<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?sub.domain.nl$
RewriteCond %{REQUEST_URI} !^/dir1/dir2/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /dir1/dir2/$1
RewriteCond %{HTTP_HOST} ^(www.)?sub.domain.nl$
RewriteRule ^(/)?$ dir1/dir2/index.php [L]
</IfModule>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /sub.domain.nl/dir1/dir2/
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /sub.domain.nl/dir1/dir2/index.php [L]
</IfModule>
# END WordPress