Spec home is a home that has been made by the construction company as a prototype to showcase the quality of their service. And, custom houses are something that is built by the choice of the customer. The differences are many in this aspect.
Matter of involvement
Usually, in a spec house, the deciding authority is the construction company. They take all the initiative in regard to the house plan, design, and budget. But in a custom house, the final decision is made by the person who owns the property and pays for it.
A spec house is generally not customizable. It is made by following the plan that the company thinks best represents the idea of a house in that particular area. But in a custom house, everything could be custom-made. The house owner has control of directing the house.
A spec house comparatively takes a little amount of expense to be constructed. As the construction company builds the house with other houses, the cost can be managed. But a custom house takes a lot of money to be built. The fact that people try to add many benefits to the house makes it more expensive.
The location of a spec house may be in the front of a particular area for the convenience of the people. But a custom house is built at the property where the owner wanted it to be.