In simple terms a party wall sits astride a boundary to land belonging to several different owners and forms section of a building using one or both properties.
https://thehearup.com/things-to-consider-before-installing-skylights-in-your-home/34988/ which sits astride the boundary but does not form section of a building is really a Party Fence Wall for the purposes of the Act.
The term Party Structure can be used in the Act. A floor structure separating flats is one example of a celebration Structure.
When does the Party Wall Act apply?
The Act will apply when:
Various works are completed to a preexisting Party Wall or Party Structure
New buildings or structures are erected up to or sat astride the boundary line
Excavations are completed up to 6 metres of a structure on a neighbouring property which have the potential to undermine the foundations of that structure
So the Act applies to my works, what do I really do now?
If the Act applies you can be obliged to serve a celebration Wall Notice on your neighbour, setting out details of the works at hand and providing key information such as for example plans, proposed commencement dates etc. In the case of adjacent excavations you may want to provide specialist details of foundations.
You may be obliged to provide your neighbours between 1 and 2 months notice of commencement of work depending on which section of the Act applies.
What goes on if my neighbour objects to my Party Wall Notice?
The Act offers you the right to complete various types of focus on or about the boundary line so provided your works are included in the Act your neighbour's objection cannot stop you going ahead but means that you are obliged to check out the Dispute procedure set out under Section 10 of the Act.
This requires one to appoint a Party Wall Surveyor to do something in your stead. Your neighbour gets the right to appoint their own surveyor or they could agree in the appointment of an individual 'Agreed' surveyor. Where two surveyors are appointed they'll acknowledge the appointment of a third surveyor to adjudicate/referee where the two surveyors cannot agree.

The surveyor prepares the Agreement/Award which sets out the rights and obligations of both parties and includes a Schedule of Condition of the neighbouring property to record its condition before works start.
Exactly what is a Party Wall Agreement/Award?
This is a written document made by the surveyor detailing the rights and obligations of the owners. The contents are agreed by the survey and upon completion the Agreement is served on the owners.
It is a legally binding document and this can be enforced by the County Court where necessary.
Evans Jones are a team of Chartered Building Surveyors in Cheltenham, who've been dealing with the Party Wall Act since its inception in 1996. You can be dealt with by way of a specialist Party Wall Surveyor with experience of all aspects of the Act who is able to advise you simply and clearly how far better meet your obligations under the Act.