File Permissions Between My User and www-data

This might be a stupid question but I honestly can't get any of the suggestions from other posts to stick so hoping someone might be able to give me some insight.

I have a web application and I'm writing some unit tests for it.

I load the web in a browser with selenium-webdriver, this hits the web server and does what ever it would do in a production environment.

The web server caches certain database queries which is desirable, but between tests I want to be able to clear the cached files.

The problem is that the permissions for the files I want to delete require sudo for me to delete them.

File permissions are this:

$ ls -l
total 16
drwxr-s--- 2 www-data www-data 4096 Aug 22 21:32 about+index
drwxr-s--- 2 www-data www-data 4096 Aug 22 21:32 blog+2016
drwxr-s--- 2 www-data www-data 4096 Aug 22 21:32 blog+index
drwxr-s--- 2 www-data www-data 4096 Aug 22 21:32 default+index

My user is part of the www-data group but the permissions don't seem to allow writing in the directory.

Main questions is, how do I set permissions on the "build" directory, and it's subdirectories for my site in such a way that it keeps it permissions for new subdirectories and files created by the web application?