One of the main reasons that building an addition can cost twice as much per square foot as remodeling an existing space is the foundation. Unless you’re building up or bumping out, every addition requires some sort of masonry foundation dug down below the frost line so it won’t get pushed around by freezing temperatures.The earthmoving and concrete-pouring required to support the new …
https://www.hgtv.com/design/remodel/mechanical-systems/choosing-an-additions-foundation

