Chances are if a spa were separate from the pool or on a different circulation system, a spill ledge would not be necessary.  In this case it would be necessary to size equipment to the scaled down body of water in the spa.   Many of the decisions regarding the shell installation, decks and jet options would be the same with the biggest criteria for equipment being efficiency and operating costs.  Spa only jobs are a lot more difficult to sell because the comparison is usually to portable units.  However, the builder that can give the homeowner a "custom spa" to their environment that resembles or is a part of the pool, the comfort and the fingertip technology of a portable unit, and then it becomes desirable because it is the best of both worlds.

By offering the pool/spa combination, it offers the cost savings of utilizing the same equipment for both components and gives the builder the flexibility of attaching a spill ledge or incorporating it into larger water features.  This is a big benefit on feature benefit list of selling a pool/spa combination.  By offering this water feature concept as an added feature at little added costs, makes it a very attractive package.  Although this seems complex, common industry technology offers the ease of operation quite simple (all you need is the right electrician).  Automatic sequencing of the control valves through an "electronic command center" makes this a realistic process for the simplest of builders.  "Manual" operation of the valve sequencing and processes to achieve heating, spa mode, turbo charging the jets and water feature make it far too complex for most homeowners to understand much less enjoy.

Therefore, based on offered options such as fiber optic lighting, water features, spa side controls, in home control panels, a certain amount of time needs to be spent finding out about the customer needs and desires.  Electronic controllers come in various models and are basically priced on what they can do.  Most major equipment manufacturers offer such controllers and representative assistance and training at no charge.  Much like a VCR, TV or DVD player, the more features the more you pay, but remember the machine seems intimidating.  "PLAY" or "ON" usually works well enough for most but, others may want to program for months at a time.  First step is getting to know the customer.  Second step is getting it hooked up right.