There are benefits to either approach. Sharing a common wall with your house can require some careful soundproofing but can reduce heat loss and exterior finishing costs like siding. A shop located in an addition is more likely to be allowed to have living space or storage space above it, and it may be easier to make use of the existing plumbing and electrical and heating systems of your home to service your shop space. Another advantage of an addition is not having to run to an outbuilding in minus-30-degree weather or a summer downpour.
Some of the most important decisions in laying out a shop involve the placement of basic machines, such as the table saw and jointer. Deciding where to put them depends in part on the scale of woodworking you do, which determines the clearance area around these machines if you need to handle long stock or large sheet goods. You’ll also want to consider work flow to avoid an excess amount of running around the shop to accomplish tasks, as well as how to power your machines without turning your shop into a snakepit of extension cords.
As the comic George Carlin might say, a shop is mostly just “a place to put your stuff.” The physical aspect of a shop is indeed simply a space of some kind — a garage, a barn, a teepee — that keeps the rain, rust, and robbers away and houses a collection of implements needed to saw, plane, slice, sand, and pound raw wood into useful objects. But just like the difference between a house and a home, what transforms a building full of tools into a comfortable workshop goes beyond the mere physical aspect.