The following is a fun little step-by-step pictorial of a 1000 sq ft addition to an older home. The original home was built on post and beam (no foundation). The pictures below show construction after a foundation was placed under the existing structure.
The original home built circa 1937 sits on a large lot with many large shade trees and other features which cried "Save Me".

The windows have been replaced, new insulation, a new heating system and other minor remodeling.

Foundation consisted of a continuous footing and a single dividing cripple wall footing to create a maximum floor span of 14ft (standard dimensional lumber to match the existing structure).
This addition looks small in its foot print, but relied on verticle space (two stories) to gain square footage without using up valuable yard space.
The second story was designed with only 5' walls and a vaulted "stick" framed ceiling. This was to keep the roof line low and prevent it from appearing as an overpowering addition.
The addition has now merged with an overframing extension joining the old and new roof. Suddenly the size changes and the focus of the house is now at the new entry. The columns at the front are for a porch that will also tie new roof to old.
Check back to see the continued progression. Coming next: roofing and closing up.