If you are editing the text file and don't want to screw up the tabs and spacing like LOS said, this is how I do it. It's a litte tedious but it's a lot less tedious than going over it again and again to try and find the mistake.
Pick a building roughly similar to your new building. I.E. if you are building a barracks pick County Watch. Highlight the text, and only the text that you want to replace, by dragging over it with the mouse. Once it is highlighted start typing, the letters will be replaced but all the spacing and tabs around it will be untouched. Don't touch the spacebar, you don't need to. This is how I did all of my mods before I got gnome editor and it works well.