Subject to section 4, this Chapter applies to any premises if—
they consist of a self-contained building or part of a building . . .;
they contain two or more flats held by qualifying tenants; and
the total number of flats held by such tenants is not less than two-thirds of the total number of flats contained in the premises.
For the purposes of this section a building is a self-contained building if it is structurally detached, and a part of a building is a self-contained part of a building if—
it constitutes a vertical division of the building and the structure of the building is such that that part could be redeveloped independently of the remainder of the building; and
the relevant services provided for occupiers of that part either—
are provided independently of the relevant services provided for occupiers of the remainder of the building, or
could be so provided without involving the carrying out of any works likely to result in a significant interruption in the provision of any such services for occupiers of the remainder of the building;
and for this purpose "relevant services" means services provided by means of pipes, cables or other fixed installations.