movement and protect the building. Raft foundations are often used on ground which was formerly used for mining. The main drawback is the high cost, as they require a lot of high-strength concrete and steel.

What is a soil survey?

A soil survey involves digging holes at various points on the site to see what the soil is like. It can be useful, but often isn’t necessary as your architect or the council’s building inspector may know already.

At Bluelime Home Design our architectural services in Kent are the perfect way to help you bring your house extension plans to life. We’ve thirteen years' experience of working on homes in south London and north Kent, and have offices in Dartford, Bromley, Erith, Bexley and Croydon. Call us at our head office on 01322 517632 for an informal chat about how we can help.

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