Neighbour having wall built with run off that could enter my garage.

[cdbe]

Just politely point out that they seem to have missed a brick. It looks to be in line with their acco, which presumably drains the other way towards their downpipe - they probably interrupted the brickwork to span over it and forgot to make good.

 

[jeds]

Is it possible that the hole has been left in a kindly way to prevent a puddle forming on your side of the wall by draining into the Acco? Tip a bucket of water there and see what happens.

On that slope, if it puddles enough to run through that brick gap, they're going to need wellies while watching telly.

 

[mrrusty]

On that slope, if it puddles enough to run through that brick gap, they're going to need wellies while watching telly.

Y totally agree, but I do wonder if that was the intent, rather than a drain out through the wall. One would presume the downpipe has a gully, so it would be a bit weird to not have the paving flowing towards the gully.

 

[RandomGrinch]

It looks like there should have been a gulley on the OP's side, where their downpipe is.

 

[jeds]

I wouldn't mind betting they left the gap because they CBA to connect the Aco to a drain so left the gap as an overflow. Judging by the brickwork it was a DIYer masquerading as a pro. Personally I'd block it up.