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Foundations are at least 100mm deep for a wall that is not more than 800mm high and 150-200mm deep for a higher wall. The foundation must be wider than the brickwork or block work, an easy way to calculate the width is to add the width of the wall to double the depth of the foundation, for instance if a 220mm wide 1 brick thick wall is built on 150mm deep foundation then the foundation should be 520mm wide. The footings for a free standing brick pillar measuring about 400mm x 400mm must be at least 200mm deep and at least 200mm wider than the pillar on all sides. For an extension or house footings they are normally 600mm wide and 1000mm deep this is the minimum though, and building inspectors can ask you to go deeper depending on the soil conditions.
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