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From `man dpkg-source`,

    -ss Specifies that the original source is available both as a directory
        and as a tarfile. dpkg-source will use the directory to create the
        diff, but the tarfile to create the .dsc.  This option must be used
        with care - if the directory and tarfile do not match a bad source
        archive will be generated.

It should also be noted that

    If no second argument is supplied then dpkg-source will look for the
    original source tarfile package_upstream-version.orig.tar.gz or the
    original source directory directory.orig depending on the -sX arguments.

Unfortunately, that build option is **only available for source format 1.0**, which is obsolete and quite limited (it doesn't even support `debian/patches`).