Hull angles aren't presenting correctly.
Gopher
Draw a square hull of 8x8, then erase a single square in each corner. In Space Haven, this should create diagonal hull walls, but this designer leaves that square open. Observent users might know to pretend there is a hull block there or think to place a wall block to remind them, but it is a failing that should be addressed for accuracy.
Jay-Nicolas Hackleman
Came here to post this - all the interior corners are able to be built on: but this is not the case in game: https://ironoakgames.gyazo.com/dab091cc6e69285bb4bcc5f7c3b05d35