Usually, rpmbuild related variables are set in ~/.rpmmacros. To change the current working directory, one could simply alter the default settings:

%_topdir      %(echo $HOME)/rpmbuild

This would change rpmbuild’s working directory on a per-user basis.

Sometimes it’s quite convenient to keep the default setting and change the working directory on a per-project basis:

$ rpmbuild --define "_topdir workingdir" -ba project.spec

To use the current directory as working directory, one could invoke rpmbuild as follows:

$ rpmbuild --define "_topdir `pwd`" -ba SPECS/project.spec

Careful: Double quotes are mandatory as well as having a proper subdirectory structure in the new working directory (BUILD, SRPM, RPM, SPECS and SOURCES).

Resources:
http://www.rpm.org/max-rpm/s1-rpm-anywhere-different-build-area.html
http://stackoverflow.com/questions/416983/why-is-topdir-set-to-its-default-value-when-rpmbuild-called-from-tcl