Flat building insurance is almost never the flat owner's purchase. A flat sits inside a structure that serves other flats, and insuring one flat's share of a roof is not a thing that can sensibly be done. The lease decides who insures the building, and in the overwhelming majority of cases that is the freeholder or a management company acting for them.
Why the building is insured as a whole
A fire, a flood or subsidence does not stop at the boundary of one flat, and a claim settled flat by flat would not rebuild the building. So leases require the whole structure to be insured under one policy, with the premium apportioned between the flats through the service charge. This is also what mortgage lenders expect to see.
What the flat owner insures instead
Contents, and anything the lease demises to the flat rather than retaining as structure, which commonly includes internal fixtures and sometimes the windows. Personal liability as an occupier. Where the flat is let, landlord contents, property owners liability, loss of rent and tenant damage. That is a flat landlord policy without a buildings section.
Reading the demise clause
The clause that says what you own is the one that decides where the block policy stops. Leases differ on the windows, the internal walls, the floorboards and any improvements made since the lease was granted. The five minutes it takes to read it is the difference between knowing who pays for a broken window and guessing.
When the flat owner does insure the structure
Occasionally, on a converted house where each flat's lease requires its own leaseholder to insure their part. Lenders dislike this and it produces apportionment arguments, so where it exists the practical answer is usually a joint policy over the whole building arranged by agreement between the owners.
Questions people ask about flat building insurance
Do I need buildings insurance for a flat?
Usually not. The freeholder or management company insures the structure under the lease and recovers the cost through the service charge.
What do I insure instead?
Contents, your own liability, and where the flat is let, landlord contents, property owners liability, loss of rent and tenant damage.
Who pays for a broken window?
It depends on the demise clause. Some leases include the windows in the flat, some leave them with the structure.
What if each flat has to insure itself?
It happens on some converted houses and lenders dislike it. A joint policy over the whole building, arranged by agreement, is the usual practical fix.