When the house next door is let
The landlord is the owner. Find them through the Land Registry title, which usually gives an address for service, or through the letting agent. Serve the landlord directly; an agent may pass the notice on, but it is safer not to rely on that.
When the owner is a company
Deliver the notice to the company secretary or clerk at its registered or principal office, or post it to them there. Companies House shows the registered office.
When the owner lives abroad
Posting under the Act means sending it to a UK address where the owner lives or works. If the Land Registry title gives a UK address for service, use it. If not, the method for empty properties below is the one to rely on, and it is still sensible to send a copy abroad so the owner actually sees it.
When the property is empty
You can make the notice out to "the owner" of the property, giving its address, and hand it to anyone you find there or, if the place is empty, fix it somewhere conspicuous on the building. Combine that with sending a copy to any address you can find.
Keep evidence of everything
Photograph a notice fixed to the property, keep certificates of posting and note the dates. If service is questioned later, your records are your proof.
Common questions
The title has no address I can use. What else can I do?
Check the electoral roll, ask other neighbours, or write to the property itself. A surveyor can help trace owners.
If an owner never replies, can the work still happen?
Yes. For shared-wall and excavation work, after the 14 days and a further 10-day request, a surveyor can be appointed on the owner's behalf.
Where this comes from in the Act
Links go to the official text of the Party Wall etc. Act 1996 on legislation.gov.uk.