Boundary line

Hi,
Me and my neighbour are looking to get our rear extension done shortly, however something has come up regarding boundary line.

My neighbour already has a 3m extension which
he is looking to increase by a further 3m.

When his existing extensions was built , His outter wall (on my side of the garden) is set back exactly one brick away from the boundary line. The boundary line is determined as mine and my neighbour pebble dash finish is slighty different in colour. He is looking to knock down his extension and rebuild his wall so its on the boundary.

Question is, is he allowed to do this as i assumed my outter wall would of started on the boundary line itself. If you have a look at the picture, it will be a lot clearer to understand.

My neighbour extension was built before we both moved in.

While he may build upto and not over the boundary, no features of his must cross it.
So any guttering etc. should not.
Windows should not open over the boundary either.

IMO it's a bit pointless to knock down and rebuild a wall just to regain a couple of inches.

But if your both having the same 6mtr extension done you will gain space inside by sharing the wall rather than you building a 300mm thick wall on your side.

You probably need to get together and determine what order the work will be done.

If he is knocking down his existing extension to build a new larger extension and you are also building an extension, you would be best to coordinate the foundations and walls. It wouldnt make sense for him to build, then for you to build, having to dig a foundation right next to his.

The current situation is that he has a wall close to the boundary, you want to build a boundary wall leaving a 100mm gap on his side.

You could build a party wall, astride the boundary, with a parapet and both build up to it. Then the would only be a single foundation to be done. The wall could be mostly blockwork so wouldnt take long to do.

If you the builds to be a separate times, get your plan drawer to detail it correctly, to avoid a dogs dinner....

seems like the perfect time to join forces and share costs. 1 architect, 1 surveyor, 1 Building team etc etc. not going to half your costs but will certainly save pennies here and there and probably end up making things looks nicer.