Predesigned house plans are often developed originally as a client project, either as a custom home for a homeowner, as a spec home for a builder, or as part of a series of homes for a developer (who built that home dozens if not hundreds of times).

The designer was paid to develop the plans by a client, from the conceptual drawings through to the construction documents. The designer owns the copyright to the design as their intellectual property and therefore offer to sell that design to others. The heavy investment in capital was made by the original client, allowing the designer to now offer that plan for a very modest fee (often 1/30th of the original cost).

In some instances, a designer may choose to invest the time and resources required to create a new design even without a client (we call this a "spec design"). A designer may do this when they are confident that the design will meet a specific need in the market, and that the investment will pay off in the long run with as that plan sells to various customers in the future.

Essentially, the house plan has already been developed, and the capital required to create it has been invested by others, meaning that the house plan can now be offered to you at a price point that is extremely attractive.