Neighbours garage on my land

So i have a parking space out the back of my property (separated by a path and stairs down to road level) and i’m wanting to build a garage on it. Lots of the other spaces have already had garages built including the space directly next to us.

their garage is 3.6 metres wide which leaves us with 3 metres of parking space to build on, we were happy to do this if we can have a party wall, using their side of the garage as one of our walls and building 3 extra etc.

They originally agreed to…

Neighbours garage on my land