Consider that if you need to underpin it can be easier/cheaper to tear down a structure like a porch and just start over.

Depends on the size, location of the property, quirks of the house. In order to enclose ours the builders need to add supports, which adds to the cost. Also depends on materials (wood vs. composite). But we were quoted between $20 and $50K, settling around $40 for some composite/some wood.

On an old row house built in the early 1900's? We would eventually like to extend the kitchen this way.