Well… in terms of Party Wall etc. Act 1996 you are now the Adjoining Owner.
The building owner MUST serve a valid notice, making you aware of the proposed works. From the date on the notice you will either have one or two months (depending upon the work involved) to decide if you agree to the works proposed. You can agree but be sure to keep the rights given to you by the Party Wall etc. Act 1996 intact.
If you disagree there are really only two options:
1) At this stage your neighbour will appoint a surveyor to negotiate the future award, you can agree to using this Surveyor although sometimes:
2) You might feel that a neutral party should be involved and in which case you appoint your own Surveyor.
The costs involved during this process are typically borne by the neighbour who intends to carry out the works, unless, you make a special request, or, if the works are of benefit to you both.