On the positive side, access to the work area is usually easy. You will already have a roof and in some cases a foundation. Lighting, ventilation and exterior egress are often available. A screened porch might have a foundation and floor, as well as electricity and the basic framework for walls in place. This will save both time and money.

On the negative side, you are probably using the space for other purposes. You will need to evaluate the advantages of the ways you use the space now as opposed to the way you will use the space in the future. One challenge when converting a covered porch is that you will often need to construct the sub-floor in order to make the porch area level with the rest of the home. You will also need to build or finish exterior walls (which is not necessary when converting a basement or attic). The exterior walls will need to either match or blend naturally with the exterior of the rest of the house. You will also need to provide heat and/or air conditioning and electricity (if not already connected).