I'm switching to singleplayer modded because my friends have lost interest, I've already tried moving the server's "world" into the singleplayer's "world", it seems the way singleplayer handles playerdata is different than multiplayer, because my inventory is the same however there are no player created houses.
You can find world data in the %appdata%/.minecraft folder. As far as I'm aware, hosted servers with FTP have basically the same folder
Just try and copy the folder where the world data is in, and paste it in your local files, exactly the same place
There is a small chance that you will lose your items in the singleplayer world, but i recommend crafting a temporary chest on the server and putting all of your items in there first, just to be safe. You'll never know when something might break ;)
Hope that helps!