I am not 100% sure what you are asking for, but you can add include directories in Project->Build options->Search directories->Compiler
this paths are relative to the project path
You can always work relative to your source file,
if you want to include a.h in a.cpp
Dir1\Dir1.1\sources\a.cpp
\includes\a.h
you can simply write
#include "../includes/a.h"