You will need at least one Party Wall Surveyor (PWS) but each property owner may insist on a second to act for them - this will be at your cost. If it is only a single storey extension then you only need a Party Wall Award (PWA) for the ground floor apartments (assuming you’re putting a full width RSJ in) - if you’re removing chimney breasts from the second and third storeys, for example, then each apartment would be impacted.
You only need to inform the owners of the properties - I’d start by knocking on the doors of those affected and asking who the owner is, introduce yourself etc. Alternatively you might need to consult the land registry if the tenants are not helpful.
I’d start by engaging a local PWS, they’ll be able to help you with all your queries. You have to serve your party wall neighbours with the notices two months before starting work, however, you can do so up to a year before you start. That said, if you need full planning permission (rather than just permitted development) I’d be inclined to apply for this first, as it can take some time.
Your neighbours cannot refuse you a PWA, however, it’s prudent to allow the required time in case they delay to the final moments as ours did. I had to ask our PWS to start the process of circumnavigating them as they pushed it to the wire.
Lots of useful info here:
https://www.gov.uk/party-walls-building-works
Generally I’d say process should be as follows;
- Get plans drawn up
- Apply for planning permission (or prior approval of permitted dev if you want to go that route)
- Apply for PWA
- Tender for builders
- Wait for builders to become available
- Start build