Conventionally framed roofs with ceiling joists of adequate width meant for
storage is probably the only type one can safely store your stuff in the
attic. A few truss designers will design these if the ceiling load specs
are provided by the builder, but its not seen very often.

Huh? Virtually any truss manufacturer will design and supply
attic trusses for "bonus space". It's not exactly a complicated
or new problem. The problem is that if the truss *WASN'T* designed
for that, then it's almost certainly because the system can't
be easily modified to do it, else they WOULD have, when it went in.

And in someone else's post:


Attic junk not included. Floor load not included.

The trusses have some extra load capabilities. Afterall, they must
support workers up there moving around.

Work that through again... If the truss has, say 500# of excess
temporary support ability, because you need that for workers,
and you use that extra 500# for storage, what happens when the
workers show up?