Remember that plans are not necessarily "as is." Builders can use a reverse set of plans (sometimes called a mirror image) to better site a house. It's also possible to hire an architect or modification service to personalize a plan. Ask if reproducible prints or electronic CADD files are available for the selected plan. Either will make the alteration process quicker and easier.
Every set of custom house plans includes several items which need to be blown up and detailed for further clarification. If a set of plans does not provide additional close-up drawings (for things such as beam connections, deck railings, stairway construction, etc.), then it is not a complete set. It is very easy to cut corners during the planning process. But the end result will be higher costs for the home owner - and lots of frustration for the building contractor.
One item to look for when evaluating a house plans' quality level is the number of structural sections which are shown on the plan. Highly detailed sets of plans will always cut lots of sectional views through the house to show every different roof framing situation. This might mean that 10 or 12 (or even more) sections need to be drawn for a large house plan. And even a small house plan should include 3 or 4 sections minimum. However, many home plans available today (especially plans purchased through inexpensive plan directories) cut corners in this department and only show one or two house section views. This means that the builder will have to guess at the rest of the house framing.
On the other hand, stock house plans are substantial past drawn designs by a skilled designer or architect ideally of a specific purpose or general use. The question is, why choose a stock plan over a custom drawn plan? It all amounts to various reasons that are discussed below.