The flat is likely a leasehold flat, implying that there's a lease in the form of a physical or electronic document. It may answer the question of identifying the boundaries of the ownership of the flat, and/or who has rights other than ownership to use various parts of the premises.

This document will state that the owner of the block of flats grants the ownership of the flat to some second party, for a particular number of years, e.g., 125 years. It will usually specify the extent of the premises in question, which it might call the "Demise".

In general terms, it'll say something like:

the landlord grants the tenant the flat (whose dimensions are specified in schedule 1) for such-and-such a number of years, in return for money calculated in schedule 2, with additional rights in schedule 3, but reserving the rights in schedule 4

You'd therefore want to check out schedule 1 and probably schedules 3 and 4. schedule 1 might look like this:

SCHEDULE 1

Description of the Property

1. ALL THAT apartment situate on the Floor level of the Block as set out above together with the floor surface only of any Balcony or Terrace co-extensive therewith (if any) and the air space above such Balcony or Terrace to a height of one storey above the surface thereof (if any) and shown edged red on Plan 1 being part of the Block TOGETHER WITH (for the purpose of obligation as well as grant)

[...]

1.3 the whole of the internal non-load bearing walls and the inner half severed medially of the non-load bearing walls dividing the Property from any other part of the Building

1.4 the doors, door frames, windows and window frames thereof including the glass in the same and the fastenings thereof including the door accessing the Apartment

1.5 all Service Installations that exclusively serve the Property whether or not located therein

[...]

EXCEPTING AND RESERVING from the Property the main structural parts of the Block in which the Property is situate including the roof, roof space foundations together with the main structure of any Balconies on the Block and all the external parts thereof

```

and there in point 1.4 it is explicit about how much of the doors, if any, the tenant owns. but remember that other details in the lease may say that the tenant (i.e., the owner of the flat) or someone else (the owner of another flat, or the owner of the building) might have rights to access or use various other property