Enclosing your porch would be a less expensive alternative to building a new one, so when you are looking for a way to live in the space all year long, that may be a possibility.
Foundation and structural support. An addition to the porch must include the ability to withstand the additional load.
Heating and cooling. Make sure the porch feeling is preserved by using cross ventilation with ceiling fans. Natural cross-ventilation enhances the porch feeling. Make sure that the porch is well insulated.
Interior finishes. In order to ensure a smooth transition from one room to the next it is best to stick to the design cues of the adjacent room and maintain some of the porch's main features.
Roof and gutters. A solid roof should be installed, preferably made of the same roofing material as the rest of the house, including the gutters and downspouts.
Energy-efficient windows. As the seasons change, consider how much sunlight you are likely to receive in your room. Window coverings can be used to screen out the summer sun and insulate the room in the winter if needed.
Relationship of interior and exterior spaces. If an existing porch is converted into an enclosed room, there is a possibility that the adjacent room will be darker. Consider carefully where your windows and skylights should go so that interior spaces don't tend to look cave-like.