There's Building, and Then There's Building
A little clarification might be in order here. When you hear someone say, “We're building a house,” that could mean one of three things:
It could mean they are having a manufactured or modular home delivered to their lot and erected on that site.
It could mean they have a home going up in a new development, where they are buying from that developer. They have a choice of a few designs and work closely with the builder and his staff on features, product choices, and the like.
It could mean they are having a home designed especially for them, either in or out of a new development.

