Part of the problem, is the substation to the right, but it'll also depend on what part of the land is yours, so you need to look at the deeds, to determine the boundaries. It's possible that you're piece of land extends to the side of the garage (but unlikely), and that might allow you to extend it sideways.
If the concrete driveway at the front is yours, then there'll very likely be a right of way marked on the plans or in the deeds, for access to the substation, but in theory, as long as it still goes down the side of the new garage, then they may not quibble too much.
You'd need to strip the roof off of your side, support the roof joists, remove the side wall, then dig the foundations down, and rebuild the new wall. Then fit new joists alongside the old ones and bolt them together (or just completely replace them), then put on a new roof. So, do you want to go for it, or look elsewhere. It's possible that the neighbour may want to do the same as you, so may join in the project.