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Lobster

wrote:

Am concerned about the stength of the wall as it is only single skin
and how much damage using an SDS and bolster will do in loosening the
surrounding brickwork. Am I right in worrying? its a single skin wall
that was rebuilt about 15 years ago on the end of our kitchen. The
flap will need to installed with one edge approx 4" away from the door
frame/wall edge.

If you do lots of chain-drilling as advised, it should be OK, as that
will minimise the jarring. At worst you might need to make up a bit of
mortar for repointing if a brick or two comes loose,

A mate has also suggested using an angle grinder instead which should
penetrate nearly all of the brick depth. Is this a better alternative?

Trouble is that if the blade is big enought to penetrate the brick
depth, then the cuts will extend way beyond the corners of the cat flap
due to the big radius on the cutting disc (think about it). This
technique is fine for larger openings, but won't work on a piddly one
like a cat hole.

That said, iy still might be useful to cut even a shallow slot around
the periphery of where the hole will be, to define nice straight edges.

David