The Party Wall Act 1996 also uses the term "party structures". This is a wide term and is applicable to all party walls and also applies to horizontal party structures (such as floors or ceilings) between, for example, two adjoining flats or maisonettes . Interestingly, While a roof may not be aparty structure, if the structure supporting the roof is "shared" between two properties, then that part is likely to be a party structure. In a similar vein, chimney stacks are often "shared" between two properties and so are likely to fall within the definition of party wall or party structure.
Any work to a party wall or party structure must comply with the act. An agreement would be needed.
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