Neighbours cannot stop you doing the conversion but can make you pay for their own surveyor.
The party wall is owned by all those whose property is connected to it. Because you are planning to do a loft conversion, you have to inform the neighbour, who has a total time limit of eight weeks to get the agreement in place with you.
It normally only takes two to three weeks if all parties are working together well and good will prevails!
There are two forms. One is the party wall notice form, used to formerly give your neighbour notice. The other is the acknowledgement form, which your neighbour has to sign and return, with their choice to use a surveyor or not.
The neighbour can’t stop you doing the conversion, they just have the right to have any work that would that is prosposed to the party wall checked out and an agreement put in place with a party wall surveyor.
Hopefully your neighbour will sign the form and decide to go ahead with out the need for a surveyor. This will save you time and money, so if you can, try to get them to agree to go ahead without a surveyor.