I've never added "../" to HEADERS.

In my opinion there is no reason to do this. If you want to use the code of a directory "upper", make a shared/static library of them. On this way you also see the headers and sources.

If you don't like that solution, create a "whatever.pri" which contains "HEADERS += $$PWD/header.h" and in your project which is using it add "include(../../../whatever/whatever.pri)"

By the way: If you ONLY add headers which don't even contain Q_OBJECT macros, this is just for beauty and doesn't mean anything for the compiler. (I suppose you know that )