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Groundwork & Foundations

New rear extension

Hello you all. A Few questions, I’m doing a rear extension, existing footing are 200mm and very shallow. It is 150mm under finished ground level and DPC is 150mm above that. So basically 200mm slab and 300mm worth of brickwork to damp course (4 bricks). My new footings will be 600mm wide and 600mm deep, di I make it level with existing footing? Beacause BC states 750mm deep from ground level (150mm +200mm + ( 400mm - dug down from bottom of existing footing) = 750mm. Do I underpin the existing footings with new footings? Involving new brickwork- do I do 4 bricks on outer leaf then 1 small concrete brick and then a block on top of that for inner leaf or can I turn a block on its side and then one block on top of that to match the DCM on inner and outer leaf? Also will a 75mm slopped lean mix be enough for infill in cavity, I ask because I’ll only have 300mm from top of concrete to DPC and minimum I need is 225mm. Thanks for your answers

3 Answers from MyBuilder Groundworkers

Best Answer

Footings these days have to be a min of 1m below ground level, if your existing footings are not adequate enough you will either under pin them or rip them out and start again. They will asses your ground and then tell you what you require, my best advise is employ a structural engineer for around £450 and he will tell you what you need to apeese the BC. There ARE different options so the engineer can be a good idea.


Answered 4th Jul 2021

You know what your doing. I take it your some sort of Enginner.


Answered 18th Jul 2021

Rule of thumb is follow existing foundation.


Answered 20th Jul 2021

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