Places where homeowners can find plans for indoor staircases include ExtremeHowTo.com and MotherEarthNews.com. As of 2015, both of these websites offer the plans and instructions for free without registering with the website for newsletters and other things.
At ExtremeHowTo.com, homeowners find detailed instructions on building a staircase. There is information to guide a homeowner in creating a layout, as well as calculating space needed and the angle of the staircase. There are also step-by-step instructions and pictures to guide the project.
MotherEarthNews.com covers the planning stages and types of staircases. There is a slide show that shows some of the important calculations and what the steps should look like during the building process.