WordPress Multisite subdirectory network: subsite URL generation when core is installed in a subdirectory

Answer: Expected URL pattern

When WordPress core is installed in a subdirectory (e.g., /blog) and the network is configured for subdirectory sites, a newly created subsite receives a URL of the form:

https://example.com/blog/subsite-slug/

The network root is treated as the path where the core resides, so functions such as get_blog_permalink() and network_home_url() automatically prepend the core subdirectory to the subsite slug.

Confirmed facts (based on core behavior)

• The siteurl option stored in the subsite's wp_options table is set to the full URL that includes the core subdirectory (e.g., https://example.com/blog/site1/).
• get_blog_permalink() returns the permalink by concatenating the network home URL (which already contains the core subdirectory) with the subsite's path, so the core subdirectory appears only once in the generated link.
• If WP_HOME and WP_SITEURL constants are defined in wp-config.php to values that include the core subdirectory, the generated URLs remain consistent and manual correction of the siteurl option is unnecessary.

Likely explanation (needs verification)

The URL generation logic assumes the network root equals the WordPress installation path. When the core lives in a subdirectory, that subdirectory becomes part of the network root, and all subsite URLs are built relative to it. This behavior holds unless:

• A domain-mapping or reverse-proxy plugin alters the perceived network root.
• Custom WP_HOME/WP_SITEURL values are set that duplicate or omit the core subdirectory.
• Permalink rewrite rules or .htaccess modifications change how the core subdirectory is reflected in URLs.

Steps to verify and standardize URL generation

1. Install WordPress 6.6+ in a subdirectory (e.g., /blog) on a test server.
2. Enable Multisite with the "Sub-directories" option via the Network Setup screen.
3. Create a new subsite (slug: site1) from Network Admin → Sites.
4. Note the "Site Address (URL)" shown; it should read https://example.com/blog/site1/.
5. In the subsite's dashboard, confirm Settings → General shows both WordPress Address (URL) and Site Address (URL) as https://example.com/blog/site1/.
6. Add a temporary PHP snippet (e.g., in a theme's functions.php) to output the permalink:

echo get_blog_permalink( get_current_blog_id(), 1 );

1. Verify the output starts with https://example.com/blog/site1/.
2. If you want environment-independent URLs, add to wp-config.php:

define( 'WP_HOME', 'https://example.com/blog' );
define( 'WP_SITEURL', 'https://example.com/blog' );

1. Reload the site and repeat the checks above; URLs should remain unchanged without manual siteurl edits.

Missing diagnostic detail

To confirm whether any manual correction is needed, verify one thing: Are WP_HOME or WP_SITEURL constants defined, or is a domain-mapping/reverse-proxy plugin active? If either is present, the URL pattern may deviate and require adjustment.