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 debootstrap.