I'm trying to get used to building as I'm really bad at it...

I made this room with a small diagonal wall (the room used to have a foundation but I took it off). Now, I got tired of it so I moved that room to a different place of the house but as I tried to change the wall I get this problem (see picture below)

I deleted the diagonal wall and closed the room but now its like that part is a different room?

When I try to give it a foundation I'm not allowed to paint the small triangle either.

I'm sorry if this is hard to understand, I don't really know how to explain this

The room has an extention with arrows you can pull. You can give the room a new extention within the walls or fix the existing extension. Among your walls you have a tool to make a room from corner to corner. The easiest with a diagonal wall is probably to drag the corner to corner walls within the room over again. I hope you understand what I say.

Rooms are really 2 different things. Separate walls can make up a room, but still it's not always true the room extension is the same as your walls. If you do changes to a room the extension of the room might not follow. Get it?

To get the extension you can click on the floor I think.