To build a loft conversion for a 1st Floor Flat – do the owners of downstairs need to sign a Party Structure Notice?
Also, if they have consented verbally but not signed the notice, and more than 14 days has elapsed since they received it, is there any way they can change their minds or say they now do not consent, or is it now too late?
A typical loft conversions, including cutting beams in to a party wall, is not notifiable to the owners of the flat below. This is because the party structure is the floor/ceiling and that is not affected. That’s not to say there isn’t a risk of damage when additional load is placed on the party wall but that risk is dealt with under the landlord/tenant procedures (landlord consent would generally be required for works of this type).
On the more general point, yes, consent must be in writing.
Is there any way I can get a next door neighbour who wants to build a Loft Conversion to contribute towards the cost I have already paid to raise the party wall in brickwork, seeing as joining his conversion onto this would make him a considerable saving in building & material costs?
He does not have to contribute until he makes use of the wall that was raised at your expense at which point the compensation will be calculated at 50% of the cost of raising the wall at that date (see section 11(11) of the Act).