You need to be able to create a chroot when you use
debootstrap. Plus if you plan on partitioning, or doing any mounts, etc. you will need root permissions.
If you check out the debootstrap manpage you should be able to use
debootstrap with the
--variant=fakechroot option to use
fakechroot, which installs the packages without root privileges. An example in your case would be something along the lines of this:
debootstrap --variant=fakechroot stable ./dir $debian_ftp
Please read the Wiki for more information on how to use