"Application for determination of the requirement for prior approval of a household extension at 18 Western Avenue" - to be precise.
To put that in plain English - rather than "planner-speak"- it's a preliminary application to see whether a formal planning one will have to be made!
There are a number of these new-style applications about these days.
If North Lincolnshire Council - the district planning authority - decides that planning permission will need to be sought for the extension at 18 Western Avenue, Brigg Town Council will then get an opportunity to comment.