Hi!
This has happened to few times now. So, I have built a building and then monster attacked my village. One of them went throught the construction site and used a ladder. That yellow square, which you can see in building menu, never get finish. And whole building remains unfinished because of that. It doesn’t give error code or anything, so this is all information I can give you.
Hello @M1N1 
Monsters can use ladders, that’s intended.
Could you upload the savefile with your unfinished building?
To upload a save, locate the folder in
C:\Program Files (x86)\Steam\steamapps\common\Stonehearth\saved_games (assuming a default Steam install), zip the individual save’s folder (not the entire saved_games folder), and upload it. If the *.zip is less than 10 MB, feel free to upload it directly to the Discourse. If larger, please upload it to a cloud storage site like Dropbox, Google Drive, File Dropper, etc. and post the sharing link here.
Something weird is happening with the building, but I’m not sure what. Perhaps @Relyss will be able to see the issue, she’s much better than I am with building bugs.