A double skinned wall with a 75 mm cavity wants to have footings 600mm wide (or thereabouts since it's not going to be subjected to very much loading).
As for the depth you need to dig - this all depends on local conditions.
The depth of concrete in the footings also depends on the above but about 200mm should be more than adequate (but don't come back to me if it all goes wrong - without knowing the soil and any other factors such as nearby trees etc. you could need to adjust accordingly).
You should also have at least a couple of courses of suitable brick below ground level for your wall, with a lean mix fill back to ground level in the cavity.
A double skinned wall with a 75 mm cavity wants to have footings 600mm wide (or thereabouts since it's not going to be subjected to very much loading).
As for the depth you need to dig - this all depends on local conditions.
The depth of concrete in the footings also depends on the above but about 200mm should be more than adequate (but don't come back to me if it all goes wrong - without knowing the soil and any other factors such as nearby trees etc. you could need to adjust accordingly).
You should also have at least a couple of courses of suitable brick below ground level for your wall, with a lean mix fill back to ground level in the cavity.
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