You will have been served one of three possible notices under the Party Wall etc. Act 1996. They are:-
Section 1- New building on line of junction
The line of junction is what we refer to as the boundary line between you and your neighbour. If your neighbour is building, for example, a single storey extension on the boundary then he has to serve you a notice to state that fact. The building owner may want to build the new wall astride the boundary that is, half on your side and half on his. Why would you want the neighbour’s wall half on your land? At some point in the future you may want to build your own extension and make use of the wall built by your neighbour. If this is the case then the cost of building the wall should be shared at the time of construction or at the time when you make use of the wall.
If you have no intention of using the wall then you can refuse for the wall to be built on your land and neighbour will have to build it wholly on his land.
A section 1 notice must be served on the adjoining owner at least one month before the intended work is begin.
Section 3- Party Structure Notice
This notice is served if any work as specified in Section 2 of the Act is be carried out. Section 2 lists thirteen cases where the building owner has rights to undertake work to an existing party wall such as inserting beams, underpinning, raising it higher etc.
A Section 3 notice must be served at least two months before any work is due to start.
Section 6- Adjacent excavation and construction
If your neighbour intends to excavate near to your own foundations then this notice must be served. There are two Section 6 notices; a 3 metre notice and a 6 metre notice.
If the adjacent excavation is within 3m of your structure (possibly the foundation if that is wider than the wall above) and the excavation depth is likely to be below the bottom of your foundations then a Section 6, 3m notice must be served.
If the excavation is within 6m of your structure and the proposed excavation is cut by a 45 degree imaginary line drawn from the lowest face of your structure then a section 6, 6m notice must be served.
A Section 6 notice must be accompanied by plans and sections showing the site and depth of any excavation the building owner proposes to make and if he proposes to erect a building or structure, its site.
In both cases a Section 6 notice must be served at least one month before the start of the intended work.
If your have been served with any of the above notices you have 14 days from the date the notice was served to act by either consenting, not consenting/ignoring or serving a counter notice.