Extension next to us not following plans- what can we do?

[RosyPosies]

I've contacted planning because an extension next to us (which we objected to) is almost finished and the tarpaulin covering it is down. Lo and behold one section of it is much wider and higher than on the plans. I am very very annoyed as it obscures a lot of our light.
Planning have said they come and look but what are the chances of them telling the builder to take it down? The house belongs to a builder and he's done the building- and in my opinion- gone over and above the plans.

If the local planning officer says it's acceptable, can we take it further and how do we do it? Is there any course of action?

If there is a significant difference in the plans, then the Council should ask for a new application to be submitted, which you should be able to comment on as before. The application should be determined as if it were a new application. If it is refused permission your neighbour has the right to appeal against the decision. The council would have to start Enforcement proceedings to make your neighbour either re-instate as per the approved drawings or to a satisfactory compromise.
If it is deemed to be only a minor breach then the Council may not deem it necessary to take any further action.
In either situation, I am afraid you do not have a right to appeal.
I would speak to your local Councillor and see if you can get them to make your case with the council officers.