Water discharge issue..flat roof

[20questions]

Idiot people built a small single storey extension right up to the neighbour's boundary with flat roof and that roof flap for drainage! The flap drains straight into the neighbour's home right by, and under their their (wooden) garage door. The grey bit on the left is start of garage. Neighbour is angry as next door built right up, and actually over boundary. He refuses to allow more encroachment so has refused to allow guttering and has insisted - by law I think - for drainage flap to be removed!
Any idea how else drainage could be put on a flat roof?

Why didn’t the neighbour call the council when it was apparent it was on the boundary?

The council do not deal with boundary disputes on private property. Nor do the police before anyone says it. You need to go to a property litigation lawyer.

If I were the neighbour I'd call Building Regs at the local council and explain the issue.

If the neighbour has legal cover on their house insurance policy then that may help as they'll want to protect the neighbour's garage from water damage.

Which way does their roof slope? Is it in the direction of the side flap next to neighbour’s garage or does it slope a different way? If it slopes a different way then they may be able to put an upstand on the top of the roof near the edge to stop the water spilling off that side. If it is sloping in the direction of that side flap then they mat have to redo the roof to slope a different way.

Thanks all - some useful advice - the slope on the roof is a thought...
Also a call to or visit to building control is a good idea!