i kind of see a few things i'd like added, instead of just one.
i noticed a bug in some of the floor tiles; if you place a water tile, it will not share that tile location with a wall like the other floor tiles do.
i also found it hard sometimes to see where your placing some tiles, so maybe an invisibility function like AoS's house builder?
having a 5th floor ability would make it better too, so you can have 4 story buildings with roofs.
being able to place roofs, like the raised ones, would be a strong addition.
i know it would probably be a good chunk of extra code, but how about making the doors working instead of immobile like the walls?
the option of hue capable creations in the designer would also be a great addition as well.
another thing i'd like is that i dont normally know what i want to call the building until i finish it, but you still need to come up with a name at the beginning; could it be possible to add a rename design function? i tried renaming it manually, but it corrupts the files and it will crash the builder if you open it after it's changed, and sometimes the server that tries to use it will crash too.
could you also make a tutorial to tell how to make the houses show up on the map for players to find them via the radar?
it would be handy if the foundation floor had a quick key like the other floors too. i like them quick keys
lastly, and the most important thing that players want to see on the houses made with this program, is the ability to have the houses be treated like a home that can have things locked down in it. i'm guessing that's what the alpha stage house on in rc8 is for?
what you have so far is a very strong layout for what will most likely be a great program that all staff will end up using in time.