If you are proposing a loft conversion, the work will be notifiable under 3(1) of the Act exercising rights under 2(2). This means in simple terms, you need to serve Party Structure Notice(s) (not Line of Junction Notice(s) on ‘Adjoining Owners’.
You should complete Land Registry searches to confirm who owns the adjoining properties. A leasehold flat not immediately adjoining (i.e. at ground floor level or below) does not count. Normally in these circumstances I would serve Notice on the adjoining freeholder(s) and top floor leaseholder(s). But, if the two are one of the same, service of Notice on the freeholder would suffice.
Loft conversions normally are straight forward in context of party wall matters however there are considerations to bear in mind:
Construction over the party wall at roof level prejudicing future rights of the adjoining owner - essentially if you build a dormer cheek on top of the party wall, this in my view is a problem
Detailing at roof level i.e flashings - poor detailing means possible roof leaks
Penetration of beams into the party wall travelling no further than 50% into wall thickness - this is an issue when the separating party walls are only a half brick thick
I would also recommend appointing a decent party wall surveyor to prepare and serve the notices for you as it is possible to get them wrong.
Further, I suggest a friendly conversion with the relevant neighbours before serving Notice(s).