When the times get tough, like now, and we look to save a few pennies, scratch building becomes an option, especially for our buildings.
Your local public library is a create source for building plans. The architectural books and historic books contain many great references, even some of the hardback railroad books contain great references.
That will help in getting ideas going and actual plans developed for the new building. Then comes the details…and speak directly to the window and doors….where can we get them. My success has come from the door and windows sold by RS Laser Kits. Also available are stair kits (try and find those elsewhere). Other manufactures have seemed to have disappeared. Check out the stair raiser kit.
Here is the link to the site: http://www.rslaserkits.com/N-Details.html