How to fill an area within a grid based wall

I am designing a system where players are able to create a grid based room, and can’t figure out a good way to fill the room once the wall has been built. I would like to fill each square within the grid with it’s own floor actor to allow editing of specific specific tiles later. I’ve tried a couple ideas, but they don’t work well with more complex room shapes like the one below. Any good solutions I may be missing? Thanks for the help.

No, that was my main issue. I figured I could just get all the vector points on the inside, but couldn’t figure out a way to determine what points are inside the walls.

Do the walls know which side is the inside? Then at least they could put tiles along that side, then other tiles can be added just because they are next to the existing ones…

What about a line trace from the wall? If it see another wall, then that face is on the inside.

I like that method. I’ve already started work on a different system, but that could still be very useful for determining inside walls and I’ll probably still use it, thanks for the help.