Confluence only allows subtractive Inheritance of Permissions, so you can restrict more on subpages (at least with view permissions).
It would be good to have this additive. So there is a bunch of pages with read only rights and for specific users can have edit rights on subpages (and from that on, on all subpages of that page).
Space (Department f1) - Everyone can read
Subpage (Projects) - Everyone can read, dep. f1 can edit
SubSubpage (Project Installations) - Some additional Members of Department installation can edit here (and on subpages)