Folder paths
Path of Exile 2 only reads .build files from a BuildPlanner folder inside its local documents path. The folder is almost the same everywhere — the prefix in front of it is what changes between Windows, Steam, Epic, and Steam Deck.
Unofficial fan-made guide. Not affiliated with, endorsed by, or sponsored by Grinding Gear Games.
Whatever the platform, you are looking for the same tail: Documents/My Games/Path of Exile 2/BuildPlanner. If that folder does not exist yet, create it — the game will not create it for you just because a file is sitting nearby.
Replace <Name> with your Windows account name. If you use OneDrive document redirection, your Documents folder may live under C:/Users/<Name>/OneDrive/Documents instead.
The Steam launcher on Windows uses the normal Windows documents path. There is no separate Steam folder for build files on Windows.
Like Steam on Windows, the Epic launcher reads from the standard documents path. The launcher you bought the game through does not move the BuildPlanner folder.
Under Proton the Windows documents folder lives inside the Proton prefix. The compatdata id (2315204395 here) can differ by install — confirm yours in the Steam app folder.
Open File Explorer, paste %USERPROFILE%/Documents/My Games/Path of Exile 2 into the address bar, and press Enter. Create a BuildPlanner folder there if it is missing.
Switch to Desktop Mode, open the file manager, and navigate into steamapps/compatdata. If several numbered folders exist, the right one contains a Path of Exile 2 documents tree under its Proton prefix.
Symptom: File sits in Path of Exile 2 but not in BuildPlanner.
Fix: Move it down into the BuildPlanner subfolder.
Symptom: Windows path looks right but the game still ignores the file.
Fix: Check whether Documents is redirected into OneDrive and use that path.
Symptom: Steam Deck path does not exist.
Fix: Open each numbered folder and find the one with the Path of Exile 2 documents tree.
Symptom: File is in the right place but still ignored.
Fix: Enable file extensions and confirm it ends in .build, not .build.txt.
Launch the game or reopen the in-game Build Planner so it re-scans the folder. A file added while the planner is open may not appear until you reopen it.
If the folder is correct and the file still does not show, the file itself may be the problem. Validate the JSON and confirm the extension.