diff --git a/tests/phpunit/tests/multisite/getSpaceAllowed.php b/tests/phpunit/tests/multisite/getSpaceAllowed.php
new file mode 100644
index 0000000000..c2393c0cf5
--- /dev/null
+++ b/tests/phpunit/tests/multisite/getSpaceAllowed.php
@@ -0,0 +1,119 @@
+<?php
+
+if ( is_multisite() ) :
+
+/**
+ * Tests specific to `get_space_allowed()` in multisite.
+ *
+ * @group multisite
+ */
+class Tests_Multisite_Get_Space_Allowed extends WP_UnitTestCase {
+	protected $suppress = false;
+
+	protected static $original_site_blog_upload_space;
+	protected static $original_blog_upload_space;
+
+	public static function setUpBeforeClass() {
+		parent::setUpBeforeClass();
+
+		self::$original_site_blog_upload_space = get_site_option( 'blog_upload_space' );
+		self::$original_blog_upload_space = get_option( 'blog_upload_space' );
+
+	}
+
+	public function setUp() {
+		global $wpdb;
+		parent::setUp();
+		$this->suppress = $wpdb->suppress_errors();
+	}
+
+	public function tearDown() {
+		global $wpdb;
+
+		/**
+		 * Reset the two `blog_upload_space` options to their original values so
+		 * they can be relied on in other test classes.
+		 */
+		update_site_option( 'blog_upload_space', self::$original_site_blog_upload_space );
+		update_option( 'blog_upload_space', self::$original_blog_upload_space );
+
+		$wpdb->suppress_errors( $this->suppress );
+		parent::tearDown();
+	}
+
+	/**
+	 * When no option exists for the site or the network, a fallback of
+	 * 100 is expected.
+	 */
+	public function test_get_space_allowed_default() {
+		delete_option( 'blog_upload_space' );
+		delete_site_option( 'blog_upload_space' );
+
+		$this->assertEquals( 100, get_space_allowed() );
+	}
+
+	/**
+	 * If an individual site's option is not available, the default network
+	 * level option is used as a fallback.
+	 */
+	public function test_get_space_allowed_no_site_option_fallback_to_network_option() {
+		delete_site_option( 'blog_upload_space' );
+		update_site_option( 'blog_upload_space', 200 );
+
+		$this->assertEquals( 200, get_space_allowed() );
+	}
+
+	/**
+	 * @dataProvider data_blog_upload_space
+	 *
+	 * @param mixed $site_option    Option to assign to the site's `blog_upload_space`.
+	 * @param mixed $network_option Option to assign to the network's `blog_upload_space`.
+	 * @param int   $expected       Expected return value.
+	 */
+	public function test_get_space_allowed( $site_option, $network_option, $expected ) {
+		update_option( 'blog_upload_space', $site_option );
+		update_site_option( 'blog_upload_space', $network_option );
+
+		$this->assertEquals( $expected, get_space_allowed() );
+	}
+
+	public function data_blog_upload_space() {
+		return array(
+			// A valid site option will be preferred over a network option.
+			array( 111,   200,   111 ),
+			array( -1,    200,   -1  ),
+			array( 222,   0,     222 ),
+
+			// Non-numeric site options should result in a fallback to the network option.
+			array( '',    333,   333 ),
+			array( false, 444,   444 ),
+			array( 'NAN', 555,   555 ),
+			array( false, -10,   -10 ),
+
+			// If neither network or site options are valid, fallback to the default.
+			array( false, false, 100 ),
+
+			// These are likely unexpected.
+			array( 0,     666,   100 ),
+			array( false, 0,     100 ),
+			array( 'NAN', 0,     100 ),
+		);
+	}
+
+	public function test_get_space_allowed_filtered() {
+		update_option( 'blog_upload_space', 777 );
+		update_site_option( 'blog_upload_space', 888 );
+
+		add_filter( 'get_space_allowed', array( $this, '_filter_space_allowed' ) );
+		$space_allowed = get_space_allowed();
+		remove_filter( 'get_space_allowed', array( $this, '_filter_space_allowed' ) );
+
+		$this->assertEquals( 999, $space_allowed );
+	}
+
+	public function _filter_space_allowed() {
+		return 999;
+	}
+}
+
+endif;
\ No newline at end of file
diff --git a/tests/phpunit/tests/multisite/site.php b/tests/phpunit/tests/multisite/site.php
index 9e01fd771d..46f5f92813 100644
--- a/tests/phpunit/tests/multisite/site.php
+++ b/tests/phpunit/tests/multisite/site.php
@@ -905,57 +905,6 @@ class Tests_Multisite_Site extends WP_UnitTestCase {
 		$this->assertEquals( '', $info['error'] );
 	}
 
-	/**
-	 * Tests to handle the possibilities provided for in `get_space_allowed()`,
-	 * which is used when checking for upload quota limits. Originally part of
-	 * ticket #18119.
-	 */
-	function test_get_space_allowed_default() {
-		$this->assertEquals( 100, self::$space_allowed );
-	}
-
-	/**
-	 * When an individual site's option is defined, it is used over the option
-	 * defined at the network level.
-	 */
-	function test_get_space_allowed_from_blog_option() {
-		update_option( 'blog_upload_space', 123 );
-		update_site_option( 'blog_upload_space', 200 );
-		$this->assertEquals( 123, get_space_allowed() );
-	}
-
-	/**
-	 * If an individual site's option is not available, the default network
-	 * level option is used as a fallback.
-	 */
-	function test_get_space_allowed_from_network_option() {
-		update_option( 'blog_upload_space', false );
-		update_site_option( 'blog_upload_space', 200 );
-		$this->assertEquals( 200, get_space_allowed() );
-	}
-
-	/**
-	 * If neither the site or network options are available, 100 is used as
-	 * a hard coded fallback.
-	 */
-	function test_get_space_allowed_no_option_fallback() {
-		update_option( 'blog_upload_space', false );
-		update_site_option( 'blog_upload_space', false );
-		$this->assertEquals( 100, get_space_allowed() );
-	}
-
-	function test_get_space_allowed_negative_blog_option() {
-		update_option( 'blog_upload_space', -1 );
-		update_site_option( 'blog_upload_space', 200 );
-		$this->assertEquals( -1, get_space_allowed() );
-	}
-
-	function test_get_space_allowed_negative_site_option() {
-		update_option( 'blog_upload_space', false );
-		update_site_option( 'blog_upload_space', -1 );
-		$this->assertEquals( -1, get_space_allowed() );
-	}
-
 	/**
 	 * Provide a hardcoded amount for space used when testing upload quota,
 	 * allowed space, and available space.