Everywhere I have seen on the internet, the default location for the rpmbuild is a user's home directory,
you can set the
%_topdir variable in the
.rpmmacros configuration file (in your home directory) like this:
echo "%_topdir ~/foo/bar/rpmbuild" >> ~/.rpmmacros
It wouldn't (shouldn't) really make much difference; since sources are to be copied there by
rpmbuild; then they are build and installed and packaged... Since all paths are supposed to be relative the outcome should be the same.
PS: more information on the rpm build area
PPS: a long time ago the default
build area was in /usr/src/packages ; but then you had permission issues etc.