The term blue print was common years ago, but with home plans drawn on white paper with black or blue ink today, the term is still widely used. The concept of the drawings remains the same, but denoting changes to the drawing has become easier.
Today, when a change is made it typically is made with a red pencil or red pen and the term usually used is redlining. Modifications made to existing house plans are clearly marked to show the change. Whether it is the placement of an interior wall or the extension of an outside wall, redlining allows the change to be shown without disturbing the existing home plans.
This serves two purposes. First, builders will know that a change to the original has been made and, second it gives the builder a flag to look for load-bearing walls or beams that may have been interrupted due to the change. When a homeowner redlines the plans, it is up to the contractor to insure safety issues are addressed as well as zoning and construction codes are still in compliance with the changes.
One of the most common changes associated with home plans is having the home turned 180-degrees on the lot. Usually, this can be a simple change by printing the plans with a mirrored effect. The problem here is that everything is turned 180-degrees, including written instructions. Imagine holding a printed paper to a mirror. This will be the outcome of mirrored prints.
Since most home plans are available in CAD form, computer aided graphics, the home plans can be mirrored and the written instructions turned back to their original position. Many companies selling blueprints will also include the prints in electronic format to allow changes to be made quickly and easily. Again, safety issues must be address prior to the beginning of construction.