Are non-restrictive relative clauses adjuncts or modifiers?

Question

In

Leo Messi, who lives next door, is the greatest football/soccer player in the world.

is "who lives next door" an adjunct or a modifier?

My thinking goes like this. If I isolate the non-restrictive relative clause

who lives next door

my intuition tells me that there is no "head" to be modified, which makes this non-restrictive relative clause an adjunct.

However, if I assume that non-restrictive relative clauses have a "head" and that this "head" is external rather than internal (meaning that the head in my sentence is "Leo Messi"), then the non-restrictive relative clause is a modifier.

My assumption, therefore, is that modifiers are attached to a "head" (i.e. modifiers "modify" a "head").

How do linguists analyse non-restrictive relative clauses?

Thanks in advance.

Answer 1

I'll refer you to, first, @BillJ's comment above, which I agree with, then to McCawley's analysis in the The Syntactic Phenomena of English, which makes restrictive relative clauses modifiers of N' (N-bar) and non-restrictive relative clauses as modifiers of sentences. The syntactic relationship between a non-restrictive relative and the NP it goes with does not appear to have a counterpart in traditional grammar -- McCawley calls it an "adposit".