User access to FTP and at same time set www-data as owner/group

In order for wordpress to read/write the owner of the files/folders need to be www-data.

Now the problem is that if the owner is www-data of group filetransfer (www-data:filetransfer) then the FTP user ftpclient1 can login but does not have write permission to folders set to www-data:filetransfer because he’s not the owner.

How can I make this work? I rather not set 777 on those folders.