Neighbour Building dividing wall

The next door neighbour is about to build a wall surrounding their garden and obviously a portion of that wall will divide our two gardens. I've no problems with this at all and I will probably do something similar in a year or so.

My question is whether I would/should be expected to share the costs of the wall that divide our gardens? I'm thinking it might be a bit cheeky to come along in a years time and build onto the dividing wall which I paid nothing for.

If you get on well with your neighbour (presumably he's consulted you about the wall, its location, materials to be used, height etc) it would be good manners to "offer" half of the cost for the construction of the portion of boundary wall that you share as neighbours. If he's building it himself you could offer to pay for some of the materials or to give some labouring assisstance.
As the saying goes: "good fences make good neighbours".
If you envisage building onto this boundary wall at some future date you should check if the new foundations for this wall will be capable of accepting superimposed loads from any such extension- by offering to cover some of the cost you can at least ensure that this will be the case.