[Desire 816 Q&A, Help & Troubleshooting] thoctor:
The whole reason I even rooted my phone (a5_chl) in the first place was to try to get more room for my apps with the limited storage(4.5ggbs useable out out 8) of the 816. My last device had 16ggbs internal storage with over 8ggbs useable and that was even low for me but I like this device a lot so I researched my options.

Option 1) after root install a write permission modification for lollipop that installs a WP_MOD.ko script into the system partition that restores write permissions to the system partition and another feature that disables the phone's SElinux security permissions from enforcing to permissive. This mod needs to do this last one because the phone is still S on and this is the workaround to be able to gain the permissions in the system partition.

Option 2) pay $25 for the Sunshine app developed by Jcase and beaups (who have a Sunshine app help forum here on XDA) and download their app from theroot.ninja.com and read and follow the instructions listed on the site for verifying compatibility with your phone and making it true S off. HTC devices are set S on from the manufacture locking access to the boot, radio and other partitions to prevent flashing different radio or boot images to your device after unlocking the bootloader and rooting. The main benefits of true S off over the write permission mod is that your phone's SElinux permissions stay set to enforcing for security purposes and any future stock updates can be done with S off now S on and permanent. With the write permission mod to do any signed stock updates you would have to first remove the mod and restore your SElinux permissions before updating which I'm sure you could do but it would be more difficult than just doing the update but it's free and true S off isn't.

Having said all this I personally prefer S off for the other benefits it gives as the write permission mod only changes the system permission issue. It is not true S off and your phone's nand storage is still locked down. After either option you then download Link2SD(which I use) and remove your external SD card and install and copy it to a folder on your computer's home screen then reparation the card using EaseUS partition manager program setting how many ggbs your want for the new app storage partition and how many for the normal SD card(usually 10 to 20 percent of the whole cards available storage but I made sure to have at least 10+ggbs for mine. Just don't partition half or more of your card. Like I said the dev of the app recommend those percentages but you can go a bit higher within reason. The app partition I named data and set to ext2 format and primary then the remaining partition I set to FAT32 format and named ExtSDcard and set to primary also but you can name them whatever. The main thing is to use the ext2 file format for stock rom and set then both to primary before finally repartitioning them in EaeseUS as that format is recommended when your are on stock roms.

Then you reinstall the card and open the link2SD app and it will have you select the format you set your app partition to (ext2) then reboot the phone to set the files in the system partition needed to load the apps from boot with the correct permissions and your done. Start moving the apps your want to the external SD storage although I leave apps that I use widgets on and root apps like link2SD ,Titanium backup,Greenify etc. on the internal storage. I move most everything else to the SD card and for a couple of bucks I pay for the link2SD paid version to be able to move all apps lib files and data to the card as well. That is where your real storage savings comes from when your can move the data from the app not just the app itself. It's the data that eats up most of your internal storage not the app itself.