Environment::extRelPath() 

\nn\t3::Environment()->extRelPath($extName = ''); 

Get the relative path (from the current script) to an extension e.g. ../typo3conf/ext/nnsite/

\nn\t3::Environment()->extRelPath('extname');

| @return string

public function extRelPath ( $extName = '' ) {
	return PathUtility::getRelativePathTo( $this->extPath($extName) );
}